4 slurry design

School of Petroleum Engineering, UNSW Open Learning - 2000

4

Slurry Design

q Wellbore temperaturesq Retardationq Densityq Filtration controlq Strength stabilityq Viscosity/suspensionq Gas migration theory and controlq Cement job simulationq Waiting-on-cement time

Each well has certain basic characteristics that dictate the required cement slurryproperties and performance. Carefully and thoroughly reviewing these wellcharacteristics is essential for designing an effective slurry. The overall goal ofdesigning a cement slurry for a specific well application is to select an economicalcement mixture that can be placed under existing well conditions. This slurry shoulddevelop and retain the properties necessary to isolate zones as well as support andprotect the casing or liner.

Designers should combine examinations of individual cementing variables to developa total cement job design. For example, to properly design cement slurries for hostilegas wells, designers must thoroughly understand the mechanism that causes loss ofhydrostatic head in a cement column. In addition, they must carefully design for fluid-loss control, cement stability and setting behaviour, as well as examine mudconditioning and spacers.

Unless proper zonal isolation is achieved, it will be impossible to independentlyproduce from the different reservoirs the well penetrates. In the case of faulty zonalisolation, it will be impossible to perform chemical treatments in the necessaryintervals. Faulty zonal isolation will also result in the fluid migration in the annulus.The remedial cementing necessary to correct uncontrolled fluid flow behind casing istime-consuming and expensive. Remedial cementing also weakens the integrity of thecasing.

This chapter examines the factors involved in designing a cement slurry, especially,for high-pressure/high temperature (HP/HT) wells.

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4.1 Wellbore TemperaturesThe wellbore temperature can affect the chemical reaction and performance of acement slurry. Circulating and static temperatures both affect cement design.Although static temperatures affect the cements curing properties, circulationtemperature has an even greater influence.

q BOTTOMHOLE CIRCULATING TEMPERATURE (BHCT)BHCT influences the thickening time or pumpability of the cement slurry. TheBCHT is normally calculated from a set of temperature schedules published inAPI Specification 10 (an example of API schedules is shown in Table 4.1. Theschedules are based on bottomhole temperatures, oF = 80oF + 0.015 x depth in feet(see Fig. 4.1).

Table 4.1 - Casing cement specification test schedules. (API RP 10, 1984)

Fig. 4.1 - Average temperature of U.S. gulf coast wells. (API RP 10, 1984)

Since accurately estimating BHCT is essential, designers should not rely only onthe API schedules alone when cementing deep wells. Temperatures should beverified by some form of actual downhole measurement, preferably during thecirculation phase. From such data the relationship of bottomhole statictemperatures (BHSTs) vs. BHCTs can be obtained to determine the pumpabilityof a cement slurry.



q BOTTOMHOLE STATIC TEMPERATURE (BHST)Knowing the BHST is important for designing and assessing a cement slurrysstability or its rate of compressive strength development. This is especially true indeep-well cementing, where the temperature differential between the top andbottom of the cement can be high. For example, cement slurries that are designedfor safe placement times may be over-retarded at top-of-cement (TOC)temperatures, resulting in poor compressive strength development. Generally, ifthe static temperature at the top of the cement column exceeds the BHCT, over-retardation is not expected.

Precise temperature readings are essential for cementing purposes. An error assmall as 5 to 100F can significantly affect results. Estimates of BHSTs may beobtained from surveys run during logging and from drillstem tests. Generally,cement sensitivity increases as BHCT increases. As a result, all laboratory testsperformed to improve slurry properties should be run using samples of the samebatch of cement, mixing water and chemical additives that will be used during thejob. Downhole conditions must be duplicated as closely as possible.

4.2 RetardationThe high temperatures in deep wells can cause cement to set prematurely unless theslurry has been properly retarded. In addition, the pressure imposed on the cementslurry by the hydrostatic load of well fluids tends to accelerate dehydration, therebyreducing the cements pumpability.

