231M018 Mud Removal.pdf

MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 1

Mud Removal Basic

Module 231M018 Oct 2000 1

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 2

Objectives of Primary Cementation • Provide complete isolation of zones (Hydraulic Bond) • To support the casing (Shear Bond) • Protect casing string

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•Ask the students for the main reasons for cementing primary casings: •The most important reason is to provide zonal isolation between different formations, or even between the formations and surface. •Other reasons that are also important are to provide casing support and protection.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 3

Mud Removal • Most important aspect of cement job • A 3-step process before cementing •

Hole cleaning

•

Conditioning the drilling fluid Displace the drilling fluid from the annulus

•

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•Ask the students what we must do in order to achieve the previous objectives. •The key thing to do is to remove the mud effectively. There is nothing we can do with a slurry that will achieve zonal isolation if we do not remove the mud. •Mud removal is a three step process that involves cleaning the hole while drilling, conditioning the drilling fluid and then displacing the drilling fluid from the hole.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 4

Mud Removal •

Hole Cleaning • • • •

•

Conditioning Mud • • • •

•

Controlled & optimized mud properties Wiper trips > 95% Total hole volume in circulation Caliper log Break gel strength Lower ty + pv Drill solids < 6% Determine MPG to find qmin for flow all-around casing

Displace Mud from Annulus • • •

Optimized slurry placement ---> CemCADE Casing centralization optimized (STO > 75%) Casing movement

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•Hole cleaning occurs during the drilling of the hole and covers: –controlling and optimizing the mud properties to maintain the hole under control and in gauge, –performing wiper trips at regular intervals to ensure the hole is being completely emptied of cuttings and that the formations are being controlled - wiper trips refer to running the drill string in and out of the hole or to the previous casing shoe without changing the drill bit. –making sure that more than 95% of the hole volume is in circulation - this will indicate if there is gelled mud in washed out areas or if there is a build up of cuttings. To be able to do this a caliper log must have been run. –running a caliper log to determine the hole volume that should be in circulation and to identify possible problem washed out zones. The actual caliper should be used to perform all the volumes calculations for the slurries. •Conditioning the drilling fluid should be done on the last wiper trip just before running the casing but can also be done when the casing is on bottom. It involves: •reducing the gel strengths, –reducing or optimizing the yield point and plastic viscosity, –reducing the solids content below 6%, –determining the Minimum Pressure Gradient to find out what is the minimum rate to achieve flow all around the casing. •Displacing the mud from the hole is done as the cement is put in place and involves: –optimizing the slurry placement using CemCADE, –optimizing the casing standoff (over 75%), –allowing for casing movement (rotation or reciprocation) if possible.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 5

Criteria for Effective Mud Removal Cementing Operation: • • • • • •

Centralize casing Casing movement Scratchers Wiper plugs Washes and spacers Flow regime selection

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•Apart from drilling the hole correctly with good drilling fluid properties to create a gauge, stabilized hole, the criteria for effective mud removal fall mainly under the cementing operation: –the casing should be centralized as much as possible, ideally 100% but as much as possible over 75%. –if possible, the casing should be moved from the start of the circulation to the end of the displacement - this movement can either be rotation or reciprocation. –if the casing is going to be moved, it has been seen that scratchers help scraping the mud filter cake off the wall and move any gelled mud. –wiper plugs must be used, both top and bottom - use more than 1 bottom plug, if possible. –Preflushes (chemical washes and spacers) should be used to separate the slurries from the drilling fluid and to perform the cleaning of the hole. –turbulent flow displacement is the preferred method of mud removal and has been seen to be the most effective - if it is not possible, another flow regime exists: Effective Laminar Flow.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 6

The Ideal Wellbore Casing BHST at top of cement >BHCT at TD

Annular gap Minimum: 3/4” Ideal: 1 1/2”

Properly conditioned hole and mud No sloughing

Gauge diameter

NO LOSSES

Uniform as possible ( no washouts or restrictions)

NO FLOW

Casing centered in borehole

Thin, impermeable mud filter cake (not gelled or unconsolidated)

Accurate BHST and BHCT

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•The ideal wellbore is represented by this drawing: •An annular gap ideally of 1 1/2” but a minimum of 3/4” - this is to ensure that there is a good sheath of cement around the pipe - a sheath of less than 3/4” will too thin and therefore very fragile. •No sloughing of the formation - this means that the formation is stable and not breaking off in pieces - if the formation is caving in, then the cuttings could block the annulus. •The hole should be as uniform as possible even if it is greater than the drilled hole - uniform hole can be effectively cleaned out whereas caves will contain gelled mud which may never be moved by any spacers. •No flow from the formation - obviously if the well is not under control and fluid is flowing into the wellbore, the cement slurry will get contaminated. •The casing should be perfectly centered in the hole - all the fluids will flow equally on all sides of the casing. •Accurate BHST and BHCT - this is necessary to determine accurate placement time avoiding premature setting of the slurry and over-retardation. •The mud filter cake should be thin and impermeable and not gelled or unconsolidated - a thin cake will not be moved by the fluids passing by it but will also not affect greatly the results of the cement job. •No losses - if there are losses part or all of the cement slurry could be lost. •Gauge diameter hole - the better the hole size, the easier turbulent flow is possible and also less volumes of fluids are required. •The hole should be conditioned to ensure that all the mud is mobile and therefore, can be removed. •The static temperature at the top of cement should be less than BHCT - the cement will set up as quickly at the top of cement as at the bottom.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 7

Fluid Calipers • •

To determine circulation efficiency or amount of fluid which is moving in the wellbore. Procedure : •

Run multi-arm open-hole caliper log and determine total hole volume.

•

Circulate at cementing rate and determine mud pump efficiency

•

Drop marker or tracer in staged intervals

•

Monitor returns for marker

•

Calculate volume circulated from rate and time (Should be ± mechanical caliper volume)

•

Increase rate and re-calculate efficiency

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•Fluid calipers are used in conjunction with the caliper log to determine how much of the hole is in circulation. •The procedure is simple and should be performed as often as possible: –The most important part is to run a multi-arm caliper log (BGT) to determine the actual open hole volume and, therefore, the total hole volume. –With the casing on bottom, circulate the well at the expected maximum cementing rate - at this time, determine the mud pump efficiency. –Drop a marker or tracer fluid in staged intervals - the markers can be different coloured fluids or by adding rice or ........... which will give off methane as it returns to surface. –Monitor the returns for the marker. –Calculate the volume that it took to pump to have the marker back on surface, from pump rate and time. –This volume should be more or less the volume from the first step - the total hole volume. –If not, then increase the pump rate and rerun the calculations. With increasing pump rate, an increase in hole volume should be seen.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 8

Influence of the Casing Stand-Off

Di Do

Vnar

Vwide

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•In an eccentric annulus with Di < Do there will be a maximum and a minimum flow velocities. The flow velocity will be highest in the widest part of the hole where the friction pressure is the lowest.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 9

Newtonian Fluid - Effect of STO The Effect of the Casing Stand-Off on the Annular Flow is Qualitatively Equivalent to the Following Flow Pattern

Q

D2

D1 L

∆P ∆L V2

V1

Q 9

•The eccentric annulus can be represented qualitatively (not Quantitatively) by two different pipe sizes hooked in parallel of diameters D1 and D2. The pressures at the entrance and exit of both pipes are the same as they are connected, however, the friction pressure in the smaller pipe is higher. This means that for a certain pump rate into the pipes, the velocity of the fluid in the smaller pipe will be lower than that in the larger pipe.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 10

Newtonian Fluid of Viscosity µ Density ρ In Laminar Flow :

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•Newtonian Fluid –For a Newtonian Fluid, the velocities of the fluid in each pipe can be estimated. The constants and viscosity cancel out leaving a relationship between the diameters & velocities. For two pipe sizes, D2 which is twice the size of D1 (which is a close approximation of 67% Standoff) the velocity in the larger pipe is 4 times the velocity of the fluid in the smaller pipe. Looking at the equivalent Reynolds Number, the velocity in the larger pipe has eight times the Re.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 11

Newtonian Fluid of Viscosity µ Density ρ In Laminar Flow : • 1. Velocity

∆P = 32µ V1 = 32µ V2 ∆L D12 D22

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 12

Newtonian Fluid of Viscosity µ Density ρ In Laminar Flow : • 1. Velocity

∆P = 32µ V1 = 32µ V2 ∆L D12 D22 V2 = (D2)2 V1 (D1)2

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 13

Newtonian Fluid of Viscosity µ Density ρ In Laminar Flow : • 1. Velocity

∆P = 32µ V1 = 32µ V2 ∆L D12 D22 V2 = (D2)2 V1 (D1)2 If D2 = 2D1

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 14

Newtonian Fluid of Viscosity µ Density ρ In Laminar Flow : • 1. Velocity

∆P = 32µ V1 = 32µ V2 ∆L D12 D22 V2 = (D2)2 V1 (D1)2 If D2 = 2D1 V2 = 4V1 (For 67%)

