9. Mud System.pdf

PVD Training October 25, 2012 Authored by: Pham Van Thien

PVD Training Section 9. Mud System

Table of Contents  .......................................................................................................................................................................... 1 9.1 Mud system  ............................................................................................................................................................................. 2 9.1.1 Mud Pumping Equipment  ................................................................................................................................................... 2 Introduction ...................................................................................................................... ......................................... 2 Mud Pumps .............................................................. ................................................................................................................................ ................................................................................................. ............................... 3 Mud Manifolds.......................................................... ............................................................................................................................. ................................................................................................. .............................. 4 9.1.2 Mud Mixing Equipment  ...................................................................................................................................................... 5 Mud Hopper. ........................................................................................................ ..................................................... 5 9.1.3 Mud Monitoring Equipment  ................................................................................................................................................ 7 Pit Volume Measuring and Recording Devices ................................................................ ......................................................................................................... ......................................... 7 Mud Logging .............................................................................................. ............................................................................................................................................................. ............................................................... 9  Automatic Drilling Fluid Weighing Devices ........................................................... .............................................................................................................. ................................................... 10 Manual Drilling Fluid Weighing Devices Devic es ................................................................ .................................................................................................................. .................................................. 11 Trip Tank .................................................................. ..................................................................................................................................... ............................................................................................... ............................ 11 Typical Trip Tank Hook-up - On a Floating Rig ................................................................ ....................................................................................................... ....................................... 11 9.1.4 Degassers  .......................................................................................................................................................................... 12 The Mud Gas Separator (Poorboy) ............................................................ ......................................................................................................................... ............................................................. 13 9.2 Circulating system  ............................................................................................................................................................... 15

PVD Training | Table of Contents

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PVD Training Section 9. Mud System

9.1 Mud system

Introduction The Rig Circulating System consists of: 1. Mud Tanks ( where the mud is stored) 2. Mud Pumps ( which are supercharged with mud from the tanks to their suction lines) 3. Standpipe manifold 4. Rotary Hose 5. Swivel and Kelly or Topdrive 6. Drillstring 7. Flowline 8. Shaleshakers mounted on top of mud tanks The mud is pumped and moved from the discharge end of the mud pumps via piping to the rig floor the mud enters the standpipe manifold, it then travels up the derrick through the standpipe and enters either the Kelly hose and swivel or the top drive. The mud then travels down the inside of the drill string and returns up the annulus, over the shaker screens where the cuttings are separated from the mud and back to the active system to be cleaned and conditioned before returning to the well. The following sections describe the equipment and explain its purpose. Refer to Figure 9.1

PVD Training | 9.1 Mud system

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PVD Training Section 9. Mud System

Fig 9.1 - Continuous Circulating System

Mud Pumps The function of the mud pump(s) is to impart power to the mud in the form of pressure and volume. The energy of the mud will assist the bit when drilling the hole (commonly called bit hydraulics) and lift the cuttings out of the hole. The most common pump in use today is the triplex pump. The three cylinders provide increased volumes of fluid at higher pressures than the older style duplex pumps. The pump pistons and liners can be changed easily to increase or decrease the pump volume and pressure output. The pumps are fitted with pulsation dampeners on the suction and discharge lines to ensure smooth displacement of fluids along with stroke counters to measure accurately the volume displaced by the pump. All pumps should be fitted with relief valves to avoid exceeding the pumps pressure rating.

PVD Training | 9.1.1 Mud Pumping Equipment

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PVD Training Section 9. Mud System

Mud Manifolds The standpipe manifold consists of: 1. Two steel pipes, clamped vertically to the derrick or mast, connecting the mud discharge lines to the rotary hoses, including one spare hose. 2. A manifold with easy operated valves, connected to the standpipes on the discharge side and to the mud pumps on the adjacent side. 3. A pressure gauge directly connected to the manifold with an isolation valve and pressure sensors for accurate pressure gauges at the drillers and choke panel. 4. Subject to design philosophy, connections to the killline, chokeline and cement pump may also be incorporated at the manifold. Note: The pressure rating of the standpipe manifold is normally the same as the mudpumps.

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PVD Training Section 9. Mud System

Mud Manifold

The circulating system is monitored and the mud properties are checked and adjusted at all times to ensure the correct margins are maintained.

Mud Hopper. The equipment used for mixing dry additives to the mud is referred to as the mud hopper. A small pump is used to circulate the mud from the active pit to the hopper and back to the pit. Jet lines and agitators in the pits are used to prevent settling out of mud solids.

PVD Training | 9.1.2 Mud Mixing Equipment

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PVD Training Section 9. Mud System

Mud Hopper 

Several pits or holding tanks are used in the mud system. The active pit or primary pit will normally have a separated settling tank or Sand trap under the shale shaker to prevent heavy solids contaminating the active pit. The additional pits are used to store pre mixed mud for ongoing operations. A small pit called a slugging pit is used to mix smaller volumes of up to fifty barrels to mix pills for specialist operations and for mixing a slug prior to tripping.

