wellcontrol-v2-1.pdf

BLOW OUT PREVENTION & WELL CONTROL Version 2.1 March 2001

Dave Hawker

Corporate Mission To be a worldwide leader in providing drilling and geological monitoring solutions to the oil and gas industry, by utilizing innovative technologies and delivering exceptional customer service.

DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

CONTENTS 1 INTRODUCTION................................................................................................................................................... 3 2 PRESSURE GRADIENTS ..................................................................................................................................... 4 2.1 FORMATION RELATED PRESSURES...................................................................................................................... 4 2.2 WELLBORE BALANCING PRESSURES................................................................................................................... 5 2.2.1 Mud Hydrostatic ......................................................................................................................................... 6 2.2.2 Equivalent Circulating Density................................................................................................................... 7 2.2.3 Surge Pressures........................................................................................................................................... 8 2.2.4 Swab Pressures ........................................................................................................................................... 8 3 KICKS AND BLOWOUTS.................................................................................................................................. 10 3.1 DEFINITIONS ..................................................................................................................................................... 10 3.2 CAUSES OF KICKS ............................................................................................................................................ 11 3.3 KICK WARNING SIGNS ...................................................................................................................................... 12 3.4 INDICATIONS OF KICKS WHILE DRILLING ........................................................................................................ 13 3.4.1 Connection Gas......................................................................................................................................... 14 3.5 INDICATORS WHILE TRIPPING........................................................................................................................... 16 3.5.1 Trip Margin............................................................................................................................................... 17 3.6 GAS EXPANSION ............................................................................................................................................... 19 3.7 FLOWCHECKS ................................................................................................................................................... 20 4 KICK CONTROL EQUIPMENT ....................................................................................................................... 21 4.1 THE BOP STACK .............................................................................................................................................. 21 4.2 PREVENTERS AND RAMS ................................................................................................................................... 22 4.2.1 Annular Preventer..................................................................................................................................... 22 4.2.2 Ram Preventers ......................................................................................................................................... 23 4.3 STACK CONFIGURATION ................................................................................................................................... 24 4.4 SUBSEA EQUIPMENT ......................................................................................................................................... 25 4.4.1 Lower Marine Riser Package.................................................................................................................... 26 4.5 CHOKE MANIFOLD ............................................................................................................................................ 27 4.5.1 Choke and Kill Lines................................................................................................................................. 28 4.6 CLOSING THE PREVENTERS ............................................................................................................................... 29 4.6.1 Pressure source......................................................................................................................................... 29 4.6.2 Accumulators ............................................................................................................................................ 29 4.6.3 Control manifold ....................................................................................................................................... 30 4.7 DIVERTERS ....................................................................................................................................................... 32 4.8 INSIDE BLOWOUT PREVENTORS ........................................................................................................................ 33 4.8.1 Kelly Rigs .................................................................................................................................................. 33 4.8.2 Top Drive Rigs .......................................................................................................................................... 33 4.8.3 Additional Preventers................................................................................................................................ 34 4.9 ROTATING PREVENTERS ................................................................................................................................... 35 5 FRACTURE CALCULATIONS ......................................................................................................................... 36 5.1 5.2 5.3 5.4

LEAK OFF TEST ................................................................................................................................................ 36 FRACTURE PRESSURE ....................................................................................................................................... 38 MAXIMUM ALLOWABLE ANNULAR SURFACE PRESSURE .................................................................................. 41 KICK TOLERANCE ............................................................................................................................................. 43

6 WELL CONTROL PRINCIPLES & CALCULATIONS ................................................................................. 48 6.1 BALANCING BOTTOM HOLE PRESSURES .............................................................................................. 48 6.2 SHUT IN FORMULAS .......................................................................................................................................... 51 DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

6.3 INFLUX HEIGHT AND TYPE ............................................................................................................................... 51 6.4 STABILIZING SHUT IN PRESSURES..................................................................................................................... 53 6.5 INDUCED KICKS ................................................................................................................................................ 54 6.6 ONE WAY FLOATS ............................................................................................................................................ 54 6.7 SLOW CIRCULATING RATES .............................................................................................................................. 55 6.8 KILL MUDWEIGHT ............................................................................................................................................ 55 6.9 CIRCULATING THE KILL MUD .......................................................................................................................... 56 6.10 PRESSURE STEP DOWN ................................................................................................................................... 58 6.11 SUBSEA CONSIDERATIONS .............................................................................................................................. 59 7 WELL CONTROL METHODS .......................................................................................................................... 60 7.1 7.2 7.3 7.5

WAIT AND WEIGHT ........................................................................................................................................... 60 DRILLER’S METHOD ......................................................................................................................................... 62 CONCURRENT METHOD .................................................................................................................................... 64 VOLUMETRIC METHOD ..................................................................................................................................... 65

8 QLOG SOFTWARE............................................................................................................................................. 67 8.1 LEAK OFF PROGRAM ........................................................................................................................................ 67 8.2 KICK/KILL PROGRAM ....................................................................................................................................... 68 9 EXERCISES.......................................................................................................................................................... 70

DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

1 INTRODUCTION

W. Wylie ERCB Many problems can be encountered when drilling wells, especially in areas previously unexplored. Most problems can be considered an inconvenience that cost operating time, and therefore money, to resolve. Kicks and blowouts are also costly in terms of time, but unlike most other problems, they are unique in that they provide a direct threat to the safety of the drilling rig and it’s personnel. It is therefore very important that anyone involved in the monitoring of the well is fully able to recognize any and all of the signs that could indicate that a kick is taking place downhole. Early identification of such an event, allowing the driller to close in the well at the earliest opportunity, will make for a safer well control procedure and reduce the danger to rig and personnel. In addition, for the mud logging engineer, it is very important to understand the theories and procedures involved in a well control situation, in order to assist and support the operation.

DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

2 PRESSURE GRADIENTS Whatever the particular method of it’s occurrence, a kick occurs when the formation pore fluid pressure exceeds the balancing pressure in the annulus. This can lead to an influx of the formation fluids into the annulus, and thus, a kick that has to be controlled. Well control then consists, essentially, of removing the influx and restoring well balance so that annular pressure exceeds formation pressure. During this process, while the well is closed, it is vital to ensure that the pressures in the annulus do not fracture the weakest formation in the open hole. If this was to happen while a kick is taken place, then a blowout has occurred and this is the most difficult and dangerous of all drilling problems, and one that can lead to the loss of rig and personnel. For effective well control, it is therefore important to have a good understanding of the formation pressures involved and the annular pressures acting against them.

2.1 Formation Related Pressures Overburden Pressure

The pressure exerted, at a given depth, by the accumulated weight of overlying sediments. It is therefore a function of both rock matrix and pore fluid.

Formation Pressure

The pressure exerted by the fluid contained in the pore spaces of rocks. It is therefore equivalent to hydrostatic pressure of the regional formation fluid; the pressure exerted by the vertical column of formation fluid(s).

Fracture Pressure

The maximum pressure a formation can sustain without failure occurring. The weakest plane of formations is typically horizontal.

OVERBURDEN STRESS

Fracture Pressure

Formation Pore Fluid Pressure

Mud Hydrostatic Pressure DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

2.2 Wellbore Balancing Pressures Mud Hydrostatic Pressure

The pressure exerted by the weight of a vertical column of static drilling fluid or mud.

Equivalent Circulating Density

Although expressed in terms of equivalent mud weight, this is actually an increase in annular pressure caused by the frictional pressure losses resulting from mud circulation.

Swab Pressure

This is a reduction in annular pressure caused by the frictional pressure losses resulting from the mud movement caused when the drillstring is lifted. It will lead to an influx if the annular pressure is reduced below the formation pressure.

Surge Pressure

Increase in annular pressure resulting from the frictional pressure surges when the drillstring is run in hole. It can lead to formation breakdown if the surge pressure exceeds the fracture pressure.

Pressure Overburden (OBG) Fracture (Pfrac) Mud Hydrostatic Formation (FP) ECD

Vertical Depth

If the formation pressure exceeds the balancing annular pressure >>> KICK If the annular pressure exceeds the fracture pressure >>> FRACTURE

Mud weight must therefore be selected so that it is high enough to balance the formation pressure and prevent a kick, but it cannot be so high that it would cause a shallower, weaker, formation to fracture. This could lead to losing circulation of fluids at the shallower depth, while kicking from a deeper formation. This is an underground blowout. DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

The “annular pressure” is therefore key to well balance and control. It is dependent on the mud weight although this “static” pressure can be increased or decreased in certain situations: • • •

Lifting pipe causes swabbing which reduces annular pressure. Running pipe causes a pressure surge, which increases annular pressure. Circulating increases annular pressure.

Formation related pressures are typically quoted in terms of “equivalent mud weight” (emw), since this provides a convenient way of “visualizing” pressures exerted downhole.

2.2.1 Mud Hydrostatic Hydrostatic Pressure is defined as the pressure exerted at a given depth by the weight of a static column of fluid. It therefore follows, that when a given drilling fluid, or mud, fills the annulus, the pressure at any depth is equal to the Mud Hydrostatic Pressure. At any depth: -

HYDmud = mudweight x TVD x g PSI = PPG x ft x 0.052 KPa = kg/m3 x m x 0.00981 PSI = SG x feet x 0.433 PSI ppg KPa SG

= pounds per square inch = pounds per gallon = kilo Pascals = specific gravity (gm/cc)

This will tell us the balancing pressure, in the wellbore, when no drilling activity is taking place and the mud column is static. As soon as any movement of the mud is initiated, then frictional pressure losses will result in either an increase, or decrease, in the balancing pressure, depending on the particular activity, which is taking place. At all times, it is important to know what the annular balancing pressure is, and the relationship with the “lithological” pressures acting against them: •

If formation pressure is allowed to exceed the wellbore pressure, then formation fluids can influx into the wellbore and a kick may result.

•

If the wellbore pressure is allowed to exceed the fracture pressure, then fracture can result, leading to lost circulation and possible blowout.

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

2.2.2 Equivalent Circulating Density During circulation, the pressure exerted by the “dynamic” fluid column at the bottom of the hole increases (and also the equivalent pressure at any point in the annulus). This increase results from the frictional forces and annular pressure losses caused by the fluid movement. Knowing this pressure is extremely important during drilling, since the balancing pressure in the wellbore is now higher than the pressure due to the static mud column. Higher circulating pressure will result in: • • • • •

Greater overbalance in comparison to the formation pressure Increased risk of formation flushing More severe formation invasion Increased risk of differential sticking Greater load exerted on the surface equipment

The increased pressure is termed the Dynamic Pressure or Bottom Hole Circulating Pressure (BHCP).

BHCP = HYDmud + ∆ Pa

where ∆ Pa is the sum of the annular pressure losses

When this pressure is converted to an equivalent mudweight, the term Equivalent Circulating Density is used. ECD = MW +

∆ Pa

(g x TVD) PPG

= PPG + (PSI / (ft x 0.052))

KPa

= kg/m3 + (Kpa / (m x 0.00981))

The weight of drilled cuttings also needs to be considered when drilling. The weight of the cuttings loading the annulus, at any time, will act, in addition to the weight of the mud, to increase the pressure at the bottom of the hole. Similar to the increase in bottom hole pressure when circulating (ECD), pressure changes are seen as a result of induced mud movement, and resulting frictional pressures, when pipe is run in, or pulled out, of the hole.

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

2.2.3 Surge Pressures Surge Pressures result when pipe is run into the hole. This causes an upward movement of the mud in the annulus as it is being displaced by the drillstring (as seen by the mud displaced at surface into the pit system), resulting in frictional pressure. This frictional pressure causes an increase, or surge, in pressure when the pipe is being run into the hole. The size of the pressure increase is dependent on a number of factors, including the length of pipe, the pipe running speed, the annular clearance and whether the pipe is open or closed. In addition to the frictional pressure, which can be calculated, it is also reasonable to assume that fast downward movement of the pipe will cause a shock wave that will travel through the mud and be damaging to the wellbore. Surge pressures will certainly cause damage to formations, causing mud invasion of permeable formations, unstable hole conditions etc.