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 15

Newtonian Fluid of Viscosity µ Density ρ In Laminar Flow : • 1. Velocity

∆P = 32µ V1 = 32µ V2 ∆L D12 D22 V2 = (D2)2 V1 (D1)2 If D2 = 2D1 V2 = 4V1 (For 67%)

•

2. Reynolds Number Re2 = ρV2 D2 = ρ4V12D1 = 8ρV1D1 µ µ µ

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 16

Newtonian Fluid of Viscosity µ Density ρ In Laminar Flow : • 1. Velocity

∆P = 32µ V1 = 32µ V2 ∆L D12 D22 2 V2 = (D2) V1 (D1)2 If D2 = 2D1 V2 = 4V1 (For 67%)

•

2. Reynolds Number Re2 = ρV2 D2 = ρ4V12D1 = 8ρV1D1 µ µ µ Re2 = 8Re1 (For 67%)

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•Newtonian Fluid –For a Newtonian Fluid, the velocities of the fluid in each pipe can be estimated. The constants and viscosity cancel out leaving a relationship between the diameters & velocities. For two pipe sizes, D2 which is twice the size of D1 (which is a close approximation of 67% Standoff) the velocity in the larger pipe is 4 times the velocity of the fluid in the smaller pipe. Looking at the equivalent Reynolds Number, the velocity in the larger pipe has eight times the Re.

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Date:Sep 99 Module: CF118 Page: 17

MUD REMOVAL

Laminar Flow in Eccentric Annulus NonNon-parallel plate model Ri/Ro Ri/Ro = 0.8 1000 500

Vwide / Vnarrow

n = 1.0 n = 0.5 n = 0.2

100 50

10 5

1 17

0

10

20

30

40

50

60

70

80

90

100

StandStand-off %

•This is a graph plotting the ratio of the velocity in the large pipe over the velocity in the small pipe verses the stand-off. A fluid with a N = 1 and 70% standoff, the velocity in the larger pipe is 50 times that of the velocity in the small pipe. This shows the importance of having as high a stand-off as possible in laminar flow. •As the fluid deviates from a Newtonian fluid (n=1), the effects of stand-off decrease but are still very important - with an n = 0.2, a stand-off of 60% will mean that the fluid will flow 5 times faster on the wide side than on the narrower side.

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Date:Sep 99 Module: CF118 Page: 18

MUD REMOVAL

In Turbulent Flow •

Velocity

∆p = ∆L =

If D2 = 2D1

2 1.75

0.241 x ρ 0.75 x µ0.25 x( V1πD1 ) 4 D14.75 V π 2 1.75 0.241 x ρ 0.75 x µ0.25 x( 2 D2 ) 4 D24.75 V2 =( D2 )0.714 V1 D 1 V2 = 1.64V1 (For 67%)

•

Reynolds Number Re2= ρV2 D2 = ρ1.64V12D1 = 3.28ρV1D1 µ µ µ Re2 = 3.28Re1 (For 67%)

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•In Turbulent Flow –Look at the same Newtonian Fluid in turbulent flow, the velocity in the large pipe is 1.64 times the velocity in the small pipe, and the Reynolds Number is 3.28 times as much.

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Date:Sep 99 Module: CF118 Page: 19

MUD REMOVAL

Turbulent Flow in Eccentric Annulus 1000 500

Vwide / Vnarrow 100 50

n = 1.0 n = 0.5 n = 0.2

10 5

1

0

10

20

30

40

50

60

70

80

90

100

API Stand - Off (%) 19

•This graph is the same as for the laminar flow graph. •Notice that the influence of a fluid deviating from a Newtonian fluid is less important, in fact, the lines tend to overlay. •For any type of fluid in turbulent flow, with 20% stand-off, the fluid will flow faster on the wide side of the annulus than on the narrower side. In comparison to the laminar flow graph, we can see that stand-off has a lower influence. •This is the fundamental reason why turbulent flow is the preferred flow regime.

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Date:Sep 99 Module: CF118 Page: 20

MUD REMOVAL

Casing Centralization • Relative Variation of flow rate ratio as a function of eccentricity eccentricity 18

RH

FLOW RATE RATIO

16 14

RC

12 10

W

8 6

% Stand-off =

4

w RH - RC

X 100

2 0

0

20

40 60 API % STANDSTAND-OFF

80

100

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•Stand-off is defined as the ratio of the smallest annular gap to the average annular gap between two diameter pipes, if one was completely centered in the other. •The graph shows the ratio of flow rates on the wide side over the narrow side versus the stand-off. Above 75%, it can be seen that there is very little difference in the flow rates ratio. Down to around 35%, the flow rate ratio changes almost linearly to about 5 times faster on the wide side than on the narrow side. Below 35% stand-off, the flow rate ratio starts to increase exponentially.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 21

Types of Centralizers •

•

•

Bow Spring (Spiral or Straight): •

Flexible bow springs

•

Centralizer OD slightly larger than OH size

Rigid Bow (or Positive) type: •

Non-flexible O.D. (Slightly less than previous casing ID)

•

Use inside cased-hole sections

•

Effective in in-gauge OH intervals only

Rigid Solid slip-on type: •

Solid body - no bows

•

Use: as per rigid type

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•There are two ways to increase the stand-off of a casing: drill perfectly straight holes - but this is very rarely possible or desired; or use centralizers on the casing. •There are basically three types of centralizes: –bow spring centralizers which have flexible bow springs (as on old leaf springs on trucks) and they usually have an outside diameter slightly larger than the diameter of the hole. –rigid bow centralizers also known as positive centralizers, which have non flexible bows and have an outside diameter slightly smaller than the smaller diameter in the well (this must be checked as they will not collapse). These centralizers are typically used inside previous casings or open hole sections which are in gauge. The are quite common in horizontal cementing. –rigid solid centralizers or turbolizers which are made up of solid material usually aluminium and have outside diameters smaller than the smallest diameter in the hole. As for the other rigid centralizers, these centralizers are used in cased or ingauge sections, and horizontal cementing. They have the added benefit of causing the fluids to take on a swirling movement as they pass the centralizer.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 22

Reciprocation •

Movement of casing up and down during the job

•

Must be done from the start of circulation to end displacement

•

20 to 40 feet stroke

•

1 to 5 minutes per cycle

•

Needs scratchers to be effective

•

Casing may become stuck during movement

•

Excessive swab and surge pressures may be created

•

Excessive pull and buckling

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Cannot be the only method of mud removal

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•Reciprocation is probably the most common type of casing movement as it is the easiest to perform. •The movement is in a vertical and as all movement should be done from the moment circulation starts to the end of the displacement. This is to ensure that if any thing is going to be freed it is freed early on before the cement is in the annulus and will not get stuck further up the pipe. •The movement is done in a stroke of 20 to 40 feet in 1 to 5 minutes per cycle (a cycle being one upward and one downward movement). •To be effective, reciprocation needs scratchers to be fitted to the casing which will scrape off the mud filter cake and move gelled mud. •Some problems may be encountered as the casing may become stuck during the movement - the casing could end up in the wrong place. Other limiting factors are the surge and swab pressures that are generated during the casing movement - surge being the pressure exerted on the downward stroke and can cause fracturing of the formation (losses); swab being the pressure as a result of the upward stroke and cause a drop in pressure below the formation pressure (kick or blowout). •Another type of problem that could be caused is excessive pull or buckling of the casing, both of which could lead to casing failure. •Casing movement should not be designed to be the only method of mud removal - good results have been seen but in combination with other good mud removal practices.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 23

