School of Petroleum Engineering, UNSW Open Learning - 2000 7 Mud Removal qWell preparat ion qMud conditioning qRunning casing ØCentralizers ØScratchers / wall cleaners qOther factors qMud displacement ØDisplacement of mud in concentric annuli ØRemoval of mud in eccentric annuli The main objective of a primary cement job is to provide complete and permanent isolation of the permeable zones located behind the casing. To achieve this, the drilling mud and the preflush (if any) must be fully displaced from the annulus, and the annular space must be completely filled with cement. Failure to remove all the mud from the interval to be cemented may result in continuous mud channels across the zones of interest, thereby favouring interzonal communication. Therefore, good mud removal and proper slurry placement are essential to o btain well isolation. Research concerning the cement process began in the 1930s. Some key factors influencing primary cement job failures were identified, and solutions were proposed as early as 1940s.
School of Petroleum Engineering, UNSW Open Learning - 2000
7
Mud Removal
q Well preparationq Mud conditioningq Running casing
Ø CentralizersØ Scratchers / wall cleaners
q Other factorsq Mud displacement
Ø Displacement of mud in concentric annuliØ Removal of mud in eccentric annuli
The main objective of a primary cement job is to provide complete and permanentisolation of the permeable zones located behind the casing. To achieve this, the
drilling mud and the preflush (if any) must be fully displaced from the annulus, andthe annular space must be completely filled with cement. Failure to remove all themud from the interval to be cemented may result in continuous mud channels acrossthe zones of interest, thereby favouring interzonal communication. Therefore, goodmud removal and proper slurry placement are essential to obtain well isolation.
Research concerning the cement process began in the 1930s. Some key factorsinfluencing primary cement job failures were identified, and solutions were proposedas early as 1940s.
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7.1 Well PreparationSeveral factors affect the efficiency of mud removal. A poorly drilled hole may haveseveral washed-out zones which are difficult to clean out, regardless of thedisplacement rate. Crooked holes make casing centralization difficult; consequently,the removal of mud from the narrow side of the annulus is problematic.
Poorly treated drilling mud might induce washouts or inversely, thicker filter cakes,which not only are difficult to remove but they reduce the annular flow area, therebyincreasing frictional pressure losses. Good drilling practices will not assure asuccessful cementing job but might prevent a failure. It is understandable that theobjective for the drilling engineer is to drill the well safely, and as fast andeconomically as possible. However, this should be accomplished bearing in mind thatone of the ultimate goals during drilling is to provide the optimum wellbore for successful cementing:
• a well with controlled subsurface pressures,• a smooth hole with a minimum number of dog legs,• an in-gauge hole,• a stabilized borehole,• a hole cleaned from cuttings and• a correctly treated mud that will give thin, dynamic filter cakes in front of
permeable zones.
Unfortunately, such an ideal situation cannot always be achieved. Therefore, cement placement techniques must often be designed to the influence of poor well preparation.
7.2 Mud ConditioningDrilling fluids have a number primary characteristics which are designed to facilitatedrilling operations and provide proper cuttings transport, but are not necessarilyconducive to efficient mud displacement. Therefore, it may be necessary to conditionthe mud, i.e., to modify its properties.
For the purposes of efficient displacement during primary cementing, the ideal mudshould be:
• a non-thixotropic fluid with little or no gel strength, low plastic viscosity andlow yield point, because the driving forces necessary to displace the mud arereduced and its mobility is increased.
• of a low density to permit easy removal by higher density fluids and allow the placement of heavier spacers and cement slurries.
• of chemical composition compatible with cement.
Care also must be taken to prevent the settling of weighting agents. This mayrepresent a major constraint for highly deviated wells.
The mud rheology can be modified by adding water (which also reduces the density)or dispersants to the mud at the surface. It is necessary to circulate for at least onehole volume, and ideally should be done before removing the drillpipe. Otherwise,
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unconditioned mud may have sufficient time to gel during the pseudostatic period(while removing drillpipe, logging and running casing).
Mud circulation is also necessary after the casing is in place. Unfortunately, it is verycommon to condition the mud only at this stage. Circulation is beneficial in the
following ways:
• ensures that the hole is cleaned from cuttings,• ensures that gas flow is not occurring,• homogenises the mud after treatment on the surface,• reduces mud yield stress and plastic viscosity because most drilling muds are
thixotropic, and• erodes the gelled and/or dehydrated mud that is trapped in washouts, on the
narrow side of an eccentric annulus, and at the walls of permeable formations.