The real danger of surge pressure, however, is that if it is too excessive, it could exceed the fracture pressure of weaker or unconsolidated formations and cause breakdown. It is a common misconception, that if the string is inside casing, then the open wellbore is safe from surge pressures. This is most definitely not the case! Whatever the depth of the bit during running in, the surge pressure caused by the mud movement to that depth, will also be acting at the bottom of the hole. Therefore, even if the string is inside casing, the resulting surge pressure, if large enough, could be causing breakdown of a formation in the open wellbore. This is extremely pertinent when the hole depth is not too far beyond the last casing point! Running casing is a particularly vulnerable time, for surge pressures, due to the small annular clearance and the fact that the casing is closed ended. For this reason, casing is always run at a slow speed, and mud displacements are very closely monitored.

2.2.4 Swab Pressures Swab Pressures, again, result from the friction caused by the mud movement, this time resulting from lifting the pipe out of the hole. The frictional pressure losses, with upward pipe movement, now result in an overall reduction in the mud hydrostatic pressure.

The mud movement results principally from two processes: 1. With slower pipe movement, an initial upward movement of the mud surrounding the pipe may result. Due to the mud’s viscosity, it can tend to “cling” to the pipe and be dragged upward with the pipe lift.

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

2. More importantly, as the pipe lift continues, and especially with rapid pipe movement, a void space is left immediately beneath the bit and, naturally, mud from the annulus will fall to fill this void.

This frictional pressure loss causes a reduction in the mud hydrostatic pressure. If the pressure is reduced below the formation pore fluid pressure, then two things can result: -

1. With impermeable shale type formations, the underbalanced situation causes the formation to fracture and cave at the borehole wall. This generates the familiar pressure cavings that can load the annulus and lead to pack off of the drill string. 2. With permeable formations, the situation is far more critical and, simply, the underbalanced situation leads to the invasion of formation fluids, which may result in a kick.

In addition to these frictional pressure losses, a piston type process can lead to further fluid influx from permeable formations. When full gauge tools such as stabilizers are pulled passed permeable formations, the lack of annular clearance can cause a syringe type effect, drawing fluids into the borehole. •

More than 25% of blowouts result from reduced hydrostatic pressure caused by swabbing.

•

Beside the well safety aspect, invasion of fluids due to swabbing can lead to mud contamination and necessitate the costly task of replacing the mud.

•

Pressure changes due to changing pipe direction, eg during connections, can be particularly damaging to the well by causing sloughing shale, by forming bridges or ledges, and by causing hole fill requiring reaming.

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

3 KICKS AND BLOWOUTS 3.1 Definitions What is a kick?

An influx of formation fluid into the wellbore that can be controlled at surface.

What criteria are necessary for a kick to occur? 1. The formation pressure must exceed the wellbore or annular pressure. Fluids will always flow in the direction of decreasing or least pressure. 2. The formation must be permeable in order for the formation fluids to flow.

What is a blowout?

A flow of formation fluids that cannot be controlled at surface.

What is an underground blowout? An underground blowout occurs when there is an uncontrollable flow of fluids between two formations. In other words, one formation is kicking while, at the same time, another formation is loosing circulation.

What is a surface blowout?

A surface blowout occurs when the well cannot be shut in to prevent the flow of fluids at surface.

Preventing a kick from becoming a blowout is paramount in well control!

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

3.2 Causes Of Kicks Not keeping the hole full when tripping out of hole When pipe is pulled from the hole, mud must be pumped into the hole to replace the steel volume removed. If not, the mud level in the hole will drop, leading to a reduction in the overall mud hydrostatic pressure. Keeping the hole full is extremely critical when pulling drill collars owing to the large steel volume.

Reducing annular pressure through swabbing Frictional forces resulting from the mud movement caused by lifting pipe, reduce the annular pressure. This is most critical at the beginning of a trip when the well is balanced by mud hydrostatic and when swab pressures are greatest.

Lost circulation If drilling fluid is being lost to a formation, it can lead to drop in mud level in the wellbore and reduced hydrostatic pressure.

Excessive ROP when drilling through gaseous sands If too much gas is allowed into the annulus, especially as it rises and starts expanding, it will cause a reduction in the annular pressure.

Underpressured formations May be subject to fracture and lost circulation which could result in a loss of hydrostatic head in the annulus.

Overpressured formations Naturally, if formation pressure exceeds the annular pressure, then a kick may result.

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

3.3 Kick Warning Signs Before an influx or kick actually occurs, there are a number of signs and indications that can give possible warnings that conditions exist for such an event to occur or, indeed, that such an event is about to take place.

Lost circulation zones

Large surge pressures should result in closer attention to possible signs of fracture and lost circulation. Weaker, fractured formations may be identified by higher ROP’s and higher, erratic torque Reduced mud returns, identified from a reduction in mud flow and decreasing pit volume, indicate a loss of fluids to the formation.

Transitional zones

Increasing ROP and decreasing drilling exponent trend. Increasing gas levels. Appearance of connection gas. Hole instability indications, tight hole, drilling torque, overpull and drag. Increasing mud temperature. Increased cuttings volume, cavings, reduced shale density.

Sealed overpressured bodies Immediate drill break resulting from the pressure differential and the higher porosity.

A Drill Break should always be Flow Checked, in order to determine whether it is associated with an overpressured zone and possible influx.

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

3.4 Indications Of Kicks While Drilling The following influx indicators are listed in the typical order that they would become apparent by surface measurements. •

Gradually decreasing Pump Pressure There may also be an associated increase in the Pump Rate. The drop in pump pressures as a direct result of lower density formation fluids entering the wellbore, reducing the overall mud hydrostatic. The pressure drop will be most significant with gas and worsened as gas expansion takes place. Initial pressure drop may be slow and gradual, but the longer the kick goes undetected, the more “exponential” the drop in pressure.

•

Increased mud flow from annulus, followed by…..

•

An associated increase in mud pit levels As formation fluids enter the borehole, an equivalent volume of mud will, necessarily, be displaced from the annulus at the surface. This is in addition to the mud volume being circulated so that the mud flow rate will show an increase. In the case of a gas influx, mud displacement will increase dramatically as gas expansion takes place

As the influx continues……. •

Variations in Hookload/WOB Although certainly not a primary indicator, these indications may be seen as the buoyancy effect on the string is modified.

If the influx reaches surface…. •

Contaminated mud, especially gas cut Reduced mud density. Change in chloride content (typically increase). Associated gas response. Pressure indicators such as cavings, increased mud temperature.

A kick should always be detected before the influx reaches surface!! EARLY DETECTION…..FLOW CHECK…..SHUT IN IF FLOWING DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

3.4.1 Connection Gas Connection gases are an extremely accurate indication of increasing formation (and therefore a warning of a possible kick) resulting from a temporary underbalance in the wellbore. The connection gas will appear as a short duration, sharp gas response, one “bottoms-up” time after the pumps are restarted following the connection.

This temporary underbalance can result as follows: •

A pressure reduction (to the ECD) due to swabbing when the pipe is lifted.

•

A reduction to mud hydrostatic when pumping is stopped and the string is set in slips.

•

A piston type suction from full gauge tools such as stabilizers and bit, as they are pulled passed permeable formations.

Swabbing results when, initially, mud is lifted with the string, due to it’s viscosity. The mud movement results in frictional pressure loss that reduces the annular pressure. This occurs for the entire length of drillstring. In addition, mud movement also results from it “dropping” to fill the void left by the pipe as it is lifted.

If Annular Pressure < Formation Pressure, then an influx can result The pressure reduction caused through swabbing increases with: • • • •

Pipe pulling speed Length of drillstring Mud viscosity Smaller annular clearance

An influx can occur from anywhere in the open hole if a formation is permeable and is brought into a condition of underbalance. However, connection gases are most likely to be generated from the bottom of the hole: • • •

This is where the pressure drop is greatest Here, there is the smallest annular clearance with the BHA and drillcollars, as opposed to drill pipe. There will be no filter cake for protection against small influxes.

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

Connection gas can also be produced from impermeable shales through fracture and caving (left), rather than through influx as with permeable formations. As cavings are generated from the borehole wall, porosity is exposed and, in the process, gas is released.

Impermeable

Permeable

FP > Phyd

Connection gases then, clearly indicate an influx of formation fluids when annular pressure is reduced temporarily. Once connection gas is recorded, subsequent connections should be very closely monitored for signs of increasing pressure and/or increased swabbing. An increasing trend could indicate that the well is getting closer and closer to balance and that a kick may eventually result, rather than a temporary influx.

Increase in Liberated Gas Produced Gas CG

This reduction in differential pressure may result from: •

Increasing formation through a transition zone,

pressure

OR •

CG

Well Flowing

A reduction in annular pressure as more gas, through increased swabbing, is allowed into the annulus.

If background gases and connection gases are increasing, the mud weight should certainly be increased to bring the well back on to balance.

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

3.5 Indicators While Tripping •

Insufficient Hole Fill When tripping out of hole, the hole is not taking enough mud fill to compensate for the pipe volume that has been pulled from the hole. This may indicate that: A kick has been swabbed into the hole, or that… Mud is being lost to the formation

•

A “wet trip” Where the influx and pressure, beneath the string, prevents mud from draining from the string as it is lifted.

•

Swabbing Excessive swabbing can be identified through the change in trip tank volume as individual stands of pipe are being lifted. The trip tank may be seen to initially gain mud before the mud level drops in the hole to allow fill to take place.

•

Pit Gain A continual increase in trip tank level clearly shows that a kick is taking place.

•

Mud Flow Similar, mud flowing at surface indicates an influx. Flow may also result from swabbed fluids that are migrating and expanding in the annulus. This in itself, may be sufficient to reduce hydrostatic further to allow an influx to take place.

•

Hole Fill Excessive hole fill (at the bottom of the hole) after a trip may show caving from an overpressured or unstable hole.

•

Pinched Bit A warning rather than an indicator, a pinched bit may be an indication of tight, under-gauged hole resulting from overpressure.

Every precaution (i.e. monitoring the well before pulling out, minimizing swabbing, flow checks) is taken to avoid taking a kick during a trip: •

Well control is more difficult if the bit is out of the hole or above the depth of influx.

•

The well cannot be shut in (pipe or annular rams) if drill collars are passing through the BOP’s.

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

3.5.1 Trip Margin The pressure reduction through swabbing is critical when tripping pipe (in comparison to that seen over a connection), since: •

The balancing pressure is the static mud hydrostatic rather than the higher ECD.

•

There is repeated swabbing as each stand is pulled.

•

The “piston” effect affects every permeable formation in open hole.

The pressure reduction can be minimized by: •

Pulling the drillstring at a slower speed.

•

Keeping mud viscosity as low as possible (bearing in mind that hole cleaning and cuttings lift properties have to be maintained while drilling).

A safety, or trip, margin can be calculated to ensure that the pressure reduction does not create an underbalance:

Pressure Reduction

Y KPa

X m/min

Running Speed

A graph can be produced that shows, for a given well profile, mud system, etc, the pressure losses (Y) that would result for a given length of drillstring being pulled at various running speeds (X). From this graph: •

For a given running speed, the additional mudweight to provide a specific trip margin over the formation pressure can be determined.

•

For a given overbalanced situation, the maximum running speed can be determined in order to avoid creating an underbalance.

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

Example: A change in formation is anticipated at 3400m. What mudweight will be required in order to provide a 500Kpa trip margin. The estimated formation pressure is 1045 kg/m3 emw.