Rotation •

Circular movement of pipe

•

Must be done from the start of circulation to end displacement

•

10 to 40 rpm

•

Scratchers help efficiency

•

Needs special rotary cement heads and power swivels

•

Torque must be very closely monitored

•

Cannot be the only method of mud removal

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•Rotation is another method of casing movement but where the casing is turned causing the mud to swirl around it. •As in reciprocation, rotation must be started at the beginning off the circulation and continued until the end of the displacement. This is to ensure that any filter cake, gelled mud, etc. that will be removed is done before cement is placed in the casing or annulus - avoiding potential blockages of the annulus. •Typically the casing should be rotated between 10 to 40 rpm with the torque being very closely monitored. •Scratchers improve the efficiency of rotation but they are less necessary then in reciprocation and in fact, some centralizers are fitted to aid the swirling movement of the mud. •The main difficulty of rotation is that it requires special surface equipment - cement head swivels, power swivels to turn the casing, etc. •Rotation causes less problems with casing, the main problem being if the torque is not closely monitored, the casing could get twisted off. •Casing movement should not be designed to be the only method of mud removal - good results have been seen but in combination with other good mud removal practices.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 24

Fluids Incompatibility •

Results In: • •

Detrimental Interface Reactions High Rheological Properties • •

•

Change in Cement Slurry Properties • • •

•

•

Thickening time altered Increase in fluid loss Reduction in compressive strength

Reduction in Hydraulic Bond

Prevented By: • • •

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Very high viscosities Very high gel strengths

•

Wiper Plugs Chemical Washes Spacers Compatibility Testing

•The mixture of a displacing fluid with the displaced fluid, e.g. cement slurry and drilling fluid, often lead to a complete failure in mud removal and zonal isolation. The mixture results in: –detrimental interface reactions which will cause further problems such as: –very high rheological properties either high viscosity or high gel strengths, –change in other slurry properties, e.g. thickening time, fluid loss and compressive strength. –which result in the loss of hydraulic bond. •This contamination can be prevented by using wiper plugs to separate the different fluids in the casing, chemical washes and spacers to separate the fluids in the casing and in the annulus, by compatibility testing to ensure that in case contamination does occur, there will be none of the detrimental effects.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 25

Cement Wiper Plugs •

Keep Fluids Separate in Casing and Reduce Contamination

•

Bottom Plugs

•

•

Remove mud ahead of cement

•

Prevent cement falling through lighter fluid ahead

•

Wipe inner casing walls clean

•

Use 2 or more if possible

Top Plugs •

Separate cement from displacing fluid

•

Positive indication of end of displacement

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•Wiper plugs are used to keep the fluids separated while they are inside the casing. •Bottom plugs are used to: –remove the mud that is ahead of the preflushes and cement, –to prevent the cement and spacers from falling through the lighter fluids that are ahead of them, –to wipe the casing wall clean from mud and debris - if the bottom plug is not used, then this cleaning will be done by the top plug, –each different fluid should be separated by a bottom plug, if possible. •Top plugs are used to: –separate the cement from the displacing fluid which is usually the drilling fluid, –as a positive indication of the end of the displacement when the plug bumps.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 26

Why Run a Bottom Plug ? •

Bottom plug wipes accumulated mud cake, scale, etc. from inner casing walls out through float equipment into annulus.

•

Volume of debris can be significant and fill-up shoetrack if not removed ahead of the top plug. •

EXAMPLE: 9 5/8” 47 lb/ft 10000 feet, collar at 9820 feet

•

Volume of 1/16” film?

•

Height corresponding to this volume?

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•The main reason to run a bottom plug is to scrape the mud cake, scale, rust, etc. from the internal casing wall and push this debris ahead of the cement out of the casing. •If a bottom plug is not run, the top plug will do the same job but this time allowing the debris to accumulate just ahead of the plug - just ahead of the plug means just inside the casing between the float collar and float shoe and possibly just outside, leaving the bottom joints free. •An example to prove this point: –Calculate the volume of a 1/16” film inside 10000 feet of 9 5/8” 47 lb/ft casing with a collar at 9820 feet.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 27

Turbulent Flow Displacement • •

Preferred and best flow regime Critical rate depends on: • • • •

•

Use Chemical Wash and/or Mudpush XT/S spacers: • •

•

10 Min. Contact time or 750 ft (use greater volume)) Spacer density to be close to that of mud

Optimize cement slurry properties: • •

•

Fluid rheologies Casing stand-off Annular gap, casing OD and bit size Formation fracture gradient

Minimum PV and TY without settling Fluid loss and free water controlled

Water wet the casing and formation

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•Turbulent flow has been found both from experiments and from statistics to be the best flow regime to remove the drilling fluid. •For a fluid to be in turbulent flow, it must be pumped above a minimum flow rate, called the critical flow rate. This critical flow rate depends on: •the fluid rheologies: the thinner the fluid, the easier it will go into turbulent flow, –the centralization of the casing or casing stand-off: the better centralized the casing the easier the fluid will go into turbulence, –the annular gap (clearance between the casing and the hole size): the smaller this gap, the easier the fluid will go into turbulence, –the fracture gradient of the formation: this is an indirect factor - the lower the frac. gradient, the harder it is to achieve turbulence without having losses. •The fluids that should be used as preflushes are chemical washes and/or MUDPUSH XT/XS spacers. Of course, water and diesel or base oil can also be used. There are some minimum requirements for these preflushes: –a contact time of at least 10 minutes or 750 feet in the annulus whichever is the bigger - under ideal conditions, less volumes can be used. –all fluids pumped must be compatible with both the drilling fluid in the well and the slurry that will be pumped. •The cement slurry properties must be optimized, e.g. minimum yield point and plastic viscosity but without causing sedimentation/free water, controlled fluid loss and free water. •All fluids must be designed in such a way to ensure that they will water wet the casing and formation.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 28

Effective Laminar Flow • •

Alternative flow regime when Turbulent flow is not possible Four criteria must be satisfied: • • • •

•

DENSITY DIFFERENTIAL (10%) MINIMUM PRESSURE GRADIENT (MPG) FRICTION PRESSURE HIERARCHY (20%) DIFFERENTIAL VELOCITY CRITERION

Viscous spacer: Mudpush XL/XLO • • • • •

Viscosity adjustable (Change D149 concentration) Volume to use: 500 ft or 60 bbls Use 20 - 40 bbls chemical wash Condition and clean mud Viscosify cement slurry when necessary

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•The Effective Laminar Flow is the alternative flow regime if turbulent flow is not possible. This flow regime should not be confused with laminar flow. The difference between the two is that E.L.F. has four criteria that must be met: •the fluid that is displacing must have a density 10% higher than the fluid being displaced, –the Minimum Pressure Gradient must be satisfied - there must be flow all around the casing, –the fluid that is displacing must have a friction pressure gradient 20% higher than the fluid being displaced, –the velocity of the fluids must be the same all around the casing. •A viscous spacer has been designed to fulfill this Effective Laminar Flow regime, MUDPUSH XL/XLO: –it has an adjustable viscosity based on changing the D149 concentration, –a minimum of 500 feet in the annulus or 60 bbls must be used, –20 to 40 bbls of chemical wash should be used ahead of the spacer to start to disperse the drilling fluid, –the drilling fluid should be conditioned to reduce the gel strengths and rheologies, and to remove any solids, –the cement slurries may have to be viscosified in order for it to follow the friction pressure gradient hierarchy - this can be done using D153.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 29

Chemical Washes •

Water based fluids, low viscosity, density of water

•

Easy to pump in turbulent flow

•

CW7 for intermediate casings, water based muds •

•

CW100 for production casings, water based muds •

•

41.25 gals water, 0.5 gals D122A, 0.25 gals J237

CW8 for intermediate casings, oil based muds •

•

41.5 gals water, 0.5 gals D122A

41.25 gals water, 0.5 gals D122A, 0.25 gals F40 last

CW101 for production casings, oil based muds •

41 gals water, 0.5 gals D122A, 0.25 gals J237, 0.25 gals F40

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•Chemical washes are the preferred fluid for turbulent flow. They are water-based fluids that have very low viscosities and densities (the same as water). For these reasons, they are very easy to get into turbulent flow. •The chemical composition of each type of chemical wash is as follows (products in gallons):

Product CW7 CW100 CW8 CW101 CW8-ES CW101-ES Water 41.5 41.25 41.25 41 41.25 - 41 41 - 39.75 D122A 0.5 0.5 0.5 0.5 0.5 0.5 J237 0.25 0.25 0.25 F40 0.25 0.25 D607 0.25 - 0.5 0.25 - 0.5 Fluid Loss No Yes No Yes No Yes Water/Oil Water Water Oil Oil Oil Oil based muds •The mixing order is: water first; D122A; J237 - agitate well; F40 or D607 just before pumping. •The chemical wash should be pre-mixed in a tank but can also be mixed on the fly by adding the products to the displacement tanks as the water is being added - this is not very efficient when preparing CW8/CW101 because there is too much product to add.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 30

Dowell Family of Spacers •

MUDPUSH XT •

•

MUDPUSH XS •

•

Turbulent flow spacer for water based muds in saline (Salt) environments

MUDPUSH XL •

•

Turbulent spacer for water based muds

Effective laminar flow spacer for water based muds

MUDPUSH XTO, XSO, XLO also exist •

Applications are same as above but in OIL BASED MUDS.