The cement design must contain just enough retarder to delay the slurry from settingand long enough to place the slurry downhole. To prevent under- or over-retarding theslurry, an accurate BHCT must be obtained for laboratory testing. Specific thickeningtime recommendations depend on largely on the type of job, the well conditions andthe volumes of cement being pumped. While temperature and pressure both influencecement setting, pump rate, casing size and depth control the placement time.

Generally, the cement thickening time for a HP/HT well should be approximatelydouble the placement time. The slurry should be designed so that the viscosity of thecement stays reasonably low during three-fourths of the thickening time test. If theviscosity increases prematurely during placement, leading to increases in pumpingpressure, the resulting high friction pressures could possibly fracture the formation(Fig. 4.2).

Fig. 4.2 - Thickening time behaviour in deep wells1.



4.3 DensityTo cement across high-pressure gas zones, a heavyweight mud system must be usedto control gas until the well can be cased off and cemented. These high-pressure gassections require mud weights up to 18 lb/gal. With the exception of a squeeze design,cement slurry designs should always have a greater density than the mud system,unless lost circulation is a concern. Even then, cement density should never be lowerthan drilling fluid density. Higher densities will increase the overbalance pressureadjacent to the gas zone, which can help prevent gas migration. Ideally, cementshould be at least 1 lb/gal heavier than drilling fluid density.

For proper cement performance, a slurry must be maintained at its correct designdensity. Since heavyweight slurries are often fairly viscous when mixed at surfaceconditions, they tend to entrain air, especially when mixed with seawater. This canmake it difficult to achieve the necessary slurry density unless proper anti-foamagents are used. Using a pressurized mud scale can help ensure that the cement slurrypumped downhole has the proper density. Properties that have the greatest effect onthe density are slurry volume, flow characteristics, thickening time, free water,suspension and fluid-loss control. Any one of these characteristics can affect theresults of a primary cement job.

Another factor involves complex slurry designs that are often used in high-temperature applications such as liner jobs. These slurries are often susceptible tosolids segregation because they commonly use dispersing additives such as polymersor lignosulfate-based retarders. Since these additives tend to reduce the slurrys low-shear rheology, yield point and static gel strength development, they increase thepotential for solids segregation. This effect is commonly offset by using viscosifyingfluid-loss additives such as cellulose derivatives.

4.4 Filtration ControlIn any deep-well cementing job, the cement slurry should contain fluid-loss additivesthat perform well at high temperatures. Fluid-loss additives help maintain designedslurry properties by preventing cement dehydration.

Good fluid-loss control is also necessary for successful squeeze cementing. In thisapplication, uncontrolled fluid loss can result in rapid cement dehydration, which canbridge off the wellbore before the entire zone of interest is squeezed (Fig. 4.3). If thefluid loss is controlled, cement can contact the entire interval, allowing small nodes ofdehydrated cement to build up across permeable areas of interest before squeezepressure develops (Fig. 4.4).



Fig. 4.3 - Uncontrolled fluid loss resulting in rapid cement dehydration1.

CEMENTFILTER CAKE

1000cc/30min

3000cc/30min

150cc/30min

25cc/30min

FILTER CAKE BUILDUP INSIDE CASING

SQUEEZE CEMENTING

Fig. 4.4 - Cement node buildup.



4.5 Strength StabilityMost API Classes of cement reach maximum strength near 2300F. Thereafter, thestrength decreases as temperature increases. This phenomenon, known as compressivestrength retrogression, takes place in unprotected cement and can occur gradually orquite rapidly. Retrogression causes permeability to increase, which causes the cementto become more susceptible to further degradation from the surrounding environment.As a result, cement jobs performed at high temperatures need special attention toachieve long-term casing support and zonal isolation. Even if static temperatures upthe hole are not 2300F, using a strength-stabilizing additive should still be considered.The need for strength stability in these cases depends on the temperature of theproducing zone and the heat transfer potential of the production casing.

To inhibit loss of cement strength where formation temperatures are between 230 and7000F, a cement design should contain at least 35% crystalline silica. Coarse silica,however, will not prevent strength retrogression in salt cement slurries subjected to atemperature of 3000F or higher.