PVD Training | 9.1.2 Mud Mixing Equipment

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PVD Training Section 9. Mud System

Mud Mixing System

Pit Volume Measuring and Recording Devices  Automatic pit volume measuring devices are available which transmit a pneumatic or electric signal from sensors on the mud pits to recorders and signalling devices on the rig floor. These instruments are valuable in detecting fluid loss or gain when circulating. The pit volume totalizer (PVT) with a calibrated read out and audio alarm system should be installed on the rig floor at the driller’s station. The PVT is used to detect a pit gain and together with a flow rate indicator is a positive indicator that a kick has been taken. The following measurement devices are required for all tanks: „ A float for the PVT system, to isolate other floats when the trip tank is in use. „ An internal calibrated ladder -type scale.  Also required: „ A remote ladder -type scale, visible from the Driller’s station for the trip tank . Visual Pit Volume Measuring Device  A float attached to a counterweight above the rig floor by a small cable and rigged up to show pit volume on a board behind the counterweight is a useful sensing device when mounted on the mud pits or trip tank. Flow line Measurement PVD Training | 9.1.3 Mud Monitoring Equipment

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PVD Training Section 9. Mud System  A flowmeter should be installed in the mud return line. This (Flo Show) device should have a read-out and alarm at the Driller’s station as shown in Figures 9.6 and 9.7 Flow Rate Sensor  A flow rate sensor mounted in the flow line is useful for early detection of influx fluid entering the wellbore or loss of returns from the wellbore. When circulating through a system too large to permit accurate measurement of a pit gain, such as when circulating through the reserve pit, a flow rate sensor mounted in the flow line is recommended.

Fig 9.6 - Flow Sensor

Communication If a transfer of mud to the active system is requested the driller will be informed, the mud logging unit if available must likewise be informed. Good communication all round is essential. Alarms The high and low settings for the pit level alarm and flow line alarm must be checked and set to appropriate values.

PVD Training | 9.1.3 Mud Monitoring Equipment

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PVD Training Section 9. Mud System

Fig 9.7 - Flow Sensor 

Mud Logging Correct installation and operation of the mud logging equipment is fundamental to perform effective primary and secondary well control. The mud logging unit and system are shown in Figure 9.8. The unit carries out some of the following services: „ Gas detection in the mud „ Gas analysis „ Cuttings density analysis „ Recording mud densities in and out „ Recording flow line temperatures „ Recording penetration rates „ Pore Pressure Trends „ Cuttings description

PVD Training | 9.1.3 Mud Monitoring Equipment

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PVD Training Section 9. Mud System

Fig 9.8 - Typical Mud Logging system

Automatic Drilling Fluid Weighing Devices  Automatic drilling fluid weighing devices are available and can improve the measurement of the drilling fluid density in the surface system. It is possible to incorporate an automatic drilling fluid weighing device with an automatic weight material system to control drilling fluid density automatically as part of the mud logging services. PVD Training | 9.1.3 Mud Monitoring Equipment

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PVD Training Section 9. Mud System

Manual Drilling Fluid Weighing Devices  A drilling fluid balance designed to weigh a compressed sample of drilling fluid can be used to read the density of gas cut drilling fluid without the necessity of diluting, degassing, weighing and calculating socalled true drilling fluid weight.

Trip Tank  As each stand of pipe is pulled out of the well the hole has to be filled with mud equal to the volume of steel removed. This is referred to as pulling dry. A trip sheet is used to record the volume of mud pumped into the well. If the well is swabbing the volume to fill the well will be less than the steel displacement, a primary indicator of a kick. While running in the well mud will be displaced from the well. The volume displaced should also be equal to the steel displacement. It is important that the rig can measure these volumes accurately. There are occasions, a float in the drill string, or the bit plugging off, pulling wet, were the fixed end displacement of the pipe tripped will have to be accounted for. In order for these volume measurements to be monitored a trip tank is used. There are many types of manually operated trip tanks but they should all have the ability to monitor volumes accurately as shown in Figure 9.10 The trip tank has a capacity of 10 to 40 barrels and built so that 30 gallons equals at least an inch of depth. It is an accurate device for measuring the influx volume or loss of fluid from the wellbore. The ability to circulate the trip tank and automatically fill the hole are those most preferred by the driller because the fluid volume used can be measured and the information displayed on the rig floor screen monitor. It is then easy to see if the hole is taking the correct amount of fluid compared with the theoretical value. The ability to switch valves over from the active pit to the trip tank for flow checking small influx volumes is an additional advantage.  All trip tanks require careful maintenance. They must be kept clean to avoid build up of solids, valves should be check for leaks and ease of operation, floats and instrumentation checked and calibrated at regular intervals.  A smaller stripping tank may be used in conjunction with the trip tank to monitor gas expansion during stripping operations.