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•There are basically 5 spacers that have been produced by Dowell: –MUDPUSH XT: for turbulent flow regime, –MUDPUSH XS: for turbulent flow regime in saline environment, –MUDPUSH XL: for Effective Laminar Flow, fresh or salt water. –MUDPUSH XEO: for high temperature, oil based environments and Effective Laminar Flow, –MUDPUSH WHT: for high temperature, water based environments, fresh or salt water and Effective Laminar Flow. •The range of these spacers can be increased by adding surfactants which will make them compatible with oil based muds. The spacers become: MUDPUSH XTO, XSO, XLO.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 31

Required Properties of Spacers •

Compatible with all other well fluids

•

Stability (good suspending capacity)

•

Controllable density and rheology

•

Good fluid loss control

•

Environmentally safe and easy to handle in the field

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•For spacers to be effective, they need to: – be compatible with the other fluids in the well, e.g. the drilling fluid, the cement slurries, –be stable and have good suspending properties even at high temperatures to avoid allowing the weighting agent to drop out of suspension, –have controllable densities and rheologies to make them repeatable from the lab to the field, –have good fluid loss control as they would be used across permeable pay zones, –be environmentally safe and easy to handle in the field.

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MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 32

MUDPUSH XL with D31

32

•Graphs in the Cementing Materials Manual are used to calculate the concentration of D149 needed to achieve a certain rheology. •Use the 100 rpm reading on the FANN 35 and use the plot. Example: –FANN 35 Reading of Mud = 60 –FANN 35 Reading of Cement = 140 –Split the Difference = 100 for 15 ppg XL –100 on graph is 30 KG of D149 to one cubic meter of water (10.5 lbs/bbl) –This will be the concentration of D149 required. –The rheology can then be checked in the lab and in the field.

32

MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 33

Composition & Field Mixing Order • •

Water: Antifoam:

•

Spacer Blend: • • •

• •

Fresh or Brackish D47 for fresh water D144 for sea and salt water

Mudpush XT - D147 (7.35 lb/bbl) Mudpush XS - D148 (5,25 lb/bbl) Mudpush XL - D149 (3.5-14 lb/bbl)

Salt (NaCl or KCl) : If required Weighting Agent : D151 - CaCO3 < 11.5 lb/gal D31 11.5-16 lb/gal D76 > 16 lb/gal

•

Surfactant: F75, D607, F40, F78, U66 (For oil based mud) - type and concentration depends on spacer / mud used.

33

•The field mixing order is as follows: •Clean the tanks and lines. •Add the correct amount of fresh or brackish water - take a 1 gl sample. •Add the antifoam agent:: D47 for fresh; D144 for seawater and salt water - 0.1 to 0.2 gal/bbl. •Add the spacer blend through the hopper and allow to prehydrate - 20 mins. to 1 hr.: –for MUDPUSH XT, use 7.35 lb/bbl D147 –for MUDPUSH XS, use 5.25 lb/bbl D148 –for MUDPUSH XL, use 3.5 to 14 lb/bbl D149 •Check the viscosity using a MARSH funnel: 32 secs. for MUDPUSH XT; 35 secs. for MUDPUSH XS; the same as determined in the lab for MUDPUSH XL (note that MUDPUSH XL can have its rheology altered by adjusting the concentration of the gelling agent). •If necessary, the salt can now be added through the hopper. Circulate for about 30 minutes for the salt to completely dissolve. •Add the required amount of weighting agent through the hopper. Check the density of the fluid as the specific gravity of some weighting agents vary. Different weighting agents may be used depending on the density required: •CaCO3 (D151) for densities less than 11.5 ppg. –Barite (D31) for densities between 11.5 and 16 ppg. –Hematite (D76) for densities above 16 ppg. •Add the required amount of surfactant just before pumping. The type and concentration of surfactant depends on lab tests and the type of drilling fluid. •Take a sample of the spacer, check it’s density and rheology.

33

MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 34

Events to be Recorded • • • • • • •

Was the mud conditioned - rate and time? How many centralizers were run and where? Was the casing rotated and/or reciprocated? Where the plugs correctly dropped? What was the density and rheology of the spacers? Was the correct volume of preflushes used? The following data must be recorded on the PRISM: • • •

•

All densities, if possible of displacement fluid as well All flow rates, if possible of displacement as well All pressures

Note any changes in flow rate, density, stoppages, pressure peaks, etc.

34

•The following questions must be answered before each job: –Was the mud conditioned, at what rate and for how long? –How many centralizers were run and where? –Was the casing rotated and/or reciprocation? –Were the plugs correctly dropped? –What was the density and rheology of the spacers? –Was the correct volume of preflushes used? •As much data must be recorded on the PACR/PAC/PRISM as possible. The minimum data must be: –The densities of the preflushes and slurries and, if possible, of the displacement fluid. –All the flow rates - the displacement pump rate is the most important but is usually the hardest to record since the displacement is often done with the rig pumps but every effort must be done. –The surface pressure throughout the job - the surface pressure during the displacement can give valuable information if losses occurred. –All notable events during the job should be recorded on the PACR chart - any changes in pump rate or density, any shutdowns, any peaks in pressure, etc. This may help explaining post-job occurrences. –All the up-to-date well data should be included on the service report, including the actual casing depth, etc. •The ST or FE that is on the rig is Dowell’s front line person in contact with the client - this person usually has contact with an enormous amount of information that could make a vital difference in the evaluation of a job or even in the design.

34

MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 35

Conclusions • • •

Condition mud prior to cementing Centralize to give optimum casing stand-off Rotate and/or Reciprocate casing •

• •

• • •

Use cable-type scratchers when reciprocating

Always use the bottom plugs: 2 preferred Optimize slurry placement using CemCADE: •

Turbulent flow preferred, or

•

Effective laminar flow technique

Use chemical wash pre-flushes Control Mudpush spacer/cement slurry properties: batch mix Compatibility mud/cement/spacer : lab/field test

35

•Conclusions for Effective Mud Removal: •Ensure that the drilling fluid has been adequately conditioned prior to cementing - choose the maximum cementing rate (usually the displacement rate) and circulate at least 1 hole volume. •Centralize the casing to ensure an optimum stand-off, higher than 75%. •Try to have some type of casing movement, either rotation or reciprocation, using scratchers to improve the efficiency. •Use bottom plugs - if possible, one for each fluid interface. •Optimize the slurry placement preferring the turbulent flow regime or if that is not possible, the Effective Laminar Flow regime. •Try to use chemical washes whenever possible - these fluids are easier to get into turbulent flow than spacers, they are more cost effective, easier to prepare and handle. •If using MUDPUSH spacers, control their properties especially density and rheology; control the cement slurry properties. Whenever possible, batch mix the slurries and spacers to ensure homogeneous properties. •Avoid adverse mud/cement/spacer reactions by lab testing all the fluids and checking again in the field. A quick field test is to pour one fluid into another slowly and stirring gently - if the fluid thickens and cannot be stirred, consult the DTE or lab. •Ensure that all the well data is correct on arrival on location. If things have changed there may still be time to rerun the design at the district. If nothing else consult and inform the district (FSM or DTE) no matter what time.

35

MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 36

36

MUD REMOVALFES/SSS MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 371

OBJECTIVES OF PRIMARY CEMENTATION •

The main reasons for cementing primary casings: – The most important reason is to provide zonal isolation between different formations, or even between the formations and surface. – Other reasons that are also important are to provide casing support and protection.

•

To achieve the previous objectives: – The key thing to do is to remove the mud effectively. There is nothing we can do with a slurry that will achieve zonal isolation if we do not remove the mud. – Mud removal is a three step process that involves cleaning the hole while drilling, conditioning the drilling fluid and then displacing the drilling fluid from the hole. – Hole cleaning occurs during the drilling of the hole and covers: – controlling and optimizing the mud properties to maintain the hole under control and in gauge, – performing wiper trips at regular intervals to ensure the hole is being completely emptied of cuttings and that the formations are being controlled - wiper trips refer to running the drill string in and out of the hole or to the previous casing shoe without changing the drill bit. – making sure that more than 95% of the hole volume is in circulation - this will indicate if there is gelled mud in washed out areas or if there is a build up of cuttings. To be able to do this a caliper log must have been run. – running a caliper log to determine the hole volume that should be in circulation and to identify possible problem washed out zones. The actual caliper should be used to perform all the volumes calculations for the slurries.