Unfortunately, an ideal conditioned mud cannot always be obtained. Nevertheless, anattempt should be made to get as close to the ideal characteristics as possible, duringthe necessary pre-cementation circulation.
7.3 Running Casing
7.3.1 CentralizersCasing should always be run with centralizers ( Fig. 7.1 ). The latter will substantiallyimprove the mud removal efficiency while increasing the likelihood of getting thecasing down when the differential sticking is expected. In addition, although
centralizers ( Fig. 7.2 ) may not actually ensure perfect centralization, they do providea more uniform annular flow area, perpendicular to the flow direction.
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Fig. 7.2 - Centralizers .
This latter advantage is of considerable importance because a cement slurry willalways follow the path of least resistance, i.e., through the widest part of the hole. If there is a considerable difference in the area of the annular flow, the mud on thenarrow side can easily be bypassed.
The effects of poor centralization on the velocity of flow (in the narrow side of theannulus) are shown in Fig. 7.3 , established for a fluid with a density of 10 lb/gal and ayield strength of 10 lb/100 ft 2, flowing through a 6 ’’ casing by 9 ’’ open hole annulus.
V e
l o c
i t y i n n a r r o w s
i d e /
A v g .
v e
l o c
i t y
Rate of flow (BPM)
6 8 10 20 40 60
0.2
0.4
0.6
0.8
3 4
1
80
1.2
75 %
50 %
33.3 %
100 % STANDOFF (CENTRED)
0
Fig. 7.3 - Effects of centralization on velocity of flow.
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Given a 50% standoff, the fluid in the narrow side will not move before the averageflow exceeds 10 bpm. Even if the average flow rate exceeds 20 bpm, the rate of flowin the narrow side will never be greater than 60% of the total flow rate. This
potentially can lead to mud (in the narrow part of the annulus) being bypassed duringthe displacement process by cement slurry ( Figs. 7.4(a)-(b) ).
Bypassed mud
Fig. 7.4(a) - Schematic diagram of bypassed mud in an eccentric annulus.
Fig. 7.4(b) – Channeling vs. pipe standoff.
Efficient centralizing may also prevent differential sticking. The main factor in the process is the filtration of water from the mud through the existing filter cake into the
surrounding permeable formations.
So long as the casing is actually moving, the possibility of differential sticking isminimized by the mud, which by circulating between the casing and the filter cake,will provide the fluid needed to maintain the latter in a soft state. When pipemovement is interrupted, however, the cake is sealed off from the mud which
previously supplied the filtrate. The residual water in the cake continues to filter intothe surrounding formation, reducing the thickness of the filter cake and increasingformation contact area with the casing. With high differential pressures acting on thecasing, it is pressed even further against the walls and the cake eventually becomescompletely dehydrated. Once this happens, the effect of pressure loading and the highfriction pressure between the casing and the solidified cake combine to prevent thecasing from being removed ( Fig. 7.5 ).
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Fig. 7.5 - Differential sticking 1.
Centralizers, therefore, should be positioned throughout the intervals that are to becemented, with special attention being paid to those intervals which are susceptible todifferential sticking (permeable zones) and through dog legs where key seats mayexist.
The specifications for casing centralizers for use with different casing weights andvarying casing and hole sizes have been established by the API. The rule of thump for spacing of centralizers applicable to vertical holes is as follows:
PLACEMENT OF CENTRALIZERSq Surface casing
Centralizers should be placed at the bottom of the casing string, preferably withone above the shoe and one at each of the bottom five or six joints. Centralizersshould also be placed in front of critical water zones.
q Intermediate casingAs well as placing centralizers as mentioned above, additional centralizers should
be run wherever a zone is to be isolated or wherever dog legs, poor holeconditions or irregular formations are suspected.
q Production casingCentralizers should be placed as recommended above when dealing with production casing. In addition to this, however, they should be placed in front of and regular spaces (100 ft above and below) across all productive zones.
q Stage cementingFor casing below the stage collar, the same rule of thump applies as for productioncasing. In addition, centralizers should be placed both one joint above and one
joint below the stage collar.
q Liners
Centralizers should be used where clearance and hole conditions permit.
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7.3.2 Scratchers / Wall CleanersScratchers are also used when running casing, being of particular importance when ahigh fluid-loss mud has resulted in a thick fluffy filter cake. The difference between agood cementing bond and a poor one is often the removal of mud cake from the
borewall at the desired depth. Two types are used – the reciprocating and the rotating
scratchers. The reciprocating scratcher cleans the wall as the casing is reciprocatedover a certain distance (normally 20 ft over a 15 ft spacing), thereby ensuring thateach scratcher works on a stretch of the wall overlapping that covered by another.The rotating scratcher cleans the wall during rotation of the casing. (see C HAPTER 8
– C EMENTING E QUIPMENT )
7.4 Other Factors
q M UD C ONTAMINATIONThe contamination of a cement slurry by mud may result in:
• Acceleration or retardation of the thickening time of the slurry.• An increase in the slurry’s fluid loss.• A reduction of the set cement’s compressive strength.• A reduction of the set cement’s final hydraulic bond strength.