Formation Pressure = 1045 x 3400 x 0.00981 = 34855 KPa BHP required = 34855 + 500 = 35355 KPa MW = 35355 / (3400*0.00981) = 1060 kg/m3

If the mud weight is now set at 1060 kg/m3, the swab/surge software can be used to determine the maximum pipe running speed, so as to avoid exceeding a 500KPa pressure drop. In this way, even with swabbing occurring, the annular pressure is never reduced below the formation pressure.

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

3.6 Gas Expansion Boyle’s Law states that the relationship between pressure, volume, and temperature (PV/T) is a constant. Gas bubbles expand as they are circulated up the annulus and the mud hydrostatic pressure (which is acting against the bubbles) decreases. As the vertical depth is halved, so too is the mud hydrostatic pressure. Correspondingly, as given by Boyle’s Law, the gas bubbles double in size. When using water base mud systems, methane gas will typically be present as free gas, rather than dissolved gas (At STP, maximum C1 in solution is 3%). There will therefore be increased expansion as a gas influx moves up the annulus:

V

4V

8V

gas volume

D/8 D/4

D/2

D depth To illustrate how significant this gas expansion can be, assume that ½ m3 (500 litres) of gas enters the borehole at 4000m. At….

2000m 1000m 500m 250m 125m 60m

V = 1 m3 V = 2 m3 V = 4 m3 V = 8 m3 V = 16 m3 V = 32 m3

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

However, oil base muds (approx 10% soluble C1 at STP), and worse still, mineral oils (~15%), have much higher bubble points, so that gas bubbles may not appear until the influx is very close to surface.

Bubble point

Gas in solution, no expansion

Therefore, SPP, MFO and pit level indicators may not be significant until the influx is close to, or at surface where expansion may be almost immediate as gas breaks out of solution. It becomes very important to try to identify the influx itself from a small volume change

3.7 Flowchecks A flow check, to determine whether the well is static or is flowing, is normally conducted in one of two ways: •

By actually looking down through the rotary table, into the wellhead, and visually determining if the well is flowing.

•

By lining the wellhead up to the trip tank and monitoring the level for any change.

They are typically conducted at the following occasions: • • • • • • •

Significant drill breaks Any kick indication while drilling, especially changes in mud flow Prior to slugging the pipe before pulling out of hole After the first few stands have been pulled, to check that swabbing has not induced flow. When the bit is at the shoe Prior to pulling drill collars through the BOP’s Constant monitoring (trip tank) while out of the hole

If the well is flowing, the well will be shut in

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DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

4 KICK CONTROL EQUIPMENT 4.1 The BOP Stack To prevent the occurrence of a blowout, there needs to be a way of closing, or sealing off the wellbore, so that the flow of formation fluids remains under control. This is achieved by the Blow Out Prevention system (BOP), an arrangement of preventers, valves and spools that is positioned on top of the wellhead. Commonly referred to as the stack, it’s purpose is to: •

Seal off the well so that the flow of formation fluids is under control.

•

Prevent fluid from escaping to surface.

•

Allow the release of fluids, from the well, under controlled conditions.

•

Allow drilling fluid to be pumped into the well under controlled conditions to balance formation pressure and prevent further influx.

•

Allow movement of the drillstring in or out of the well

The size and arrangement of the BOP stack will be determined by the hazards expected and the protection required, together with the size and type of pipe being used. BOP’s have various pressure ratings established by the American Petroleum Institute (API). This will be based on the lowest pressure rating of a particular item in the stack, such as a preventer, casing head or other fitting. A suitably rated BOP can therefore be installed depending on the rating of the casing and the expected formation pressures below the casing seat. BOP’s commonly have ratings of 5, 10, or 20,000 psi.

The requirements for a BOP stack are as follows: •

There must be sufficient casing to provide a firm anchor for the stack.

•

It must be able to close off and seal the well completely, with or without string in the hole.

•

It must have a simple and rapid shut in procedure.

•

It must have controllable lines through which to bleed off pressure.

•

It must provide the ability to circulate fluids through both the string and the annulus.

•

There must be the ability to hang or shear pipe, shut in a subsea stack, detach the riser and abandon the location.

•

Subsea stacks cannot be affected by the lateral movement of the riser caused by current movement and tidal variations. This is achieved through a ball joint connection.

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4.2 Preventers and Rams These are the names applied to the various “packers” that can be closed to seal the wellhead. A small BOP arrangement for a shallow land well is shown below.

Annular preventer

Ram preventers

Manual closure possible on land rigs and jack ups

4.2.1 Annular Preventer This, simply, is a reinforced packer (rubber seal) that surrounds the wellbore. It can close around pipe, of any size, when pressure is applied, thus closing off the annulus. With increasing pressure, it will close around pipe of any diameter, including drillpipe, smooth collars and kelly. However, it cannot be used on irregularly shaped pipe, or tools such as spiral drillcollars or stabilizers. It allows slow rotation and vertical movement of the pipe while the well remains sealed off. Tripping into the hole with closed annular preventer is known as snubbing. Pulling out of the hole while the annular preventer is closed is known as stripping. An annular preventer can also close across an open wellbore when there is no pipe in the hole. DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

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4.2.2 Ram Preventers Ram Preventers have a more rigid rubber seal that fits around specific, pre-designated shapes.

Pipe/Casing Rams

Here, the rubber seals match, exactly, tubing of specific diameter, so that the annulus is completely sealed off with pipe in the hole. The BOP stack must therefore include pipe rams for each size of pipe in the hole.

Blind/Shear Rams

Blind or shear rams are used to close off an open annulus, i.e. when there is no pipe in the hole. If there is pipe in the hole, the blind rams will crush it when closed. When equipped with shear blades, the pipe will be cut. These are more typical in subsea stacks so that pipe can be held by pipe rams, and cut through by shear rams allowing the rig to abandon location.

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4.3 Stack Configuration Choke & Kill Lines

Annular preventer

Blind/Shear Pipe Ram Pipe Ram Pipe Ram

Casing Head

Simple BOP stack schematic The annular preventer is always positioned on top of the BOP stack. The positioning of the various rams, and lines, is dependent on the expected operations. The following summarizes the benefits/disadvantages of positioning the blind, or shear, rams beneath, or above, the pipe rams. •

Lower blind rams

The well can be shut in to allow other rams to be repaired or changed i.e. used as a master valve. The string cannot be hung off on pipe rams. •

Upper blind rams

The string can be hung from pipe rams, backed off and then the well shut in by the blind ram. Pipe rams can be closed with pipe in hole and blind rams replaced with pipe rams. This will minimize wear and also allow ram to ram stripping of the pipe.

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4.4 Subsea Equipment

Marine Riser, Choke and Kill Lines

Ball/Flex Joint

Lower Marine Riser Package Flex lines or loops (Choke + Kill)

Annular Preventer, often two

BOP Stack

Pipe and Shear Rams

Temporary and Permanent Guidebases

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4.4.1 Lower Marine Riser Package

Riser Connection

Flex/Ball Joint

Annular Preventor

Flexible lines connecting to choke/kill

Control Pod

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4.5 Choke Manifold Following a kick and shut in, back pressure is applied, in order to balance the well, by routing returns through adjustable chokes. Release of fluids and pressure can therefore be controlled safely. A soft shut-in is where the choke is open before the rams are closed, in order to minimize the shock exerted on the formation. A hard shut-in is where the choke is closed prior to shut in.

The chokes are connected to the BOP stack through a series of lines and valves that provide a number of different flow routes and the ability to stop fluid flow completely. This arrangement is known as the choke manifold.

Again, there are specific requirements for the choke manifold: •

The manifold should have a pressure capability equal to the rated operation pressure of the BOP stack (equal to the weakest component).

•

The choke line connecting the manifold to the stack should be as straight as possible and firmly anchored.

•

Alternative flow and flare routes should be available downstream of the choke line in order to isolate equipment that may need repair.

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4.5.1 Choke and Kill Lines

Choke lines are typically used to release fluids from the annulus. Kill lines are typically used to pump mud into the wellbore if it is not possible through the drillstring.

The placement or configuration of the rams determines the positioning of the kill lines. They will be placed directly beneath one or more of the rams, so that when the rams are closed, fluid and pressure can be bled off under control (choke line). The choke line is routed to the choke manifold where pressures can be monitored. An adjustable choke allows for the ‘back pressure’ being applied to the well to be adjusted in order to maintain control. They also allow for an alternative way of pumping drilling mud or cement into the wellbore, should it not be possible to circulate through the kelly and drillstring (kill line). The kill line will normally be lined up to the rig pumps, but a ‘remote’ kill line may often be employed in order to use an auxiliary, highpressure, pump. Although preventers may have side outlets for the attachment of choke and kill lines, separate drilling spools are often used. This is a drill-through fitting that fits between the preventers creating extra space (which may be required in order to hang off pipe and have enough room for tool joints between the rams) and allowing for the attachment of the kill lines. On floating rigs, when the BOP stack is on the seabed, the choke and kill lines are attached to opposite sides of the marine riser. The lines have to flexible at the top and the bottom of the riser to allow for movement and heave.

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4.6 Closing the Preventers Preventers are closed hydraulically with fluid supplied under pressure. Manual closure is possible if the stack is accessible. There are three main system components to close the preventers: 1. Pressure source 2. Accumulators 3. Control manifold.

4.6.1 Pressure source •

The hydraulic fluid must be supplied under sufficient pressure to close the rams.

•

Electric or pneumatic pumps are usually used to deliver the hydraulic fluid under said pressure.

•

In addition, there should always be backup pumps and an alternative source of electricity or air to power them.

4.6.2 Accumulators Accumulator bottles are a series of pre-charged nitrogen bottles that store and supply the hydraulic fluid, under pressure, necessary to close the preventers •

Different preventers have different operating pressures and require different volumes of hydraulic fluid in order to function.

•

The total volume of hydraulic fluid required to operate the entire stack must be known.

•

Accumulator bottles are linked together in order to store the necessary volume.

•

The bottles are pre-charged with nitrogen (typically 750 - 1000 psi).

•

Hydraulic fluid is pumped into the bottles, compressing the nitrogen and increasing the pressure in the bottle.

•

This operating pressure (minimum typically 1200psi, maximum typically 3000psi) determines the amount of hydraulic fluid available from each bottle and therefore the total number of bottles required.

For example: -

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A

A Pre-charge

P = 1000psi

B Maximum Fluid Charge

B

C

V = 40litres

P = 3000psi

N2 volume = (1000*40)/3000

= 13.33litres

C Minimum Operating Pressure P = 1200psi N2 volume = (1000*40)/1200 = 33.33litres

Therefore, usable hydraulic fluid, per bottle, is 20litres 4.6.3 Control manifold This is basically the well control operations center. The control manifold directs the flow of hydraulic fluid to the correct ram or preventer. Regulators reduce the pressure from the accumulator operating pressure to the preventer operating pressure, typically 500-1500psi. The master control panel is typically situated in the doghouse, with a second panel in another safe area. Typically, pneumatic operation is used to open and close preventers, choke and kill lines and to monitor and regulate pressures.

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Subsea stacks require slightly different operation from the control panel, in that: •

They also require signal or pilot lines in addition to hydraulic fluid lines.

•

Subsea regulators and valves control the flow and pressure of hydraulic fluid upon receiving the signals from surface.