•

Conditioning the drilling fluid should be done on the last wiper trip just before running the casing but can also be done when the casing is on bottom. It involves: – reducing the gel strengths, – reducing or optimizing the yield point and plastic viscosity, – reducing the solids content below 6%, – determining the Minimum Pressure Gradient to find out what is the minimum rate to achieve flow all around the casing.

•

Displacing the mud from the hole is done as the cement is put in place and involves: – optimizing the slurry placement using CemCADE, – optimizing the casing standoff (over 75%), – allowing for casing movement (rotation or reciprocation) if possible.

37

Date:Sep 99 Module: CF118 Page: 382

MUD REMOVALFES/SSS MUD REMOVAL CRITERIA FOR EFFECTIVE MUD REMOVAL •

Apart from drilling the hole correctly with good drilling fluid properties to create a gauge, stabilized hole, the criteria for effective mud removal fall mainly under the cementing operation: – the casing should be centralized as much as possible, ideally 100% but as much as possible over 75%. – if possible, the casing should be moved from the start of the circulation to the end of the displacement this movement can either be rotation or reciprocation. – if the casing is going to be moved, it has been seen that scratchers help scraping the mud filter cake off the wall and move any gelled mud. – wiper plugs must be used, both top and bottom - use more than 1 bottom plug, if possible. – Preflushes (chemical washes and spacers) should be used to separate the slurries from the drilling fluid and to perform the cleaning of the hole. – turbulent flow displacement is the preferred method of mud removal and has been seen to be the most effective - if it is not possible, another flow regime exists: Effective Laminar Flow. – THE IDEAL WELLBORE CASING

•

The ideal wellbore is represented by this drawing:

• An annular gap ideally of 1 1/2” but a minimum of 3/4” this is to ensure that there is a good sheath of cement around the pipe - a sheath of less than 3/4” will too thin and therefore very fragile. • No sloughing of the formation - this means that the formation is stable and not breaking off in pieces - if the formation is caving in, then the cuttings could block the annulus. • The hole should be as uniform as possible even if it is greater than the drilled hole - uniform hole can be effectively cleaned out whereas caves will contain gelled mud which may never be moved by any spacers. • No flow from the formation - obviously if the well is not under control and fluid is flowing into the wellbore, the cement slurry will get contaminated.

Annular gap Minimum: 3/4” Ideal: 1 1/2”

BHST at top of cement >BHCT at TD Properly conditioned hole and mud

No sloughing Uniform as possible

Gauge diameter NO LOSSES

( no washouts or restrictions)

NO FLOW

Casing centered in borehole Thin, impermeable mud filter cake (not gelled or unconsolidated)

Accurate BHST and BHCT

• The casing should be perfectly centered in the hole - all the fluids will flow equally on all sides of the casing.

• Accurate BHST and BHCT - this is necessary to determine accurate placement time avoiding premature setting of the slurry and over-retardation. • The mud filter cake should be thin and impermeable and not gelled or unconsolidated - a thin cake will not be moved by the fluids passing by it but will also not affect greatly the results of the cement job. • No losses - if there are losses part or all of the cement slurry could be lost. • Gauge diameter hole - the better the hole size, the easier turbulent flow is possible and also less volumes of fluids are required. • The hole should be conditioned to ensure that all the mud is mobile and therefore, can be removed. • The static temperature at the top of cement should be less than BHCT - the cement will set up as quickly at the top of cement as at the bottom.

38

MUD REMOVALFES/SSS MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 393

FLUID CALIPERS •Fluid calipers are used in conjunction with the caliper log to determine how much of the hole is in circulation. •The procedure is simple and should be performed as often as possible: –The most important part is to run a multi-arm caliper log (BGT) to determine the actual open hole volume and, therefore, the total hole volume. –With the casing on bottom, circulate the well at the expected maximum cementing rate - at this time, determine the mud pump efficiency. –Drop a marker or tracer fluid in staged intervals - the markers can be different coloured fluids or by adding rice or ........... which will give off methane as it returns to surface. –Monitor the returns for the marker. –Calculate the volume that it took to pump to have the marker back on surface, from pump rate and time. –This volume should be more or less the volume from the first step - the total hole volume. –If not, then increase the pump rate and rerun the calculations. With increasing pump rate, an increase in hole volume should be seen.

INFLUENCE OF THE CASING STANDOFF • In an eccentric annulus with Di < Do there will be a maximum and a minimum flow velocities. The flow velocity will be highest in the widest part of the hole where the friction pressure is the lowest.

39

The Effect of the Casing Stand-Off on the Annular Flow is Qualitatively Equivalent to the Following Flow Pattern

NEWTONIAN FLUID- THE EFFECT OF CASING STAND-OFF

Q

• D2

D1 L ∆P ∆L

V2

V1

The eccentric annulus can be represented qualitatively (not Quantitatively) by two different pipe sizes hooked in parallel of diameters D1 and D2. The pressures at the entrance and exit of both pipes are the same as they are connected, however, the friction pressure in the smaller pipe is higher. This means that for a certain pump rate into the pipes, the velocity of the fluid in the smaller pipe will be lower than that in the larger pipe.

Q

39

Date:Sep 99 Module: CF118 Page: 404

MUD REMOVALFES/SSS MUD REMOVAL NEWTONIAN FLUID

– For a Newtonian Fluid, the velocities of the fluid in each pipe can be estimated. The constants and viscosity cancel out leaving a relationship between the diameters & velocities. For two pipe sizes, D2 which is twice the size of D1 (which is a close approximation of 67% Standoff) the velocity in the larger pipe is 4 times the velocity of the fluid in the smaller pipe. Looking at the equivalent Reynolds Number, the velocity in the larger pipe has eight times the Re.

LAMINAR FLOW IN ECCENTRIC ANNULUS •

•

Non-parallel plate model Ri/Ro = 0.8

This is a graph plotting the ratio of the velocity in the large pipe over the velocity in the small pipe verses the stand-off. A fluid with a N = 1 and 70% standoff, the velocity in the larger pipe is 50 times that of the velocity in the small pipe. This shows the importance of having as high a stand-off as possible in laminar flow.

1000 500

Vwide/ Vnarrow

n = 1.0 n = 0.5 n = 0.2

100 50

As the fluid deviates from a Newtonian fluid (n=1), the effects of stand-off decrease but are still very important - with an n = 0.2, a stand-off of 60% will mean that the fluid will flow 5 times faster on the wide side than on the narrower side.

10 5

1

0

10

20

30

40

50

60

70

80

90

100

Stand-off %

• Velocity∆p = ∆L =

If D2 = 2D1

2 1.75 0.241 x ρ 0.75 x µ0.25 x ( V1πD1 ) 4 4.75 D1 V π 2 1.75 0.241 x ρ 0.75 x µ 0.25 x( 2 D2 ) 4 D24.75

V 2 =( D 2 ) 0.714 V1 D1 V2 = 1.64V1 (For 67%)

IN TURBULENT FLOW – Look at the same Newtonian Fluid in turbulent flow, the velocity in the large pipe is 1.64 times the velocity in the small pipe, and the Reynolds Number is 3.28 times as much.

• Reynolds Number Re2= ρV2 D2 = ρ1.64V12D1 = 3.28ρV1D1 µ µ µ Re2 = 3.28Re1 (For 67%)

1000

TURBULENT FLOW IN AN ECCENTRIC ANNULUS •

This graph is the same as for the laminar flow graph.

•

Notice that the influence of a fluid deviating from a Newtonian fluid is less important, in fact, the lines tend to overlay.

•

For any type of fluid in turbulent flow, with 20% stand-off, the fluid will flow faster on the wide side of the annulus than on the narrower side. In comparison to the laminar flow graph, we can see that stand-off has a lower influence.

•

This is the fundamental reason why turbulent flow is the preferred flow regime.