It may also result in a high-viscosity/high-gel strength mixture generating highdisplacement pressures. Flowing in laminar regime at normal rates, such a mixturemay induce mud bypassing, resulting in further contamination of the cementslurry. In extreme cases, the flow may be stopped by the highly gelified plug.
Cement-mud contamination may be prevented by using plugs to separate thefluids as they move down the casing, and spacers and/or washers to minimize thecement-mud contact in the annular space.
q R ECIPROCATING OR R OTATING C ASINGBoth laboratory and field observations have indicated that pipe movement, byrotation or reciprocation, in combination with mechanical devices (such asscratchers, scrapers, or cable wipers) during cement placement helps remove themud that would otherwise be trapped on the narrow side of an eccentric annulus.
McLean et al. (1967) performed an extensive study on how pipe movementaffected the displacement efficiency. They found that rotation appeared to exert adrag force on the cement and pull it around to displace mud, as illustrated in Fig.7.6 . In contrast, they found reciprocation to be of little effect, however, theyemphasized that lateral motion was not allowed in their experiments, which islikely to happen in the field. Smith (1990), in contrast to McLean et al. , stated thatcasing reciprocation would increase the displacement efficiency.
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Fig. 7.6 - Effect of casing rotation .
Casing reciprocation is a relatively easy undertaking in most single-stage jobs.One must be careful, however, to ensure that the reciprocation rate is controlled inorder to avoid pressure surges during the down stroke, which could break weak formations, or a swabbing effect during the upstroke, which may cause a blowout.
In a highly eccentric annulus, rotation may be a more effective means of removingmud. However, while casing rotation during primary cementing may be easilyaccomplished in shallow (6000-ft) straight holes, fear of twisting the casing often
prevents the use of this technique in deep, crooked or directional wells.
Pipe movement is not generally considered in liner cementing. However, thequality of cement around the liner can be greatly improved by using special liner hangers, allowing one to rotate the liner after the hanger is set.
q F LUIDS AHEAD OF THE CEMENT SLURRYThe fluids used to separate cement and mud and to facilitate the removal of mudfrom the annular space can be classified as follows:
• Chemical washes• Spacers
Chemical washesWashes are thin, usually water-based fluids containing surfactant and mudthinners, designed to thin and disperse the mud so that it can be efficientlyremoved from the hole. Because they tend to be thin and disperse the mud
particles, they are primarily designed for use in turbulent flow conditions. Washesare available for water and oil-based muds.
SpacersSpacers are fluids of a controlled viscosity, density and gel strength that form a
buffer between the cement slurry and the drilling fluid. They help in the removalof drilling fluid during cementing operations.
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7.5 Mud Displacement EfficiencyMud displacement is much more complicated than mud circulation. In addition to the
parameters mentioned earlier, mud displacement is dependent upon the relative propert ies of the fluids involved (density and rheology), their relative flow regimes,and their eventual interaction when mixed together.
7.5.1 Displacement of Mud in Concentric AnnuliOne of the first parameters found to have an influence on mud displacementefficiency is the flow regime of the displacing fluid. In laminar flow, fluid velocity ismuch higher near the centre of the annular space than near the walls ( Fig. 7.7 ) and,hence, fluid in mid-stream is always bypassing the fluid near the boundaries. In aturbulent flow, on the other hand, the profile of fluid velocity is more uniform acrossthe annular space with the fluid swirling in a continuous state of instability. If theattainment of turbulent flow is achieved, the potential for “viscous fingering”, inwhich the displacing fluid fingers through the mud leaving channel, is minimized.
Fig. 7.7 - Velocity profiles for laminar annular flow 2.
From pilot-scale studies, Howard and Clark (1948) concluded that when the Reynoldsnumber of the cement slurry was low only 60% of the “circulatable” mud weredisplaced, whereas 90 to 95% could be displaced when the cement slurry was in theupper laminar or turbulent-flow regime. This issue has subsequently been raised by
several authors, but there is still no consensus today concerning the best displacementregime for optimum mud removal.