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4.7 Diverters The diverter is a low pressure system installed beneath the bell nipple and flow line assembly to direct well flow away from the rig and personnel. They are typically employed prior to installing a BOP stack in order to provide safety in the event of shallow gas being encountered. They are essential in offshore drilling, but the diverter system is only designed to handle low pressures. It is designed to pack off, or close around, the Kelly or drillpipe and direct fluid flow away. If it were attempted to be control high pressures, or completely shut in the well, the likely result would be failure and uncontrolled flow, with the breakdown of formations around the shallow casing or conductor pipe. Typically, two diverter lines are installed and, in the event of a kick: •

One or both diverter lines will be opened

•

A packer is closed around the drillpipe, or Kelly, in order to close off the annulus

•

Gas will then be directed away from the rig until it loses pressure

Rig Structure Diverter assembly

Response must be quick since, with shallow gas, there is little hydrostatic head and gas will quickly blowout at surface. One vent line must be open before closing the packer, in order to prevent gas from blowing out around conductor pipe.

Marine Riser This schematic shows a typical installation for drillships and semisubmersibles. LMRP

Annular preventer

It is mounted to the drill floor substructure at the top of the marine riser assembly.

Ram preventers

Relative motion between the BOP stack and the rig is accounted for by a flex/ball joint positioned above the stack. A second flex/ball joint may be installed between the diverter and the riser’s telescopic joint.

Seabed

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4.8 Inside Blowout Preventors This refers to equipment that can be used to close off the drillstring in order to provide additional control. They may be manual shut off valves that can be inserted into the string at the surface, or they may be automatic check valves actually located inside the drillstring downhole. There are slight differences in the equipment depending on the rotary system of the rig: -

4.8.1 Kelly Rigs Upper kelly valve or cock

This valve is positioned between the kelly and the swivel, in order to isolate drilling fluid in the drillstring.

Lower kelly valve or cock

This is installed at the base of the kelly and will most likely be used if the upper kelly valve is damaged or inaccessible.

Safety valve

This is actually identical to the lower kelly valve. Rather than being installed as part of the string, it is kept on the rig floor in order to be quickly “stabbed” into the top of the drillpipe should a kick occur during a trip when the kelly is racked.

4.8.2 Top Drive Rigs Top drives utilize an Upper Remote Safety Valve and a Lower Safety Valve, the two valves connected together. •

The upper valve is operated remotely, since the top drive location is likely to be inaccessible (height) should a kick take place.

The advantage of this arrangement is that kick protection is immediately available should a kick occur during a trip.

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4.8.3 Additional Preventers

Inside BOP

This is a check valve that is used to close off the top of the drillpipe. It allows the string to be stripped into the hole, under pressure, in the event that a kick occurs when the string is off bottom. It is physically difficult to stab the valve against mud flow from the drillpipe, so a safety valve is usually installed first.

Drop In Check Valve

This valve is actually pumped or dropped into the drillpipe, setting itself in a landing sub situated in or close to the BHA. Some models can be retrieved on wireline, otherwise, the drillstring has to pulled out to retrieve the valve. They are typically used in stripping operations. If abandoning location offshore, they must be deployed prior to shearing the pipe.

Float Valve

This check valve is installed in the bit sub to prevent backflow of mud through the drillstring. Simple models are one-way valves, which prevent pressures being transmitted as well as fluid flow. Unfortunately, this results in the disadvantage that the shut in drillpipe pressure would not be known. A “slotted” or “vented flapper” type minimizes backflow but allows for stabilized shut in pressure to be recorded.

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4.9 Rotating Preventers These may be known as rotating heads, or rotating BOP’s.

Kelly driver

•

Top rubber

•

They are mounted on top of the standard BOP stack and act as a rotating flow diverter. This allows rotation and vertical movement of the drillstring at the same time that a rubber stripper seals around, and rotates with, the pipe or kelly.

•

Mud flow is therefore contained and can be diverted away through a bowl and bearing assembly.

•

Annular pressures up to 3500psi can be controlled with such equipment.

•

Applications include underbalanced drilling applications and even facilitating the drilling with high pressures while well is flowing.

Bearing assembly Bottom rubber

Bowl

While well pressures are contained by the rubber seal around the drillstring or kelly, flow is diverted by way of a steel bowl and bearing assembly. The bearing assembly enables the inner part to rotate with the drillstring while the outer part is stationary with the bowl.

Seals are typically of two types: -

1. A cone shaped rubber that seals around the drillstring. The inside diameter of the seal is slightly smaller than the outside diameter of the pipe, so that the seal stretches to provide an exact seal around the pipe. No hydraulic pressure is required to complete the seal, since the pressure is provided by the actual wellbore pressures acting on the cone rubber. The rubber is therefore selfsealing, the higher the wellbore pressure the greater the seal.

2. A packer type seal requiring an external hydraulic pressure source to inflate the rubber and provide a seal. A seal will be given as long as the hydraulic pressure is greater than the wellbore pressure.

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5 FRACTURE CALCULATIONS 5.1 Leak Off Test This is a pressure test that is typically carried out after drilling out casing/cement, prior to drilling the next hole section. There are two principle reasons for this test. Cement Integrity

Before drilling the next hole section, it is critical to determine that the cement bond is strong enough to prevent high pressure fluids from flowing through to shallower formations or to surface.

Fracture Pressure

If, as intended, the cement retains the pressure exerted during the test, then formation fracture will occur, under controlled conditions. The formation at this depth, since it will be the shallowest in the next hole section, is assumed to be the weakest point. The fracture pressure determined from the test will therefore be the maximum pressure that can be applied in the wellbore, without causing fracture.

Two types of test may be performed: A Formation Integrity Test is often performed when there is a good knowledge of the formation and fracture pressures in a given region. Rather than inducing fracture, this pressure test is taken to a predetermined maximum pressure, one considered high enough to safely drill the next hole section. A complete Leak Off Test leads to the actual fracture of the formation.

Procedure: •

After drilling out the casing shoe, a small section of new formation, perhaps 10m, is drilled.

•

Shut in the well

•

Pump mud, at a constant rate, into the wellbore in order to increase the pressure in the annulus.

•

Monitor pressure for indication that mud is injected into the formation. A linear increase will be seen initially, with a drop in pressure occurring when leak off is reached. At this point, stop pumping.

The pressure plot against time, or mud volume pumped, shows that there are 3 principle stages to a complete Leak Off Test. It must be the operator who makes the decision as to which particular value is taken as the ‘leak off” pressure, but obviously, it should be the lowest value. This way well be the initial Leak Off Pressure, if the test hasn’t been taken further to cause complete rupture. If it has, then the Propagation Pressure is likely to be the lowest, indicating that the formation has actually been weakened as a result of the test.

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Rupture Pressure Complete and irreversible failure has occurred when pressure drops - stop pumping

Surface Shut In Pressure

Propagation Pressure If pumping is stopped at the point of failure, the formation may recover, but weakened Leak Off Pressure Slower pressure increase - reduce pump rate as mud begins to inject into the formation

Mud Volume Pumped With a LOT, mud is actually injected into the formation until fracture occurs. The formation is therefore weakened allowing less tolerance for the next hole section. Full Leak Off’s should be conducted on wildcat wells where

no pressure/fracture information is known. If regional pressure and fracture gradients are known, then an FIT can be conducted to a pressure that is known to be above the maximum anticipated pressure requirement during the next hole section. By not increasing the pressure to actual leak off, an FIT provides a built in safety margin against shoe fracture.

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5.2 Fracture Pressure All materials have a finite strength. Fracture Pressure can be defined as the maximum pressure that a formation can sustain before it’s tensile strength is exceeded and it fails. Factors affecting the fracture pressure include: Rock type In-situ stresses Weaknesses such as fractures, faults Condition of the borehole Relationship between wellbore geometry and formation orientation Mud characteristics

If a rock fractures, a potentially dangerous situation exists in the wellbore. Firstly, mud loss will result in the fractured zone. Depending on the mud type and the volume lost, this can be extremely costly. Mud loss may be reduced or prevented by reducing annular pressure through reduced pump rates, or, more expensive remedial action may be required, using a variety of materials to try and “plug” the fractured zone and prevent further loss. Obviously, all of this type of treatment is extremely damaging to the formation and is to be avoided if at all possible. However, if mud loss is so severe, then the mud level in the wellbore may actually drop, reducing the hydrostatic pressure exerted in the wellbore. This may result in a zone, elsewhere in the wellbore, becoming underbalanced and flowing – we now have an underground blowout!

Knowledge of the fracture gradient is therefore essential while planning and drilling a well.

LOP

The fracture pressure is determined from the leak off test performed at the casing shoe. During this test, a combination of two pressures provide the pressure, at the shoe, to cause fracture: •

The hydrostatic pressure exerted by the drilling fluid, at the shoe.

•

The shut-in pressure applied by pumping mud into a closed well…i.e. the leak off pressure.

HYD

Fracture

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Pfrac = HYDshoe + LOP

where Pfrac = fracture pressure HYDshoe = mud hydrostatic at the shoe = MW x TVDshoe x constant LOP = shut-in pressure applied at surface, whether determined from LOT or FIT

Pfrac (emw) = MW + LOP/(TVDshoe x g) Example - imperial A LOT is performed at a shoe depth of 4000ft TVD, and with a mudweight of 10.5 ppg. Leak off occurs when the surface shut in pressure is 1500psi. LOP = 1500psi HYDshoe = 10.5 x 4000 x 0.052 = 2184psi Pfrac = 2184 + 1500 = 3684psi Pfrac emw = 3684 / (4000 x 0.052) = 17.71ppg emw

Example - metric An FIT is performed at a shoe depth of 2500m TVD, and with a mudweight of 1035 kg/m3. The FIT is held at a surface shut in pressure of 10500 KPa. LOP = 10500KPa HYDshoe = 1035 x 2500 x 0.00981 = 25383 KPa Pfrac = 25383 + 10500 = 35883 KPa Pfrac emw = 35883 / (2500 x 0.00981) = 1463 kg/m3 emw

It is very important to understand, however, that although the pressure test is the only way of determine the fracture pressure (other than actually losing circulation), there are certain circumstances that can lead to inaccuracy or unreliability: •

A Formation Integrity Test gives no determination of actual fracture pressure, only an accepted maximum value for the drilling operation. Although not providing accurate data, this test does provide a safety margin.

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•

Well consolidated formations are typically selected to set the shoe – this formation may not be the weakest if subsequent unconsolidated, or overpressured, formations are encountered within a short interval from the shoe.

•

Apparent leak off may be seen in high permeability, or highly vugular formations, without fracture actually occurring.

•

Poor cement bonds may result in leak off through the cement, rather than the formation.

•

Localized porosity or micro-fractures can result in lower recorded fracture pressures.

•

Well geometry, in relation to horizontal or vertical stresses, can also lead to deceptive fracture pressures, with different results being produced, in the same formations, between vertical and deviated wells.

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5.3 Maximum Allowable Annular Surface Pressure When a well has to be shut in, in order to control a kick, surface shut-in pressure is required to balance the bottom hole pressure. At the time of shut-in, there are two pressures acting at the shoe: • •

mud hydrostatic shut-in pressure applied from surface.

These two pressures, combined, cannot exceed the fracture pressure of the formation at the shoe (Pfrac determined from the leak off test).

i.e.

Pfrac > HYDshoe + Shut-in Pressure

MAASP is the maximum shut in pressure that can be applied without fracturing the weakest zone, assuming this is the shoe: Pfrac = HYDshoe + MAASP MAASP = Pfrac - HYDshoe

At the time of a LOT, the MAASP is clearly equal to the Leak Off Pressure, since this is the shut-in pressure that actually causes fracture.

Example – imperial A LOT is performed at a shoe depth of 4000ft TVD, with a mudweight of 10.5 ppg. Leak off pressure is 1500psi. Pfrac = hyd + LOP = (10.5 x 4000 x 0.052) + 1500 Pfrac = 2184 + 1500 = 3684psi MAASP therefore, with 10.5ppg mud, also equals 1500psi; any shut-in pressure higher than this will fracture the shoe.