500

Vwide / Vnarrow

100 50

n = 1.0 n = 0.5 n = 0.2

10 5

1

0

10

20

30

40

50

60

70

80

90

100

API Stand - Off (%)

40

MUD REMOVALFES/SSS MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 415

CASING CENTRALIZATION •Stand-off is defined as the ratio of the smallest annular gap to the average annular gap between two diameter pipes, if one was completely centered in the other. •The graph shows the ratio of flow rates on the wide side over the narrow side versus the stand-off. Above 75%, it can be seen that there is very little difference in the flow rates ratio. Down to around 35%, the flow rate ratio changes almost linearly to about 5 times faster on the wide side than on the narrow side. Below 35% stand-off, the flow rate ratio starts to increase exponentially. 41

TYPES OF CENTRALIZERS • There are two ways to increase the stand-off of a casing: drill perfectly straight holes - but this is very rarely possible or desired; or use centralizers on the casing. • There are basically three types of centralizes: – bow spring centralizers which have flexible bow springs (as on old leaf springs on trucks) and they usually have an outside diameter slightly larger than the diameter of the hole. – rigid bow centralizers also known as positive centralizers, which have non flexible bows and have an outside diameter slightly smaller than the smaller diameter in the well (this must be checked as they will not collapse). These centralizers are typically used inside previous casings or open hole sections which are in gauge. The are quite common in horizontal cementing. – rigid solid centralizers or turbolizers which are made up of solid material usually aluminium and have outside diameters smaller than the smallest diameter in the hole. As for the other rigid centralizers, these centralizers are used in cased or in-gauge sections, and horizontal cementing. They have the added benefit of causing the fluids to take on a swirling movement as they pass the centralizer. – RECIPROCATION • Reciprocation is probably the most common type of casing movement as it is the easiest to perform. • The movement is in a vertical and as all movement should be done from the moment circulation starts to the end of the displacement. This is to ensure that if any thing is going to be freed it is freed early on before the cement is in the annulus and will not get stuck further up the pipe. • The movement is done in a stroke of 20 to 40 feet in 1 to 5 minutes per cycle (a cycle being one upward and one downward movement). • To be effective, reciprocation needs scratchers to be fitted to the casing which will scrape off the mud filter cake and move gelled mud. • Some problems may be encountered as the casing may become stuck during the movement - the casing could end up in the wrong place. Other limiting factors are the surge and swab pressures that are generated during the casing movement - surge being the pressure exerted on the downward stroke and can cause fracturing of the formation (losses); swab being the pressure as a result of the upward stroke and cause a drop in pressure below the formation pressure (kick or blowout). • Another type of problem that could be caused is excessive pull or buckling of the casing, both of which could lead to casing failure. • Casing movement should not be designed to be the only method of mud removal - good results have been seen but in combination with other good mud removal practices.

41

MUD REMOVALFES/SSS MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 426

ROTATION •

Rotation is another method of casing movement but where the casing is turned causing the mud to swirl around it.

•

As in reciprocation, rotation must be started at the beginning off the circulation and continued until the end of the displacement. This is to ensure that any filter cake, gelled mud, etc. that will be removed is done before cement is placed in the casing or annulus - avoiding potential blockages of the annulus.

•

Typically the casing should be rotated between 10 to 40 rpm with the torque being very closely monitored.

•

Scratchers improve the efficiency of rotation but they are less necessary then in reciprocation and in fact, some centralizers are fitted to aid the swirling movement of the mud.

•

The main difficulty of rotation is that it requires special surface equipment - cement head swivels, power swivels to turn the casing, etc.

•

Rotation causes less problems with casing, the main problem being if the torque is not closely monitored, the casing could get twisted off.

•

Casing movement should not be designed to be the only method of mud removal - good results have been seen but in combination with other good mud removal practices.

•

INCOMPATIBILITY BETWEEN DISPLACED AND DISPLACING FLUIDS

•

The mixture of a displacing fluid with the displaced fluid, e.g. cement slurry and drilling fluid, often lead to a complete failure in mud removal and zonal isolation. The mixture results in:

•

detrimental interface reactions which will cause further problems such as: – very high rheological properties either high viscosity or high gel strengths, – change in other slurry properties, e.g. thickening time, fluid loss and compressive strength. – which result in the loss of hydraulic bond.

•

This contamination can be prevented by using wiper plugs to separate the different fluids in the casing, chemical washes and spacers to separate the fluids in the casing and in the annulus, by compatibility testing to ensure that in case contamination does occur, there will be none of the detrimental effects.

•

CEMENT WIPER PLUGS

•

Wiper plugs are used to keep the fluids separated while they are inside the casing.

•

Bottom plugs are used to: – remove the mud that is ahead of the preflushes and cement, – to prevent the cement and spacers from falling through the lighter fluids that are ahead of them, – to wipe the casing wall clean from mud and debris - if the bottom plug is not used, then this cleaning will be done by the top plug, – each different fluid should be separated by a bottom plug, if possible.

•

Top plugs are used to: – separate the cement from the displacing fluid which is usually the drilling fluid, – as a positive indication of the end of the displacement when the plug bumps.

42

MUD REMOVALFES/SSS MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 437

WHY RUN A BOTTOM PLUG ? •

The main reason to run a bottom plug is to scrape the mud cake, scale, rust, etc. from the internal casing wall and push this debris ahead of the cement out of the casing.

•

If a bottom plug is not run, the top plug will do the same job but this time allowing the debris to accumulate just ahead of the plug - just ahead of the plug means just inside the casing between the float collar and float shoe and possibly just outside, leaving the bottom joints free.

•

An example to prove this point: – Calculate the volume of a 1/16” film inside 10000 feet of 9 5/8” 47 lb/ft casing with a collar at 9820 feet.

TURBULENT FLOW DISPLACEMENT •

Turbulent flow has been found both from experiments and from statistics to be the best flow regime to remove the drilling fluid.

•

For a fluid to be in turbulent flow, it must be pumped above a minimum flow rate, called the critical flow rate. This critical flow rate depends on:

•

the fluid rheologies: the thinner the fluid, the easier it will go into turbulent flow, – the centralization of the casing or casing stand-off: the better centralized the casing the easier the fluid will go into turbulence, – the annular gap (clearance between the casing and the hole size): the smaller this gap, the easier the fluid will go into turbulence, – the fracture gradient of the formation: this is an indirect factor - the lower the frac. gradient, the harder it is to achieve turbulence without having losses. – The fluids that should be used as preflushes are chemical washes and/or MUDPUSH XT/XS spacers. Of course, water and diesel or base oil can also be used. There are some minimum requirements for these preflushes: – a contact time of at least 10 minutes or 750 feet in the annulus whichever is the bigger - under ideal conditions, less volumes can be used. – all fluids pumped must be compatible with both the drilling fluid in the well and the slurry that will be pumped.

•

The cement slurry properties must be optimized, e.g. minimum yield point and plastic viscosity but without causing sedimentation/free water, controlled fluid loss and free water.

•

All fluids must be designed in such a way to ensure that they will water wet the casing and formation.

EFFECTIVE LAMINAR FLOW •

The Effective Laminar Flow is the alternative flow regime if turbulent flow is not possible. This flow regime should not be confused with laminar flow. The difference between the two is that E.L.F. has four criteria that must be met:

•

the fluid that is displacing must have a density 10% higher than the fluid being displaced, – the Minimum Pressure Gradient must be satisfied - there must be flow all around the casing, – the fluid that is displacing must have a friction pressure gradient 20% higher than the fluid being displaced, – the velocity of the fluids must be the same all around the casing.

43

MUD REMOVALFES/SSS MUD REMOVAL •

Date:Sep 99 Module: CF118 Page: 448

A viscous spacer has been designed to fulfill this Effective Laminar Flow regime, MUDPUSH XL/XLO: – it has an adjustable viscosity based on changing the D149 concentration, – a minimum of 500 feet in the annulus or 60 bbls must be used, – 20 to 40 bbls of chemical wash should be used ahead of the spacer to start to disperse the drilling fluid, – the drilling fluid should be conditioned to reduce the gel strengths and rheologies, and to remove any solids, – the cement slurries may have to be viscosified in order for it to follow the friction pressure gradient hierarchy - this can be done using D153. – CHEMICAL WASHES

•

Chemical washes are the preferred fluid for turbulent flow. They are water-based fluids that have very low viscosities and densities (the same as water). For these reasons, they are very easy to get into turbulent flow.

•

The chemical composition of each type of chemical wash is as follows (products in gallons):

•

Product CW7 CW100 CW8 CW101 CW8-ES CW101-ES Water 41.5 41.25 41.25 41 41.25 - 41 41 - 39.75 D122A 0.5 0.5 0.5 0.5 0.5 0.5 J237 0.25 0.25 0.25 F40 0.25 0.25 D607 0.25 - 0.5 0.25 - 0.5 Fluid Loss No Yes No Yes No Yes Water/Oil Water Water Oil Oil Oil Oil basedorder mudsis: water first; D122A; J237 - agitate well; F40 or D607 just before pumping. The mixing

•

The chemical wash should be pre-mixed in a tank but can also be mixed on the fly by adding the products to the displacement tanks as the water is being added - this is not very efficient when preparing CW8/CW101 because there is too much product to add.