However, the choice of the proper displacement regime cannot be made outside thegeneral context of the primary cementing job. Hole and pipe sizes, fluid densities andrheological properties, and operational constraints must be taken into account whendesigning a cementing job for optimum mud removal. For example, the technique of turbulent-flow displacement may prove to be impractical or impossible in cases like:
• For weighted displacing fluids, the critical pumping rate to achieve turbulentflow may exceed the capabilities of available equipment.
• The pressures associated with high displacement rates may be well abovefracture gradients of weak formations.
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q L AMINAR -FLOW DISPLACEMENTEvery thing being equal, and at least at low flow rates, the displacement of a densefluid by a lighter one leads to an unstable phenomenon known as buoyant plume .Conversely, when the displacing fluid is heavier than the displaced fluid, buoyantforces tend to flatten the interface and promote efficient displacement.
Differences in rheological properties are also likely to play a role in this process.Everything else being equal, the laminar-flow displacement of a “thin” fluid by a“thick” one will always be more efficient than the reverse situation, which isknown to give rise to an unstable interface.
q T URBULENT -FLOW DISPLACEMENTTurbulent-flow displacement technique becomes commonplace in the 1960s withthe introduction of cement formulations that allowed turbulence at achievable
pump rates. In 1964, Brine and Holmes published the results of a surveyconcerning 46 cement jobs performed in the southwest Louisiana, an area that wasnotorious for primary-cementing failures. They indicated the need for turbulentflow, and suggested that the annular space should be in contact with the turbulentdisplacing fluid for a sufficient time.
Today some claim that a four-minute contact time is sufficient, while others claimthat 10 minutes are necessary. However, such an assumption would ignore other important factors such as casing movement and centralization. Even though theturbulent-flow technique has improved the success rate of cement jobs in someareas the basic fundamentals underlying this practice are not well understood.
Good mud displacement can often be obtained with a low-viscosity, unweightedfluid such as water, diesel oil or a chemical wash. Although the stresses generated
by such fluids are extremely low, even at a very high Reynolds number, their ability to displace mud supports the following mechanism – turbulent eddies in thedisplacing fluid cause a drag/erosion/dilution process at the mud/displacing fluidinterface. For weighted turbulent fluids (spacers or scavenger slurries) which aremore viscous, the intensity of turbulence is smaller, but the turbulent stresses are
bound to be much higher, and the erosion of the mud may be enhanced by the presence of solid particles.
Generally speaking, gravitational forces are not important when displacing in
turbulent flow. This may be attributed to the fact that, in the absence of densitydifference, the interface between the fluids is already “flat”. Therefore, when thedisplacing fluid is heavier, gravitational forces cannot greatly enhance thedisplacement efficiency. On the other hand, good displacement efficiencies can beobtained with chemical washes which are up to 4 lb/gal lighter than the mud. Suchresults imply that unstable density differences can be countered by the turbulenceof a wash.
For the turbulent-flow displacement technique to be successful, several criteriamust be met:
• The displacing fluid must be sufficiently thin for the critical pumping rateto be achievable with field equipment.
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• The displacing fluid must exhibit excellent fluid-loss properties, especiallywhen its solid-to-liquid ratio is high; otherwise, losses of the base fluid(water or oil) may increase the viscosity, and raise the critical pumpingrate for turbulent flow beyond the capabilities of the field equipment.
• The chemical and physical properties of the displacing fluid must be
carefully designed. It is of utmost importance for the displacing fluid to befully compatible with the mud. In addition, a weighted displacing fluidmust be able to suspend the solids required to achieve the designeddensity.
7.5.2 Removal of Mud in Eccentric AnnuliIn highly-deviated or horizontal wells, the casing tend to lie on the low side of thehole. If the casing is poorly centralized, cement slurry will tend to favour the path of least resistance, which is in the wide side of the annulus where wall friction isminimum, creating severe mud channeling on the narrow side as shown in Fig. 7.8 .
Fig. 7.8 - Mud channeling in eccentric annulus.
There are two major schools of thought on how to improve mud displacementefficiency in eccentric annuli.
• The yield point, density and eventual plastic viscosity of the displacing fluidshould be higher than the corresponding properties of the displaced fluid.
• Based on the effects of turbulent flow or partially turbulent flow, thindisplacing fluids should be pumped at a rate such that at least partial turbulentflow is obtained.
With few exceptions (e.g., in the absence of density differences), theoreticalapproaches tend to favour the first approach. This is not surprising, because mostmodels did not take into account the mechanisms which are known to underlay theturbulent-flow technique – erosion and dilution. On the other hand, the experimentalstudies agree with one or the other, the great majority supporting the second approach.