Typical Trip Tank Hook-up - On a Floating Rig  As illustrated in Figure 9.10, a centrifugal pump takes suction from the trip tank and fills the hole through a line into the bell nipple. The pump runs constantly while the drill string is pulled from the hole. The hole stays full as each stand of pipe is pulled and excess mud returns to the trip tank through an outlet on the main flow line. A valve must be installed in the flow line downstream of this outlet to block all flow to the shale shakers while making a trip. A closed circulation system can be monitored by a float system and a digital read-out in 1-barrel increments on the Driller’s console.

PVD Training | 9.1.3 Mud Monitoring Equipment

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PVD Training Section 9. Mud System

Fig 9.10 - Trip Tank Hook Up (sub-sea)

 Although gas will be released as it passes over the shale shakers while drilling there may be additional volumes of gas entering the well bore which must be removed from the mud. Recirculation of gas cut mud reduces the hydrostatic head in the well if not properly removed from the circulating system. Degassers are mounted over the active pit. They are in effect a one stage liquid-gas separator. Mud enters the degasser from submerged pipe work in the mud pit. The maximum lift to the inlet is about 10 feet. A vacuum pump mounted on top of the degasser is driven by a three hp electric motor. This vacuum is applied to the vapour space by this pump. The vacuum applies ranges from 8 to 15 inches of mercury, or between two and five pounds per square inch, depending on the density of the mud drawn into it. Degasses can extract gas from mud flows up to 900 gallons per minute. PVD Training | 9.1.4 Degassers

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PVD Training Section 9. Mud System  A degasser may be used to remove entrained gas bubbles in the drilling fluid (gas cut mud) that are too small to be removed by the mud/gas separator. Most degassers make use of some degree of vacuum to assist in removing any entrained gas. The drilling fluid inlet line to the degasser should be placed close to the drilling fluid discharge line from the mud/gas separator to reduce the possibility of gas breaking out of the drilling fluid in the pit. Also, the drilling fluid throughput capacity of the degasser should exceed the maximum flow rate from the well to ensure that all drilling fluid is subjected to degassing. The mud enters near the top of the degasser and flows through a pipe closed in at the far end. The top of this pipe is cut away horizontally to form an open trough. The mud spills over the trough through out the length of the pipe over inclined plates. These plates spread the mud over the large surface area releasing the gas. As the mud streams evenly over the plates the vacuum in the vapour space causes the gas to leave the mud. The gas is then vented from the tank to a vent line or burned safely away from the rig. The degassed mud is returned back to its normal weight and drops to the bottom of the cylinder of the degasser. The degassed mud passes through an extractor jet [5/8th inch] entering the mud stream. This allows the pressured mud to be reduced to lesser pressure than the vacuum pressure. Mud is returned to the active pit. Safety valves prevent mud from entering the vacuum pump, backing up into the degasser if the working pressure reduces.

Fig 9.11 - Vacuum Degasser

The Mud Gas Separator (Poorboy) The mud/gas separator is situated downstream of the choke manifold. The mud/gas separator is used to separate gas from the drilling fluid that is gas cut. The separated gas can then be vented a safe distance

PVD Training | 9.1.4 Degassers

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PVD Training Section 9. Mud System from the rig (land rigs) or through the vent line in the derrick. Generally, two basic types of mud/gas separators are in use. The most common type is the atmospheric mud/gas separator, sometimes referred to as a “gas buster” or “poor -boy” separator. Another type of mud/gas separator is   designed such that it can be operated at moderate back pressure, usually less than 100 psig, although some designs are operated at gas vent line pressure which is atmospheric, plus the vent line friction drop. See Figure 9.12  All separators with a liquid level control may be referred to as pressurized mud/gas separators. Both the atmospheric and the pressurized mud/gas separators have advantages and disadvantages. Some requirements are common to both types of mud/gas separators. A by-pass line to the flare stack must be provided in case of malfunction or in the event the capacity of the mud/gas separator is exceeded. Precautions must also be taken to prevent erosion at the point the drilling fluid impinges on the wall of the vessel. Provisions must be made for easy cleanout of the vessels and lines in the event of plugging. Unless specifically designed for such applications, use of the rig mud/gas separator is not recommended for well testing operations. The mud gas separator or poorboy, should be lined up at all times when a kick is being displaced. The separator is used to remove large gas bubbles from the mud and to deal with a flow of gas once the influx is at surface. There will be a limit to the volume of gas that each separator can safely deal with. When this limit is exceeded, there exists the possibility that gas will blow through into the shaker header box.  An estimation can be made of the maximum gas flowrate that the separator can handle. The limiting factors will be the back pressure at the outlet to the vent line in relation to the hydrostatic head of fluid at the mud outlet of the separator. When the back pressure due to the gas flow is equal to, or greater than, the hydrostatic head available at the mud outlet, the gas will blow through to the shaker header tank. In order to minimise the possibility of a gas blow-through, the vent line should be as straight as possible and have a large ID. The mud outlet should be configured to develop a suitable hydrostatic head (minimum recommended head is 10 feet). The back pressure due to the flow of gas should be monitored with a pressure gauge. Some warning of the possibility of a gas blow-through will be given when the registered pressure approaches the hydrostatic head of the fluid in the discharge line. It should be noted that the maximum hydrostatic head available may not be that of the mud in the event that large volumes of oil or condensate are displaced to surface. If the safe operating limit of the separator is approached, the choke can be closed in (while ensuring that the well is not overpressured) or the flow switched to the overboard line or the burn pit.