MAASP will only change if mud weight changes: Drilled depth is unimportant, since we are dealing with weakest zone at the shoe. Of the two pressures acting at the shoe: Mud hydrostatic only changes if the mud weight changes. Pfrac obviously does not vary.

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What is the MAASP, if at 6000ft MD, mudweight has to be increased to 11.2ppg? MAASP = Pfrac - HYDshoe = 3684 - (11.2 x 4000 x 0.052) = 1354psi

The form of this calculation will only change if a weaker zone, at a greater depth, is encountered.

Example – metric Since Pfrac remains constant, if mudweight is increased, the MAASP has to decrease correspondingly. At the time of the leak off test, a table of mudweight versus MAASP should be constructed.

A leak off is performed at a shoe TVD of 3000m; the mudweight is 1020kg/m3 and the recorded leak off pressure is 8000 Kpa. Pfrac = (1020 x 3000 x 0.00981) + 8000 = 38019 Kpa MAASP = Pfrac – HYDshoe

MAASP @ 1020kg/m3 = 8000 Kpa MAASP @ 1030kg/m3 = 38019 - (1030 x 3000 x 0.00981) = 7706KPa MAASP @ 1040kg/m3 = 38019 - (1040 x 3000 x 0.00981) = 7412KPa MAASP @ 1050kg/m3 = 38019 - (1050 x 3000 x 0.00981) = 7117Kpa

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5.4 Kick Tolerance Mud weight must, clearly, be sufficient to exert a pressure that will balance the formation pressure and prevent a kick, but it cannot be so high that the resulting pressure would cause a formation to fracture. This would lead to lost circulation (mud being lost to the formation) in the fractured zone. This, in turn, would lead to a drop in the mud level in the annulus, reducing the hydrostatic pressure throughout the wellbore. Ultimately then, with reduced pressure in the annulus, a permeable formation at another point in the wellbore may begin to flow. With lost circulation at one point and influx at another, we now have the beginnings of an underground blowout! A critical condition exists should the wellbore has to be shut in.

While drilling, high formation pressures can be safely balanced by the mudweight. However, if a kick is taken (either through a further increase in formation pressure, or through a pressure reduction cause by swabbing, for example), then the well would have to be shut in. If the pressure caused by the mudweight is too high, then weaker formations at the shoe may fracture when the well is shut in. This situation would be worsened if higher shut-in pressures are required to balance low density influxes, especially expanding gas! KICK TOLERANCE is the maximum balance gradient (i.e. mudweight) that can be handled by a well, at the current TVD, without fracturing the shoe should the well have to be shut in.

KICK TOLERANCE = TVDshoe x (Pfrac – MW) TVDhole Where Pfrac MW

= fracture gradient (emw) at the shoe = current mudweight

If the mudweight, that is required to balance the formation pressures while drilling, would result in shoe fracture during well shut in, then a deeper casing shoe (with greater fracture pressure) must be set. In order to account for a gas influx, the formula is modified as follows: -

KT = [TVDshoe x (Pfrac – MW)] - [influx height x (MW – gas density)] TVDhole TVDhole

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The method illustrated is based on three criteria: •

A maximum influx height and volume (zero kick tolerance) – Point X

•

A typical or known gas density (from previous well tests for example)

•

The maximum kick tolerance (liquid influx with no gas) – Point Y

This defines limits on a graphical plot, which provides easy reference to this important parameter.

The values are determined as follows:

Maximum Height = TVDshoe x (Pfrac – MW) MW – gas density If gas density is unknown, assume 250 kg/m3 (0.25 SG or 2.08ppg)

Maximum Influx Volume is determined from the maximum height and the annular capacities – this defines Point Y on the graph.

Maximum KT, as shown before, = TVDshoe x (Pfrac – MW)

TVDhole This defines Point X on the graph, a liquid influx without any gas.

The graph is completed by dividing it into the different annular sections covered by the influx, i.e. in the event that there are different drill collar sections, or if the influx passes above the drill collar section, or even if the influx passes from open hole to casing. This is necessary since the same volume of influx will have different column heights in each annular section.

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Kick Tolerance, worked example Using the following well configuration: Casing Shoe = 2000m Hole Depth = 3000m Pfrac at shoe = 1500 kg/m3 emw Current MW = 1150 kg/m3 Drill Collar length = 200m Annular Cap = 0.01526m3/m (216mm open hole, 165mm drill collars) Annular Cap = 0.02396m3/m (216mm open hole, 127mm drillpipe) Gas Density = 250 kg/m3

Maximum Height = TVDshoe x (Pfrac – MW) MW – gas density

= 2000 (1500 – 1150) = 777.8m 1150 – 250

Maximum Volume, determined from 200m around the drill collars, and 577.8m around drillpipe: DC DP

= 200 x 0.01526 = 577.8 x 0.02396

Max Vol = 3.05 + 13.84

Maximum KT = TVDshoe x (Pfrac – MW)

= 3.05m3 = 13.84m3 = 16.89m3

= 2000 (1500 – 1150) = 233.3 kg/m3

TVDhole

3000

Therefore, Point X = 16.7m3, Point Y = 233 kg/m3

Now, determine the ‘break point” of the graph, for the drill collar / drill pipe annular sections: To do this, calculate the KT related to a 3.05m3 gas influx, which would reach the top of the 200m length of drill collars:

KT = [TVDshoe x (Pfrac – MW)] - [influx height x (MW – gas density)] TVDhole TVDhole = 2000 (1500 – 1150) 3000

- 200 (1150 – 250) 3000

= 173.3 kg/m3 Therefore, 3.05m3 and 173.3 kg/m3 define the “break point” on the graph. The graph can now be plotted, as follows: DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

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KT kg/m3 Drill Collars

240

Drill Pipe

Y 200 173 160

120

80

40

Influx Volume m3

0 0

2

3.05 4

6

8

10

12

14

16

X

18

From this graph, the following information can be determined:

For a liquid influx, with no gas: •

The kick tolerance is 233 kg/m3 above the present mudweight.

•

This would mean that the maximum formation pressure that can be controlled, by well shut-in, without resulting in fracture, is 1383 kg/m3 (1150 + 233).

•

If formation pressures greater than this are anticipated, then a new casing shoe would have to be set.

Lighter and expanding gas changes this scenario dramatically: •

If more than 16.7 m3 of gas was allowed into the annulus, there is no kick tolerance on well shutin, the shoe would fracture!

•

Operators will often work on an acceptable maximum kick influx to determine kick tolerance:

•

For example, a 10m3 gas influx would give a kick tolerance of 86 kg/m3 above the present mudweight.

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This can be verified with the formula: Of the 10m3, 6.95m3 would be around the drillpipe annular section, since 3.05m3 fill the drill collar section: Height around DP = 6.95 / 0.02396 = 290m Height around DC = 200m Total Height = 490m

KT

= 2000 (1500 – 1150) - 490 (1150 – 250) 3000 = 86.3 kg/m3

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6 WELL CONTROL PRINCIPLES & CALCULATIONS 6.1 BALANCING BOTTOM HOLE PRESSURES Assuming a kick at the bottom of the hole, during well control the bottom hole pressure (BHP) must be balanced on both the drillstring side and annulus side. The well can be considered to behave along the lines of a U-tube.

Take a normal well situation:

SIDP = 0

SICP = 0

Assume a normally balanced well, where the mud hydrostatic pressure exceeds the formation pressure. In a normal drilling situation, the u-tube is open at the surface with the pressure at the bottom of the hole equal to the mud hydrostatic. This pressure would be the same on both sides of the u-tube, so that the well is balanced. If the well is shut in, the pressures are the same and no additional surface pressure is required to achieve balance.

BHP = HYDmud

Now, consider actual depths and pressures:

SIDP = 0

SICP = 0 MW = 1020 kg/m3 TVD = 1000m HYDmud = 1020 x 1000 x 0.00981 = 10006 KPa If this is greater than Pform, then the BHP = 10006 KPa. Shut in pressures would be zero since the well is balanced.

BHP = HYD = 10006KPa

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Now, let the annulus be partially (half in this case) filled with a lighter mud:

SIDP = 0

The string is still filled with 1020 kg/m3 mud, so exerts a BHP pressure of 10006 KPa.

SICP = 98

However, the hydrostatic in the annulus has been reduced:

1020 500m

HYD1020 = 1020 x 500 x 0.00981 = 5003KPa HYD1000 = 1000 x 500 x 0.00981 = 4905KPa

1000

Annular Pressure = 5003 + 4905 = 9908KPa

1000m 10006KPa

This does not balance the BHP, so if the well was shut in, an additional 98KPa would have to be imposed at surface.

9908KPa

BHP = 10006KPa

(98 + 5003 + 4905 = 10006)

Returning to our well with 1020 kg/m3 mud:

At 1000m, a formation is penetrated with a pressure of 10400KPa. A kick results in the well being shut in.

SIDP=394KPa

BHP now equals 10400KPa On the drillstring side, it is assumed that the influx does not enter the pipe: HYDmud = 10006KPa

influx SIDP of 394KPa will therefore balance the well:

BHP = Pform = 10400Kpa

10400 = 10006 + 394

In the annulus, the overall hydrostatic has been reduced by the influx, so that a higher SICP will be required to balance the well

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Example: Drilled depth = 3500m TVD, MW = 1030kg/m3. A formation kicks….FP = 38000KPa; oil of density 850 kg/m3 influxes to a height of 500m.

SIDP = 2635KPa

SICP=3518KPa

HYDmud = 35365KPa

1030

HYDmud = 30313KPa

850

HYDinflux = 4169KPa

Pform = 38000KPa BHP = the higher FP = 38000 KPa HYD = 1030 X 3500 X 0.00981 = 35365KPa

To balance the drillstring side, SIDP = 38000 – 35365 = 2635KPa To balance the annulus, SICP = 38000 – (HYDmud + HYDinflux) = 38000 – [(1030 x 3000 x 0.00981) + (850 x 500 x 0.00981)] = 38000 – [30313 + 4169] = 3518KPa

From these U-tube principles, the following shut-in formulas can be determined: -

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6.2 Shut In Formulas Annular or Drillpipe Pressure + Shut In Pressure = Formation Pressure

The SIDP provides the additional pressure to the mud hydrostatic in the drillstring, in order to balance the increased BHP resulting from the formation pressure. Mud Hydrostatic + SIDP = Formation Pressure

The same principle applies to the annulus side of the u-tube, but here, the mud column is contaminated by the influx. This reduces the overall hydrostatic in the annulus, so that a greater CSIP is required in order to provide balance. If it is assumed that the influx is concentrated at the bottom of the hole and the height of the influx can be determined: NewMud HYD + InfluxHYD + SICP = Formation Pressure Where influx hydrostatic = influx gradient x influx height

6.3 Influx Height and Type Despite the formula shown above, because of too many uncertainties, the SICP is not used to determine formation pressure, but it can be used as an early estimation as to what type of influx needs to be controlled. The influx volume is normally assumed to be equal to the pit volume increase, i.e. the volume of mud displaced at surface, as a result of the influx downhole. Height of influx = pit gain * annular capacity

clean mud

contaminated mud influx

Pit volume increase, once the well has been shut in and lined up to the trip tank, will be due to an influx volume situated at the bottom of the hole. Prior to shut in, however, while circulating, the influx would have been dispersed further up the annulus, contaminating the mud and with reduced height due to larger annular capacity. These possible errors are ignored and the influx assumed to occupy the bottom of the hole, with a reduced mud column above.

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The height reached by an influx is dependent on: • • • •

The pressure differential and permeability, i.e. effectiveness of influx flow The fluid type The time taken to shut in the well, allowing for influx Annular capacities

Naturally, the greater the height of the influx, then the greater the reduction in hydrostatic pressure, and the greater the CSIP that will be required to balance the well.