•

SPACERS- THE DOWELL FAMILY OF SPACERS

•

There are basically 5 spacers that have been produced by Dowell: – MUDPUSH XT: for turbulent flow regime, – MUDPUSH XS: for turbulent flow regime in saline environment, – MUDPUSH XL: for Effective Laminar Flow, fresh or salt water. – MUDPUSH XEO: for high temperature, oil based environments and Effective Laminar Flow, – MUDPUSH WHT: for high temperature, water based environments, fresh or salt water and Effective Laminar Flow.

•

The range of these spacers can be increased by adding surfactants which will make them compatible with oil based muds. The spacers become: MUDPUSH XTO, XSO, XLO.

44

MUD REMOVALFES/SSS MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 459

REQUIRED PROPERTIES OF SPACERS • For spacers to be effective, they need to: –

be compatible with the other fluids in the well, e.g. the drilling fluid, the cement slurries,

–

be stable and have good suspending properties even at high temperatures to avoid allowing the weighting agent to drop out of suspension,

–

have controllable densities and rheologies to make them repeatable from the lab to the field,

–

have good fluid loss control as they would be used across permeable pay zones,

–

be environmentally safe and easy to handle in the field.

MUDPUSH XL WEIGHTED WITH D31 • Graphs in the Cementing Materials Manual are used to calculate the concentration of D149 needed to achieve a certain rheology. • Use the 100 rpm reading on the FANN 35 and use the plot. Example: – FANN 35 Reading of Mud = 60 – FANN 35 Reading of Cement = 140 – Split the Difference = 100 for 15 ppg XL – 100 on graph is 30 KG of D149 to one cubic meter of water (10.5 lbs/bbl) – This will be the concentration of D149 required. – The rheology can then be checked in the lab and in the field.

COMPOSITION AND FIELD MIXING ORDER • The field mixing order is as follows: • Clean the tanks and lines. • Add the correct amount of fresh or brackish water - take a 1 gl sample. • Add the antifoam agent:D47 for fresh; D144 for seawater and salt water - 0.1 to 0.2 gal/bbl. • Add the spacer blend through the hopper and allow to prehydrate - 20 mins. to 1 hr.: –

for MUDPUSH XT, use 7.35 lb/bbl D147

–

for MUDPUSH XS, use 5.25 lb/bbl D148

–

for MUDPUSH XL, use 3.5 to 14 lb/bbl D149

• Check the viscosity using a MARSH funnel: 32 secs. for MUDPUSH XT; 35 secs. for MUDPUSH XS; the same as determined in the lab for MUDPUSH XL (note that MUDPUSH XL can have its rheology altered by adjusting the concentration of the gelling agent). • If necessary, the salt can now be added through the hopper. Circulate for about 30 minutes for the salt to completely dissolve. • Add the required amount of weighting agent through the hopper. Check the density of the fluid as the specific gravity of some weighting agents vary. Different weighting agents may be used depending on the density required: • CaCO3 (D151) for densities less than 11.5 ppg. –

Barite (D31) for densities between 11.5 and 16 ppg.

–

Hematite (D76) for densities above 16 ppg.

• Add the required amount of surfactant just before pumping. The type and concentration of surfactant depends on lab tests and the type of drilling fluid. • Take a sample of the spacer, check it’s density and rheology.

45

MUD REMOVALFES/SSS MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 4610

EVENTS TO BE RECORDED • The following questions must be answered before each job: – Was the mud conditioned, at what rate and for how long? – How many centralizers were run and where? – Was the casing rotated and/or reciprocation? – Were the plugs correctly dropped? – What was the density and rheology of the spacers? – Was the correct volume of preflushes used? • As much data must be recorded on the PACR/PAC/PRISM as possible. The minimum data must be: – The densities of the preflushes and slurries and, if possible, of the displacement fluid. – All the flow rates - the displacement pump rate is the most important but is usually the hardest to record since the displacement is often done with the rig pumps but every effort must be done. – The surface pressure throughout the job - the surface pressure during the displacement can give valuable information if losses occurred. – All notable events during the job should be recorded on the PACR chart - any changes in pump rate or density, any shutdowns, any peaks in pressure, etc. This may help explaining post-job occurrences. – All the up-to-date well data should be included on the service report, including the actual casing depth, etc. • The ST or FE that is on the rig is Dowell’s front line person in contact with the client - this person usually has contact with an enormous amount of information that could make a vital difference in the evaluation of a job or even in the design. • CONCLUSIONS FOR EFFECTIVE MUD REMOVAL: • Ensure that the drilling fluid has been adequately conditioned prior to cementing - choose the maximum cementing rate (usually the displacement rate) and circulate at least 1 hole volume. • Centralize the casing to ensure an optimum stand-off, higher than 75%. • Try to have some type of casing movement, either rotation or reciprocation, using scratchers to improve the efficiency. • Use bottom plugs - if possible, one for each fluid interface. • Optimize the slurry placement preferring the turbulent flow regime or if that is not possible, the Effective Laminar Flow regime. • Try to use chemical washes whenever possible - these fluids are easier to get into turbulent flow than spacers, they are more cost effective, easier to prepare and handle. • If using MUDPUSH spacers, control their properties especially density and rheology; control the cement slurry properties. Whenever possible, batch mix the slurries and spacers to ensure homogeneous properties. • Avoid adverse mud/cement/spacer reactions by lab testing all the fluids and checking again in the field. A quick field test is to pour one fluid into another slowly and stirring gently - if the fluid thickens and cannot be stirred, consult the DTE or lab. • Ensure that all the well data is correct on arrival on location. If things have changed there may still be time to rerun the design at the district. If nothing else consult and inform the district (FSM or DTE) no matter what time.

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MUD REMOVALSEC MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 47 1

OBJECTIVES OF PRIMARY CEMENTATION •The main reasons for cementing primary casings: –The most important reason is to provide zonal isolation between different formations, or even between the formations and surface. –Other reasons that are also important are to provide casing support and protection. •To achieve the previous objectives: –The key thing to do is to remove the mud effectively. There is nothing we can do with a slurry that will achieve zonal isolation if we do not remove the mud. –Mud removal is a three step process that involves cleaning the hole while drilling, conditioning the drilling fluid and then displacing the drilling fluid from the hole. –Hole cleaning occurs during the drilling of the hole and covers: –controlling and optimizing the mud properties to maintain the hole under control and in gauge, –performing wiper trips at regular intervals to ensure the hole is being completely emptied of cuttings and that the formations are being controlled - wiper trips refer to running the drill string in and out of the hole or to the previous casing shoe without changing the drill bit. –making sure that more than 95% of the hole volume is in circulation - this will indicate if there is gelled mud in washed out areas or if there is a build up of cuttings. To be able to do this a caliper log must have been run. –running a caliper log to determine the hole volume that should be in circulation and to identify possible problem washed out zones. The actual caliper should be used to perform all the volumes calculations for the slurries. –Conditioning the drilling fluid should be done on the last wiper trip just before running the casing but can also be done when the casing is on bottom. It involves: –reducing the gel strengths, –reducing or optimizing the yield point and plastic viscosity, –reducing the solids content below 6%, –determining the Minimum Pressure Gradient to find out what is the minimum rate to achieve flow all around the casing. •Displacing the mud from the hole is done as the cement is put in place and involves: –optimizing the slurry placement using CemCADE, –optimizing the casing standoff (over 75%), –allowing for casing movement (rotation or reciprocation) if possible. CRITERIA FOR EFFECTIVE MUD REMOVAL •Apart from drilling the hole correctly with good drilling fluid properties to create a gauge, stabilized hole, the criteria for effective mud removal fall mainly under the cementing operation: •the casing should be centralized as much as possible, ideally 100% but as much as possible over 75%. –if possible, the casing should be moved from the start of the circulation to the end of the displacement this movement can either be rotation or reciprocation. –if the casing is going to be moved, it has been seen that scratchers help scraping the mud filter cake off the wall and move any gelled mud. –wiper plugs must be used, both top and bottom - use more than 1 bottom plug, if possible. –Preflushes (chemical washes and spacers) should be used to separate the slurries from the drilling fluid and to perform the cleaning of the hole. –turbulent flow displacement is the preferred method of mud removal and has been seen to be the most effective - if it is not possible, another flow regime exists: Effective Laminar Flow.