PVD Training | 9.1.4 Degassers

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PVD Training Section 9. Mud System

Fig 9.12 - Example Mud Gas Separator – Operating at Maximum Capacity

9.2 Circulating system 9.2.1 Pump pressure Let examine what happens when a fluid is moved or pumped along a pipe.

 As we start to pump fluid will move along the pipe. To make this happen we must overcome the fluids resistance to flow. This will caused by the friction of the fluid against the pipe and the viscosity of the fluid. PVD Training | Circulating system

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PVD Training Section 9. Mud System For example, the more viscous the fluid or the smaller the pipe, the more resistance to flow. To overcome this resistance will require a pressure to exerted the at the pump.

Now the mud is flowing at a constant rate of 100 gpm, we should see a steady pressure at the pump. This “Pump Pressure” is equal to the amount of friction that must be overcome to move the fluid along the pipe at this flow rate.

This pressure would change if any of the following change: 

Flow rate



Fluid properties (viscosity, density)



Pipe diameter



Pipe length

Increase the flow rate, fluid viscosity, fluid density and length of the pipe would all cause an increase in pump pressure. Increase the diameter of the pipe would cause a decrease in the pump pressure.

Note that whilst there is a pressure at the pump (pump pressure) of 500psi, the fluid is leaving the pipe at 0 psi gauge pressure. There is therefore a pressure loss or pressure drop across this section of pipe.  As in this example we only have one section of pipe, this pressure loss is the same as pump pressure. 1.

The Circulating System

When looking at circulating pressures we can split the circulating system into four component parts.

PVD Training | Circulating system

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PVD Training Section 9. Mud System

Surface lines

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from the mud pump to the top drive or kelly

Drill string

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the drill string including drill pipe, HWDP and drill collar.

Bit

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the nozzles of flow path through the bit

 Annulus-

from the bit back up to surface including, for example;

Drill

collar in open hole

Drill

pipe in the open hole

Drill

pipe in casing

When we pump mud around the circulating system, each section of the system will have an associated pressure loss. The sum of these pressure losses will be seen at the pump as the pump pressure.

Pump pressure = The sum of all the pressure loss in the system from the pump back to surface.

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PVD Training Section 9. Mud System

9.2.2 Changes in pump rate If we change the pump rate then the pump pressure will also change 

Increase the spm will cause the pressure to increase



Decrease the spm will cause the pressure to decrease

The actual pressure for a new pump rate can be estimated using the following formula;

Change in mud weight  A change in mud weight will also affect pump pressure: 

Increase the mud weight will cause the pressure to increase



Decrease the mud weight will cause the pressure to decrease

This effect can also be calculated using the formula;

The effect of circulating pressure on bottom hole pressure

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PVD Training Section 9. Mud System

If we start with the pump pressure and subtract each individual pressure loss in turn, this will show us the circulating pressure at different points in the system. The important pressure to note here is that there is 200 psi remaining to move the mud up the annulus. This pressure loss in the annulus is usually termed the Annular Pressure Loss (APL)

Looking back at the circulating system it can be seen that most of the pressure losses take place prior to the mud reaching the bit. In fact the only pressure loss remaining in the open hole is the annulus pressure loss (APL). Thus the APL is the only pressure which will have any effect on Bottom Hole Pressure (BHP).

So when the mud is being circulated, a pressure equal to the APL acts on the bottom of the hole. This will have the effect of increasing BHP. Equivalent circulating density We have seen that bottom hole pressure increases when circulating by the amount of the annular pressure losses (APL) BHCP = Mud hydrostatic + APL So while circulating the bottom hole circulation pressure (BHCP) is higher than the mud hydrostatic. BHCP can be expressed as a value in pounds per gallon (ppg), this is known ass the Equivalent Circulating Density ECD can be calculated from the bottom hole circulating pressure by converting this to an equivalent mud density.

 Another more common formula used to calculate ECD uses the annular pressure loss and the original mud weight.

PVD Training | Circulating system

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