BHP = Pform Gas expansion will reduce the hydrostatic even further! With reliable data, the influx gradient can be determined as follows:

Fluid Gradient (psi/ft) = (MWppg x 0.052) - (SICP - SIDP (psi)) influx height (ft)

Fluid Grad (KPa/m) = (kg/m3 x 0.00981) - (SICP - SIDP (KPa)) influx height (m)

Fluid Gradient (psi/ft) 0.05 – 0.15 0.15 – 0.40 0.433 0.433 – 0.48

Fluid Type Gas Condensate – Oil Fresh water Salt water

Fluid Gradient (KPa/m) 1.131 – 3.393 3.393 – 9.048 9.795 9.795 – 10.858

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6.4 Stabilizing Shut In Pressures When a well is shut in, both CSIP and SIDP will steadily increase until the well is balanced on their respective sides. Once pressures have stabilized, the shut in pressures reflect the reduction in hydrostatic (in the drillstring and in the annulus) pressure and the additional pressure required to balance the formation. Before the pressures have stabilized, BHP does not balance the formation pressure, so further influx is possible. At the same time, gas can continue to migrate while the well is shut in, although it is not possible to expand since there is nowhere to displace mud from the annulus. If pressures (CSIP) do not stabilize, but continue to gradually increase, then there IS gas in the hole, which is migrating. To determine the degree of underbalance if pressures don’t stabilize, record the pressure against time, typically every minute. Plot on a graph, pressure against time. If rate of increase slows down, this can be taken as the amount of underbalance

Pressure

P

Time In this event, it is useful to know how fast the gas is rising in the shut-in annulus. This can be gauged by how quickly CSIP is rising:

Migration Speed = pressure increase per time / hydrostatic pressure gradient = (psi/hr) / (psi/ft) = ft/hr eg; MW = 11.1ppg; Pressure increases by 200 psi over 30 minutes Migration Speed

= 400 / (11.1*0.052) = 693 ft/hr

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6.5 Induced Kicks If a kick results from formation pressure exceeding the mud hydrostatic pressure, shut in pressures will be seen on both sides of the u-tube, SICP > SIDP. If zero SIDP is recorded, then clearly, the mud hydrostatic (inside the drillstring side of the u-tube) still balances the formation pressure. Since an increase in formation pressure is not the cause of the kick, a reduction in annular pressure must be the cause, therefore the kick must have been induced by some other mechanism: •

Through swabbing, temporarily reducing the annular pressure below the formation pressure, to allow an influx.

•

Not keeping the hole full in a trip, again reducing the mud hydrostatic in the annulus.

A CSIP will still be recorded, since the influx reduces the mud hydrostatic in the annulus and the reduction must be balanced by the back pressure applied at surface. Control, in this situation, simply requires circulating the influx out.

6.6 One Way Floats Unless “flapper or vented” designs are used, floats prevent the transmission of fluids and pressure up the string. Therefore, an increase in formation pressure will not be transmitted to surface, through the drillstring, and SIDP will be recorded as zero! Without knowledge of the SIDP, then the formation pressure (and kill mudweight as will be seen shortly) cannot be determined. In order to determine this required information, the following procedure can be used: After shut-in and stabilization, record the SICP Pump at the planned kill rate (this is based on the Slow Circulating Rate and known pressure) Maintain a constant SICP by using the choke Record the pressure on the drillpipe while circulating Stop pumping and close the choke

The effective shut-in drillpipe pressure can then be calculated as follows: -

SIDP = drillpipe pressure - SCR pressure

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6.7 Slow Circulating Rates Well control is always performed at slow pump rates in order to minimize additional pump pressure in the annulus. It is important to know what pressure results from this pump rate in a ‘normal” situation. SCR pressures should be recorded, for both pumps, at a range of pump rates e.g. 20, 30 and 40 strokes per minute. They are typically recorded: • • • •

At the beginning (or end) of every shift, if the drilled depth has changed If mud weight changes significantly If the hole/string profile has changed Before drilling ahead with a new bit

SCR pressures are used to determine the following: • •

Initial and Final circulating pressures when circulating the kill mud to the bit SIDP when using a one-way float (as just illustrated)

6.8 Kill Mudweight As already seen, the SIDP is the additional pressure (to the mud hydrostatic pressure) required to balance the BHP due to the increase in formation pressure: -

FP = HYDmud + SIDP The kill mudweight is the mudweight required to balance the formation pressure. i.e.

Kill MW = Initial MW +

SIDP (TVD x g)

PPG

=

PPG +

PSI ft x 0.0052

kg/m3

=

kg/m3 +

KPa m x 0.00981

Example - Imperial Drilling at 8000ft with a mudweight of 10.6ppg, a kick is taken. On shut in, a SIDP of 350psi is recorded: KMW = 10.6 +

350 8000 x 0.052

= 10.6 + 0.84 = 11.5ppg FP

= 11.44ppg emw = 11.44 x 8000 x 0.052 = 4759PSI

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Example - Metric Drilling at 3700m with a mudweight of 1045kg/m3, a kick results in a SIDP of 2500KPa: KMW = 1045 +

2500 3700 x 0.00981

= 1045 + 68.9 = 1114 kg/m3 FP

= 1114 kg/m3 emw = 1114 x 3700 x 0.00981 = 40,435 KPa

6.9 Circulating The Kill Mud The kill mud is circulated at a constant pump rate, the Slow Circulation Rate. At the start of the well control process, the drillstring is full of the original mud (again, making the assumption that no influx has found it’s way up inside the drillstring).

The recorded pump pressure will therefore be the recorded SCR pressure and the additional SIDP required to balance the well: Therefore, at the start of the well kill operation:

ICP = SIDP + SCR pressure where ICP = Initial Circulating Pressure

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As the heavier kill mud is pumped down the string, the original lighter mud is displaced from the string, around into the annulus.

Since heavier mud is replacing lighter mud, the hydrostatic pressure in the string is increasing. Correspondingly, less SIDP is required to maintain BHP balance. The pump rate is maintained at the SCR The SIDP therefore has to be decreased manually by opening the choke, so that, at all times….

HYDmud + SIDP = BHP (Formation Pressure)

Continuing the circulation, lets assume that the kill mud has reached the bit, displacing all of the original mud from the drillstring: Since the kill mud has been calculated to control the well, the hydrostatic pressure from the kill mud now balances the BHP, so that no additional back pressure needs to be applied from surface. The “SIDP” recorded is now equivalent to the SCR pressure, but for the heavier kill mud. This can be determined, by the ratio method, from the SCR pressure recorded for the original mud weight: -

Final Circulating Pressure FCP = SCRpress x (KMW/MW) FCP balances the string side of the u-tube and should be maintained for the remainder of the well kill operation.

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6.10 Pressure Step Down During the initial stages of the well control procedure, a pressure step down is therefore followed while circulating kill mud to the mud, gradually reducing the pressure from the initial circulating pressure to the final circulating pressure. Once the kill mud is at the bit, and the pressure is at the FCP, we know the drillstring side of the u-tube is balancing the formation pressure. The pressure step down is determined for the pressures required at regular intervals determined from the total pump strokes required to fill the string with kill mud (i.e string capacity and down strokes). Required pressure (i.e. choke) adjustments are therefore determined from the total number of strokes (surface to bit) and the difference between ICP and FCP.

For example: Down strokes = 1000 ICP = 1100PSI FCP = 700PSI

1100

ICP

1000 900 800

FCP

700

strokes

0

100

200

300

400

500

600

700

800

900

1000

pressure 1100 1060

1020

980

940

900

860

820

780

740

700

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6.11 Subsea Considerations Depending on the water depth, very long choke and kill lines have to run down the length of the riser. Higher frictional losses in the choke line may lead to a variation in slow circulation rate pressures, therefore the initial and final circulating pressures. The marine riser is isolated and returns are circulated back to the rig through the choke line. This has two consequences: •

You must account for the significant capacity and stroke change during well kill procedure.

•

Once the well is controlled, it has only actually been displaced to kill mud as far as the sea bed. The marine riser is still filled with the original mud, therefore, before proceeding, the marine riser also needs to be displaced to kill mud.

6.12 Horizontal Well Considerations Less hydrostatic head to balance formation pressures Gas will not start expanding until it reaches the vertical section of the well. Therefore, a kick may not be identified early on and once in the vertical section, gas has shorter distance/time before reaching surface. Tapered drillstrings, with drillcollars providing weight in the vertical section of the well, result in smaller annular capacity and therefore a greater height to the gas column, lowering hydrostatic pressure and increasing CSIP.

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7 WELL CONTROL METHODS 7.1 Wait and Weight The well is shut in while the mud is weighted up to the required kill weight, calculations and kill sheets are prepared. Only one circulation is required to kill the well. Advantages • • • •

Lower pressures are imposed on the well This method is generally faster since the kick is circulated out and the well killed in a minimum of one circulation. Safer Less wear on surface gas equipment and choke

Disadvantages • •

The well control process must wait until the kill mud is ready More calculations are required

Procedure •

Shut the well in and weight up the required volume of mud to the kill mudweight.

•

Open the choke and bring the pump up to the designated kill pump rate (from SCR).

•

Maintain a constant kill rate as the kill mud is pumped down the string. Follow the “SIDP” step down procedure by adjusting the casing choke (A). If the actual stabilized ICP is not the same as the calculated ICP, the step down sequence should be adjusted accordingly. A reduction in the CSIP will be seen as the influx passes from drill collars to drillpipe (B), since the larger annular capacity reduces the influx height, increasing the overall hydrostatic in the annulus.

•

When the kill mud is at the bit, the drillpipe pressure should equal the calculated FCP (C).

•

Adjust the choke to maintain this pressure for the remainder of the operation. A reduction in CSIP will be seen as kill mud enters annulus, increasing the overall hydrostatic in the annulus (D)

•

Bring the influx to surface - as the gas expands, both CSIP and pit levels will be seen to increase (E). The gas needs to bled off in order to maintain drillpipe pressure and keep CSIP within operational limits so as not to fracture the shoe (F).

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Kill mud at bit

Kill mud at surface

Influx arrives at surface

CSIP Drillpipe pressure

Influx removed

F B D

E

A C

Post kill procedure •

When the kill mud reaches surface, pumping can stop and the well shut in.

•

At this point, the influx should have been removed from the annulus and the well should be killed. If some CSIP is still recorded, then continue circulating until the remaining influx is removed.

For offshore rigs •

The riser now has to be displaced to kill mud

•

Open diverter and flow check well

Throughout the well kill operation, constant BHP is maintained with: Constant kill mudweight Constant slow circulation pump rate Constant drillpipe pressure once the string is displaced to kill mud

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7.2 Driller’s Method Under controlled conditions, the existing mud is circulated to bring the influx out of the hole. During this circulation, calculations are made, kill sheets completed and the mud weighted up to the required kill weight. A second circulation then displaces the well to kill mud, killing the well. This method is typically used in situations such as circulating out large gas shows, trip gases, or kicks that have been swabbed into the hole, since a mudweight increase will not be required. Advantages • •

Circulation begins immediately It is a simpler technique, requiring fewer calculations

Disadvantages • • •

More time is required for the two circulations Higher pressures are imposed on the annulus More wear on choke and gas equipment

Procedure - Circulation 1 •

Open the choke and bring the pump up to the desired slow circulation rate.

•

Circulate the influx to surface at the constant pump rate and maintain constant drillpipe pressure (A) by adjusting the choke. This should provide sufficient BHP to prevent further influx.

•

Gas must be allowed to expand and mud displaced at surface.

•

Correspondingly, CSIP increases (B). This will help to prevent further influx, but it cannot be allowed to exceed fracture pressures.