47

MUD REMOVALSEC MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 482

INCOMPATIBILITY BETWEEN DISPLACED AND DISPLACING FLUIDS • The mixture of a displacing fluid with the displaced fluid, e.g. cement slurry and drilling fluid, often lead to a complete failure in mud removal and zonal isolation. The mixture results in: • detrimental interface reactions which will cause further problems such as: – very high rheological properties either high viscosity or high gel strengths, – change in other slurry properties, e.g. thickening time, fluid loss and compressive strength. – which result in the loss of hydraulic bond. • This contamination can be prevented by using wiper plugs to separate the different fluids in the casing, chemical washes and spacers to separate the fluids in the casing and in the annulus, by compatibility testing to ensure that in case contamination does occur, there will be none of the detrimental effects. • CEMENT WIPER PLUGS • Wiper plugs are used to keep the fluids separated while they are inside the casing. • Bottom plugs are used to: – remove the mud that is ahead of the preflushes and cement, – to prevent the cement and spacers from falling through the lighter fluids that are ahead of them, – to wipe the casing wall clean from mud and debris - if the bottom plug is not used, then this cleaning will be done by the top plug, – each different fluid should be separated by a bottom plug, if possible. • Top plugs are used to: – separate the cement from the displacing fluid which is usually the drilling fluid, – as a positive indication of the end of the displacement when the plug bumps. TURBULENT FLOW DISPLACEMENT • Turbulent flow has been found both from experiments and from statistics to be the best flow regime to remove the drilling fluid. • For a fluid to be in turbulent flow, it must be pumped above a minimum flow rate, called the critical flow rate. This critical flow rate depends on: • the fluid rheologies: the thinner the fluid, the easier it will go into turbulent flow, – the centralization of the casing or casing stand-off: the better centralized the casing the easier the fluid will go into turbulence, – the annular gap (clearance between the casing and the hole size): the smaller this gap, the easier the fluid will go into turbulence, – the fracture gradient of the formation: this is an indirect factor - the lower the frac. gradient, the harder it is to achieve turbulence without having losses. – The fluids that should be used as preflushes are chemical washes and/or MUDPUSH XT/XS spacers. Of course, water and diesel or base oil can also be used. There are some minimum requirements for these preflushes: – a contact time of at least 10 minutes or 750 feet in the annulus whichever is the bigger - under ideal conditions, less volumes can be used. – all fluids pumped must be compatible with both the drilling fluid in the well and the slurry that will be pumped. • The cement slurry properties must be optimized, e.g. minimum yield point and plastic viscosity but without causing sedimentation/free water, controlled fluid loss and free water. • All fluids must be designed in such a way to ensure that they will water wet the casing and formation.

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MUD REMOVALSEC MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 49 3

EFFECTIVE LAMINAR FLOW • The Effective Laminar Flow is the alternative flow regime if turbulent flow is not possible. This flow regime should not be confused with laminar flow. The difference between the two is that E.L.F. has four criteria that must be met: • the fluid that is displacing must have a density 10% higher than the fluid being displaced, – the Minimum Pressure Gradient must be satisfied - there must be flow all around the casing, – the fluid that is displacing must have a friction pressure gradient 20% higher than the fluid being displaced, – the velocity of the fluids must be the same all around the casing. • A viscous spacer has been designed to fulfill this Effective Laminar Flow regime, MUDPUSH XL/XLO: – it has an adjustable viscosity based on changing the D149 concentration, – a minimum of 500 feet in the annulus or 60 bbls must be used, – 20 to 40 bbls of chemical wash should be used ahead of the spacer to start to disperse the drilling fluid, – the drilling fluid should be conditioned to reduce the gel strengths and rheologies, and to remove any solids, – the cement slurries may have to be viscosified in order for it to follow the friction pressure gradient hierarchy this can be done using D153. CHEMICAL WASHES • Chemical washes are the preferred fluid for turbulent flow. They are water-based fluids that have very low viscosities and densities (the same as water). For these reasons, they are very easy to get into turbulent flow. • The chemical composition of each type of chemical wash is as follows (products in gallons):

Product CW7 CW100 CW8 CW101 CW8-ES CW101-ES Water 41.5 41.25 41.25 41 41.25 - 41 41 - 39.75 D122A 0.5 0.5 0.5 0.5 0.5 0.5 J237 0.25 0.25 0.25 F40 0.25 0.25 D607 0.25 - 0.5 0.25 - 0.5 Fluid Loss No Yes No Yes No Yes Water/Oil Water Water Oil Oil Oil Oil • The mixing order is: water first; D122A; J237 - agitate well; F40 or D607 just before pumping. based muds • The chemical wash should be pre-mixed in a tank but can also be mixed on the fly by adding the products to the displacement tanks as the water is being added - this is not very efficient when preparing CW8/CW101 because there is too much product to add. • SPACERS- THE DOWELL FAMILY OF SPACERS • There are basically 5 spacers that have been produced by Dowell: – MUDPUSH XT: for turbulent flow regime, – MUDPUSH XS: for turbulent flow regime in saline environment, – MUDPUSH XL: for Effective Laminar Flow, fresh or salt water. – MUDPUSH XEO: for high temperature, oil based environments and Effective Laminar Flow, – MUDPUSH WHT: for high temperature, water based environments, fresh or salt water and Effective Laminar Flow. • The range of these spacers can be increased by adding surfactants which will make them compatible with oil based muds. The spacers become: MUDPUSH XTO, XSO, XLO.

49

MUD REMOVALSEC MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 50 4

REQUIRED PROPERTIES OF SPACERS •For spacers to be effective, they need to: – be compatible with the other fluids in the well, e.g. the drilling fluid, the cement slurries, –be stable and have good suspending properties even at high temperatures to avoid allowing the weighting agent to drop out of suspension, –have controllable densities and rheologies to make them repeatable from the lab to the field, –have good fluid loss control as they would be used across permeable pay zones, –be environmentally safe and easy to handle in the field. COMPOSITION AND FIELD MIXING ORDER •The field mixing order is as follows: •Clean the tanks and lines. •Add the correct amount of fresh or brackish water - take a 1 gl sample. •Add the antifoam agent:D47 for fresh; D144 for seawater and salt water - 0.1 to 0.2 gal/bbl. •Add the spacer blend through the hopper and allow to prehydrate - 20 mins. to 1 hr.: –for MUDPUSH XT, use 7.35 lb/bbl D147 –for MUDPUSH XS, use 5.25 lb/bbl D148 –for MUDPUSH XL, use 3.5 to 14 lb/bbl D149 •Check the viscosity using a MARSH funnel: 32 secs. for MUDPUSH XT; 35 secs. for MUDPUSH XS; the same as determined in the lab for MUDPUSH XL (note that MUDPUSH XL can have its rheology altered by adjusting the concentration of the gelling agent). •If necessary, the salt can now be added through the hopper. Circulate for about 30 minutes for the salt to completely dissolve. •Add the required amount of weighting agent through the hopper. Check the density of the fluid as the specific gravity of some weighting agents vary. Different weighting agents may be used depending on the density required: •CaCO3 (D151) for densities less than 11.5 ppg. –Barite (D31) for densities between 11.5 and 16 ppg. –Hematite (D76) for densities above 16 ppg. •Add the required amount of surfactant just before pumping. The type and concentration of surfactant depends on lab tests and the type of drilling fluid. •Take a sample of the spacer, check it’s density and rheology. EVENTS TO BE RECORDED •The following questions must be answered before each job: –Was the mud conditioned, at what rate and for how long? –How many centralizers were run and where? –Was the casing rotated and/or reciprocation? –Were the plugs correctly dropped? –What was the density and rheology of the spacers? –Was the correct volume of preflushes used?

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MUD REMOVALSEC MUD REMOVAL

Date:Sep 99 Module: CF118 Page: 51 5

• As much data must be recorded on the PACR/PAC/PRISM as possible. The minimum data must be: – The densities of the preflushes and slurries and, if possible, of the displacement fluid. – All the flow rates - the displacement pump rate is the most important but is usually the hardest to record since the displacement is often done with the rig pumps but every effort must be done. – The surface pressure throughout the job - the surface pressure during the displacement can give valuable information if losses occurred. – All notable events during the job should be recorded on the PACR chart - any changes in pump rate or density, any shutdowns, any peaks in pressure, etc. This may help explaining post-job occurrences. – All the up-to-date well data should be included on the service report, including the actual casing depth, etc. • The ST or FE that is on the rig is Dowell’s front line person in contact with the client - this person usually has contact with an enormous amount of information that could make a vital difference in the evaluation of a job or even in the design.

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