•

Once the influx is out of the well, shut the well in and record pressures (C). If SIDP and SICP are zero

The well is dead and the mud density is sufficient to balance the well

If SIDP and SICP are equal (>0)

Mudweight must be increased to balance the formation pressure

If SICP > SIDP

There is still influx in the annulus and a second kick, or further influx has occurred during the initial circulation.

Repeat this procedure until the influx has been completely removed.

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Procedure - Circulation 2 An assumption is made, that prior to the second circulation, all influx fluids have been removed from the annulus during the first circulation. •

Open the choke and bring the pump up to the slow circulation rate.

•

Pump the kill mud at constant rate, maintaining constant CSIP by adjusting the choke (D). This will allow the drillpipe pressure to fall as the kill mud is pumped down to the bit and hydrostatic increases.

•

When the kill mud reaches the bit, the well is dead on the drillpipe side. Record the drillpipe pressure, FCP (E).

•

Continue circulation, displacing the annulus to kill mud, while maintaining constant drillpipe pressure (F). CSIP will decrease as kill mud displaces the annulus.

•

Once kill mud reaches surface, stop pumping, shut in the well and confirm that it is dead.

STEP 1

Influx removed

Kill mud at bit

STEP 2

Well Dead

Influx at surface

B CSIP Drillpipe pressure

A

D

E

F

C

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7.3 Concurrent Method With this method, circulation commences immediately and the mud is gradually weighted up as circulation proceeds. This will continue until the final required kill mud reaches surface and the well is dead. Disadvantages • •

Higher pressures are imposed on the annulus Barite mixing and mud weight may not be consistent throughout

Procedure •

With the well shut in, calculate the ICP, the kill mudweight and the FCP.

•

Rather than stroke increments from surface to bit, determine the pressure reduction required in terms of incremental mudweight until the final kill mud is being circulated. Increasing the mud weight and reducing drillpipe pressure will take place over several circulations.

•

Bring the pump up to the slow circulation rate, ensuring the drillpipe pressure is equal to the ICP by adjusting the choke.

•

As the mud density reaches each incremental increase, the drillpipe pressure is reduced through the choke, following the step down chart.

•

When kill mud reaches surface, the well is dead

1100 ICP 1000 900 800

FCP

700

MW 10.0

10.2

10.4

10.6

10.8

11.0

11.2

11.4

11.6

11.8

12.0

pressure 1100

1060

1020

980

940

900

860

820

780

740

700

For each incremental increase in mudweight, drillpipe pressure is reduced. When the final kill mud is at the bit, the drillpipe pressure should be at the FCP. eg ICP = 1100psi; FCP = 700psi; MW = 10.0ppg; KMW = 12.0ppg DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

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7.5 Volumetric Method This technique is used when normal kill procedures are not possible from the bottom of the hole. This may be due to the following reasons: •

If the drillstring is out of the hole and drillpipe pressure is meaningless.

•

If effective kill circulation is not possible due to the drillstring being washed out or twisted off, or if the bit nozzles are plugged.

Two main principles are followed during this procedure: •

Constant BHP is maintained by allowing a measured volume of drilling fluid to escape from the annulus as the influx moves up the hole.

•

As gas expands, CSIP increases. Excessive pressure is avoided by bleeding off controlled amounts of drill fluid, without reducing BHP to a point that would allow further influx.

Information required 1. The degree of underbalance from the CSIP gauge - the CSIP pressure obviously reflects the additional pressure required to balance the formation pressure.

2. The mud column height that, when bled off from the annulus, reduces the hydrostatic pressure by a given amount, e.g. 100psi or 700KPa.

height (ft) = 100psi / (MWppg x 0.052) height (m) = 700KPa / (MWkg/m3 x 0.00981) 3. The mud volume that would produce the same pressure drop when bled off from the annulus.

volume (bbls) = height (ft) x casing capacity (bbls/ft) volume (m3) = height (m) x casing capacity (m3/m)

Procedure - Step 1 The first step is to volumetrically bleed off mud from the annulus, while maintaining BHP, allowing influx to rise and gas to come to surface. •

Allow the CSIP to increase to 200psi above the underbalance. This provides a BHP which is 200psi over formation pressure, preventing further influx.

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•

Slowly bleed off the mud volume required to reduce the hydrostatic pressure by 100psi. This is done with the choke while maintaining constant CSIP. The CSIP still reflects the underbalance + 200psi, while the BHP, reduced by 100 psi, now provides 100psi margin over the formation pressure.

•

Close the choke and allow the pressure to increase by a further 100psi. The CSIP now reflects the underbalance + 300psi, while the BHP provides 200psi margin over the formation pressure.

•

Again, maintaining constant CSIP, bleed off the mud volume required to reduce the hydrostatic by a further 100psi. The CSIP still reflects underbalance + 300psi, while the BHP again provides the 100psi margin over the formation pressure.

•

Repeat until gas is at surface P > Underbalance 500 CSIP 400 300 200 BHP 100 ORIGINAL CSIP

Procedure - Step 2 With the gas now at surface, it is necessary to pump mud into the well through the kill line, replacing the gas and maintaining BHP to balance the formation pressure. As this is done, the gas will compress, increasing the CSIP. •

Record CSIP

•

Slowly pump the mud volume necessary to increase the hydrostatic by 100psi, into the well.

•

Wait for the gas to separate from the mud (perhaps 15 minutes).

•

Slowly bleed gas from the choke, lowering the CSIP to the initial value.

•

Continue bleeding until a further 100psi drop is recorded, in order to compensate for the 100psi increase in hydrostatic pressure due to the mud pumped into the well.

•

Repeat this procedure until all of the gas is removed from the annulus.

•

Flow check the well. If it is static, run pipe to the bottom.

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8 QLOG SOFTWARE 8.1 Leak Off Program During a leak off test, this program will read and record the pressure changes realtime and, at the end of the test, will calculate the fracture pressure and equivalent mudweight. By default, the casing pressure sensor will be the one monitored for pressure readings, so you should ensure that the test is being conducted on the same manifold as your sensor.

Required information: Sampling interval

i.e. how often data will be recorded. seconds.

TVD

Taken from realtime system hole depth - this may need to be edited for the depth of the test.

Mud Density

Taken from the realtime system - this may need to be edited to show the value determined by the mud engineer and thus the value to be used for calculations.

Input by the user, typically 5

Mud Pump or Auxiliary pump Pump number

The pump output can then be determined from the pump data file.

Volume or Time

The parameter that the pressure will be plotted against. If Mud Pump is selected above, you can select either volume or time so that the pressure will be plotted against either the mud volume pumped or time. If Auxiliary is selected, you have to select time here, since you will not have a stroke indicator.

Once all the data has been entered, press F3 to start. The program will then start collecting data based on the sample interval selected. Once the test has finished, press any key to stop the data acquisition. Press F7 to calculate. The program will determine the maximum pressure recorded, and from that it will calculate the Fracture Gradient in terms of Equivalent Mud Density. Beware that the ‘Fracture Pressure’ quoted is the maximum Applied Pressure recorded during the test, not the actual Fracture Pressure. Use F2 or F8 to produce a printout or plot of the test. DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

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8.2 Kick/Kill Program This program takes data both from the realtime system and from user input. Any data taken from the realtime system can be edited if required.

Page 1 Data Pump speed and pressure for Slow Circulation Rates. These should be performed regularly by the driller and the mudlogger should update this program every time they are performed. The pump output will be calculated automatically from the pump speed and the output stored in Realtime-Pump Data. Use ‘enter’ to update the calculation. Pump to use

i.e. which pump are they going to use to circulate kill mud.

Drillpipe and Annular Capacities

Calculated automatically from hole and pipe profiles

Original Mudweight

Taken from the realtime system.

Trip Margin

Enter the required pressure if a certain overbalance on the kill mudweight is required.

Down Strokes and Lag Strokes

Calculated from the current profiles, but they will only be updated if the rig is circulating and the system is registering pump strokes. Since, when running this program, the well is likely to be shut in, you may have to enter the correct strokes manually.

Casing Burst Pressure

Obtain from the drilling engineer

Depth of Last Casing Shoe

Taken from the hole profile but remember that this will be measured depth. If the well is deviated, the True Vertical Depth should be entered here.

Formation Fracture Gradient

Manually entered from the last Leak Off or Formation Integrity Test.

Page 2 Data Shut in Pressures

Drillpipe and Casing - these will be read from the realtime system, but should be confirmed with the driller when the pressures have stabilized.

Pit Volume Increase

i.e. the pit gain due to the kick. Remember to subtract surface line run off, if applicable.

Pit Volume Total

This should be the total volume of the pits that will be used to make up and circulate the kill mud. This volume is required to

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determine how much barite is required to increase the mudweight. Total Vertical Depth

Taken from the system (hole depth) but it will have to be edited if the kick does not occur at the bottom of the hole.

Kill Method

1 for Drillers, 2 for Wait and Weight, 3 for Concurrent

Stroke/MW increment

For the Drillers and Wait and Weight methods, this is the stroke increment for the pressure step down when the kill mud is being circulated to the bit (as the kill mud goes from surface to bit, the pressure should be reduced from the Initial to the Final Circulating Pressure). For the Concurrent Method, it is the incremental increase in mudweight that should be entered - the program will then determine how many circulations will be required.

Options F7 to calculate:

Initial Circulating Pressure Kill Mudweight Final Circulating Pressure Maximum Allowable Casing Pressure Total Barite Required Sacks of Barite to Add Fluid Invasion Type Trip Margin Mudweight ie kill mudweight + increment necessary to give the defined pressure overbalance Trip Margin Sacks (of barite)

F3 for Table: For Driller/Wait and Weight methods, this will be a table of strokes vs pressure for the pressure step down (Initial to Final) as the kill mud is circulated to the bit. For Concurrent method:- for each circulation required with an incremental increase in the mudweight, the final circulating pressure is shown.

F2 to Print:

Prints out the table above

F8 for Plot:

Shows pressure reduction vs strokes for the above step down.

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9 EXERCISES EXERCISE 1 - Fracture Gradient and Maximum Allowable Annular Surface Pressure

A Formation Integrity Test was performed at a shoe depth of 8800 ft (TVD 8502 ft) The mudweight during the test was 9.6 ppg, and for the purposes of the test, surface pressure was held at 3000 psi. 1) Calculate the Hydrostatic Pressure at the shoe. 2) Calculate the Fracture Pressure at the shoe. 3) Calculate the Fracture Gradient at the shoe. 4) Calculate the Fracture Gradient Equivalent Mud Weight. 5) What is the value of MAASP taken from the test ?

At 10000 ft ( TVD 9620 ft ), the mud density has to be raised to 10.2 ppg. 6) What is the Hydrostatic Pressure at the shoe ? 7) What is the new MAASP ?

EXERCISE 2 - Fracture Gradient and MAASP An FIT is performed at 4000m (3850m TVD) with a mudweight of 1100 kg/m3. The pressure is held at 20000 Kpa. 1. Calculate the hydrostatic pressure at the shoe. 2. Calculate the fracture pressure. 3. Calculate the fracture gradient EMW. 4. What is the MAASP at the time of the FIT ? 5. Calculate the MAASP at the following incremental mudweights:1150 kg/m3 1200 kg/m3 1250 kg/m3 DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

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Exercise 3 Well Control A kick is taken while drilling a 12 1/4” hole at 7500 feet (MD and TVD). The present mud weight is 10.2ppg The 13 3/8” casing shoe (ID 12.72”) is set at 4000 feet. A leak off test performed with 9ppg mud gave a leak off pressure of 1500psi. The pump capacity is 0.102 bbls/stroke The drill string is composed of:

drillpipe HWDP 500ft DC 600ft

OD 5.0”, ID 4.28” OD 5.0”, ID 3.0” OD 8.5”, ID 3.0”

The last SCR’s taken gave 220psi at 30 spm. A pit gain of 8 bbls was taken and the shut in pressures are

SIDPP 280psi SICP 330 psi

1) Calculate (bbls/ft) the pipe capacity for each section 2) Calculate (bbls/ft) the annular capacity for each section 3) Calculate the fracture gradient at the shoe. 4) At the time of the kick, calculate

a) the present hydrostatic pressure b) the present MAASP

5) Calculate the density of mud required to kill the well 6) Calculate the initial and final circulating pressures 7) Calculate

a) strokes from surface to bit b) strokes from bit to casing shoe c) strokes from casing shoe to surface

8) Calculate the height of the influx 9) Calculate the gradient of the influx 10) What type of influx produced the kick?

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Exercise 4 Well Control

SIDP 710psi

CSIP 1035psi

Bit Size: 8.5” Casing size: 9 5/8” LOT: 2875psi performed with 10ppg mud Pump output: 0.119 bbls/stk Surface line drain off: 23 bbls Last SCR @ 30spm = 400psi Ann Ploss is 40psi when killing well at this rate Drilling @ 80spm = 2900psi Ann Ploss = 300 psi

Pipe lengths: HWDP 490 ft DC 750 ft Shoe 10350ft MD 9800ft TVD

Pipe capacities (bbls/ft): DP HWDP DC

Bit 15670 ft MD 14760 ft TVD

0.01776 0.0088 0.008

Annular Capacities (bbls/ft) DP/CSG DP/OH HWDP/OH DC/OH

0.0562 0.0505 0.0505 0.0292

The well is being drilled with a mud weight of 10.7 ppg, providing a sufficient overbalance over the formation pressure. At 15670ft MD, a pit gain is observed. The pumps are stopped, and a total pit gain of 43bbls is taken before the well is shut in. The shut in pressures are recorded as shown.

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1) Calculate the surface to bit strokes 2) Calculate the bit to casing shoe strokes 3) Calculate the shoe to surface strokes 4) Calculate the total annular capacity 5) Calculate the drillstring capacity 6) Calculate the fracture gradient (EMW) at the shoe 7) Calculate the ECD, prior to the kick being taken

Once the kick has been taken, 8) Calculate the height of the influx 9) Calculate the gradient of the influx 10) What type of kick is it ? 11) Calculate the kill mudweight 12) Calculate the formation pressure 13) Calculate the initial circulating pressure 14) Calculate the final circulating pressure 15) Calculate the ECD while killing the well at 30 spm. Assume the annulus is completely displaced to kill mud. 16) At the kill rate, calculate the

downtime bit to shoe time shoe to surface time

17) Calculate the MAASP with kill mud in the hole 18) Before drilling ahead with this new mudweight, calculate the mudweight required to produce a trip margin of 500psi.

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Verify your calculations by using the QLOG kick/kill program. i.e. confirm the following

Kill Mudweight Initial and Final circulating pressures MAASP with kill mud Influx type Trip Margin

You will need to input the following: SCR data for pump 1 Drillpipe Capacity (Q5) Annular Capacity (Q4) Mudweight Trip Margin required Bit to Surface Strokes (Q2 + Q3) Surface to Bit Strokes (Q1) TVD depth of Casing Formation Fracture Gradient (Q7) Shut in Pressures Pit Volume Increase TVD

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Exercise 5 Well Control

SIDP 1500KPa

CSIP 1800KPa

Bit Size: 311mm Casing size: 339mm FIT: 10000 KPa, with 1020 kg/m3 mud Pump output: 0.016 m3/stk Last SCR @ 40spm = 2500 KPa

Pipe lengths: HWDP 250m DC 150m

Pipe capacities (m3): Shoe 1000m MD 1000m TVD

DP HWDP DC

23.82 1.13 0.68

Annular Capacities (m3) Bit 3000m MD 2650m TVD

DP/CSG DP/OH HWDP/OH DC/OH

70.29 101.28 15.82 6.54

The well is being drilled with a mudweight of 1045 kg/m3 when a 5m3 pit gain is taken. The well is shut in and pressures recorded as shown above.

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1. Calculate the surface to bit strokes 2. Calculate bit to casing shoe strokes 3. Calculate shoe to surface strokes

From the FIT.... 4. What is the fracture pressure at the shoe? 5. What is the maximum equivalent mudweight to avoid fracturing the shoe?

When the kick has been taken... 6. Calculate the mudweight required to kill the well 7. Calculate the initial circulating pressure 8. Calculate the final circulating pressure 9. Calculate the MAASP when kill mud is in the hole 10. Calculate the height of the influx 11. Calculate the gradient of the influx 12. What is the fluid causing the kick? 13. Before drilling ahead with the kill mud, what mudweight would provide a trip margin of 2000 KPa?

Exercise 6 Kick Tolerance Use the following well profiles and information: Hole depth Hole size

3700m TVD 216mm

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Present MW

13.5 ppg

Shoe depth Pfrac

3000m TVD 15.5 ppg emw

278m of 165mm drill collars 127mm drill pipe

Annular Capacity DC/OH = 0.0949 bbls/m Annular Capacity DP/OH = 0.1508 bbls/m

Gas influx, density 2.08 ppg

1. What is the maximum influx height that can be safely controlled without fracturing the shoe? 2. What is the annular volume around the drill collars? 3. For the maximum influx height, what is the maximum influx volume? 4. What is the kick tolerance, assuming a liquid influx? 5. What is the kick tolerance for a gas influx reaching the top of the drill collars? 6. Plot a graph of kick tolerance against influx volume. 7. Assuming a liquid kick, what is the maximum formation pressure (emw) that can be safely controlled without fracturing the shoe? 8. From the graph, given a 20bbls influx of gas, what is the maximum formation pressure that can be safely controlled? 9. Determine the height of a 20bbls influx and then verify the answer to question 8 by using the formula.

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Exercise 7 Kick Tolerance Use the following well profiles and information: Hole depth Hole size Present MW

3200m TVD 216mm 1350 kg/m3

Shoe depth Pfrac

2000m TVD 1700 kg/m3 emw

250m of 165mm drill collars 127mm drill pipe

Annular Capacity DC/OH = 0.01525 m3/m Annular Capacity DP/OH = 0.02396 m3/m

Gas influx, density 250 kg/m3

1. What is the maximum gas influx height that can be safely controlled without fracturing the shoe? 2. What is the annular volume around the drill collars? 3. For the maximum influx height, what is the maximum influx volume? 4. What is the kick tolerance, assuming a liquid influx? 5. What is the kick tolerance for a gas influx reaching the top of the drill collars? 6. Plot a graph of kick tolerance against influx volume. 7. Assuming a liquid kick, what is the maximum formation pressure (emw) that can be safely controlled without fracturing the shoe? 8. From the graph, given a 5m3 influx of gas, what is the maximum formation pressure that can be safely controlled? 9. Determine the height of a 5m3 influx and then verify the answer to question 8 by using the formula.

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Exercise 8 Subsea Well Control 12 ¼” hole; Drilled Depth = 8500 ft TVD Current MW = 10.2 ppg

Air Gap = 80ft Water Depth = 120ft

13 3/8” Casing Shoe = 6000 ft TVD,

Riser ID = 20”

CSG ID = 12.87”

LOT performed with 9.5 ppg mud

Choke line = 200ft Choke ID = 2 ¾”

Leak Off Pressure = 2100psi Pump Output = 0.115 bbls/stroke at 97% efficiency

DC length = 200ft DC OD = 8” DC ID = 3”

Pit Gain = 20bbls SIDP = 500psi; SICP = 800psi

HWDP length = 300ft HWDP OD = 5” HWDP ID = 3”

Last SCR pressure @ 30 SPM = 350psi DP OD/ID = 5” / 4.28”

1. 2. 3.

Calculate the strokes from surface to bit Calculate strokes from bottom to shoe Calculate strokes from shoe to BOP

4.

Calculate strokes, through choke, from BOP to RT

5.

What is the fracture pressure at the shoe?

6.

Prior to taking the kick, what is the MAASP

7.

What is the kill mudweight?

8.

What is the formation pressure?

9.

What are the initial and final circulation pressures?

10.

What is the MAASP with kill mud in the hole?

11.

What is the mud weight required to provide a trip margin of 150 psi over the formation pressure?

12.

What would the new MAASP be?

Kick Tolerance, assuming gas influx of 2.1ppg, and using the new kill mud (+trip margin) 13.

What is the maximum height of the influx?

14.

What is the maximum kick pressure?

15.

What is the maximum volume of influx allowable?

16.

What is the kick tolerance for a 10bbls gas kick?

17.

Once the well has been controlled, what volume of kill mud is required to displace the riser?

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Exercise Answers Exercise 1 Fracture Gradient and MAASP. 1) 2) 3) 4) 5) 6) 7)

4244 psi 7244 psi 0.852 psi/ft 16.39 ppg EMW 3000 psi 4509 psi 2735 psi

Exercise 2 Fracture Gradient and MAASP 1. 2. 3. 4.

41545 Kpa 61545 Kpa 1630 kg/m3 EMW 20000 Kpa

5. 18111 Kpa 16223 Kpa 14334 KPa

Exercise 3 Well Control 1)

drillpipe HWDP DC

0.0178 bbls/ft 0.00874 bbls/ft 0.00874 bbls/ft

2)

drillpipe/casing drillpipe/hole HWDP/hole DC/hole

3)

mud hydrostatic = 1872psi fracture pressure = 3372 psi fracture gradient = 0.843 psi/ft EMW = 16.21 ppg

4)

a) hydrostatic = 3978 psi b) MAASP = 1250 psi

5)

10.92 ppg

6)

ICP = 500psi FCP = 236psi

0.13289 0.12149 0.12149 0.07559

bbls/ft bbls/ft bbls/ft bbls/ft

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7)

a) 1216 b) 3902 c) 5211

8) 9) 10)

106 feet 0.058 psi/ft gas

Exercise 4 Well Control 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)

2240 2123 4888 834.3 bbls 266.6 bbls 0.813 psi/ft, 15.64 ppg emw 11.1 ppg emw 685 ft 0.082 psi/ft Gas 11.63 ppg 8923 psi 1110 psi 435 psi 11.68 ppg emw

16)

75 mins 71 mins 2 hrs 43 mins

17) 18)

2044 psi 12.3 ppg

Exercise 5 Well Control

1. 1602 2. 7727 3. 4393 4. 20006 KPa 5. 2039 kg/m3 6. 1103 kg/m3 7. 4000 KPa 8. 2639 KPa 9. 9186 KPa 10. 114.7m 11. 7.636 KPa / m 12. Condensate or Oil 13. 1180 kg/m3 DATALOG: BOP & WELL CONTROL MANUAL, Version 2.1, issued March, 2001

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Exercise 6 Kick Tolerance 1. 2. 3. 4. 5.

525.4m 26.4 bbls 63.7 bbls 1.62 bbls 0.76 bbls

6. 1.62

0.76

26.4

63.7

7. 15.12 bbls 8. ~ 15.5 bbls emw 9. 210.7m, 0.97 bbls (Max FP = 14.47 bbls emw)

Exercise 7 Kick Tolerance 1. 2. 3. 4. 5. 6.

636m 3.8125 m3 13.06 m3 218.75 kg/m3 132.81 kg/m3 218.7

132.8

3.81

13.1

7. 1569 kg/m3 8. ~ 1470 kg/m3 9. 299.6m, 1466 kg/m3

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Exercise 8 Subsea Well Control 1. 1281 2. 2575 3. 6890 4. 13 5. 5064psi, 16.23ppg emw 6. 1882psi 7. 11.4ppg 8. 5008psi 9. ICP=850psi, FCP=391psi 10. 1435psi 11. 11.7ppg 12. 1342psi 13. 14. 15. 16.

2831ft 3.2ppg 336.4bbls 3.1ppg

17. 72.9bbls

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