4.6 Viscosity / SuspensionIn primary cementing, a cement slurry should have a reasonable viscosity duringmixing operations and should maintain good suspension properties under downholeconditions. Suspension properties are important, since solid segregation causes fluidto migrate upward through the slurry after it is placed in the wellbore, creating anuncemented area in the annulus. Free fluid is particularly harmful in deviated orhorizontal wells where segregation/separation is more likely. This free fluid cancollect along the high side of the annulus and form a channel that contributes to zonalcommunication or gas migration. Even in vertical wells, pockets of free fluid locatednear a corrosive water zone can eventually result in casing leaks. Therefore, it isimportant to design slurries that are stable under downhole conditions.

It is more difficult to formulate a cement design with good suspension properties forhigh temperature conditions. The downhole thermal thinning effect makes it difficultto design a slurry with a reasonable viscosity during mixing that will still have enoughsuspension properties downhole to prevent solids segregation (Fig. 4.5).

Fig. 4.5 - Thermal thinning effect.



When a cement design has settling problems that result in upward migration of freefluid, the slurrys low shear rheology, yield point, and/or static gel strengthdevelopment must be increased. Many different additives can be used to do this. Anymaterial that viscosifies a cement design will improve the cements ability to suspendsolids and hinder fluid from migrating upward through the slurry.

4.7 Gas Migration Theory and ControlHalliburton Research has identified two primary types of gas migration: short-termand long-term gas migration. Short-term migration occurs before the cement sets,and long-term migration develops after the cement has set.

q SHORT-TERM GAS MIGRATIONThe most widely accepted cause for gas migration through unset cement is that thecement is unable to maintain overbalance pressure (Fig. 4.6). Initially, after thecement slurry is placed downhole, it behaves as a fluid and fully transmitshydrostatic pressure to the gas-bearing formation. This overbalance pressureprevents gas from percolating through the cement slurry (Fig. 4.7-A).

CEMENT FLUID

CEMENTGELS

CEMENTSETS

CEMENTHARDENS

TIME

C

B

A

OVERBALANCEPRESSURE

FORMATION GAS PRESSURE

Fig. 4.6 - Maintaining overbalance pressure.

Sometime after the cement slurry is placed in the annulus, it will begin to developgel strength (Fig. 4.7-B). Gelation causes the cement to become increasinglycapable of supporting its own weight, reducing the columns ability to transmithydrostatic pressure to the gas zone. As this is occurring, the cement loses filtrateto permeable formations, causing a loss of overbalance pressure, which thenallows gas to enter the annulus and percolate through the gelled cement (Fig. 4.7-C). If gas begins to migrate, it will continue to percolate at a rate proportional tothe volume reductions that are occurring in the slurry, until the cement hasdeveloped enough gel strength to prevent further percolation (Fig. 4.7-D).



PERMEABLEZONE

GAS ZONE

FILTRATELOSS

FILTRATELOSS

FILTRATELOSS

A B

C D

Fig. 4.7 - How a gas channel is formed.

Gas migration can be controlled by adjusting cement column length to increasehydrostatic pressure, and/or using fluid-loss control additives, delayed gellingagents or special cements.

q LONG-TERM GAS MIGRATIONLong-term gas leakage occurs sometime after the cement job was performed andconsidered successful. As with short-term gas migration, the best method ofeliminating long-term gas migration is squeeze cementing. However, carefullydesigning the cement slurry, planning the job and using specific cement additives,particularly expansion additives can help prevent long-term gas migration.

Long-term gas migration is indicated by gas flow at the surface through theannulus, sometimes as early as few weeks after the cement job was performed. Anoise log is probably the most reliable method of locating the source of theproblem. Cement bond logs (CBL) may not be sensitive enough to detect adiscontinuity in the cement sheath.

There are two suspected causes of long-term gas migration: (1) inadequate drillingfluid displacement and (2) the cement debonding from the casing after setting.



Incomplete displacement or excessive filter-cake buildup can also create drillingfluid channels in the cement. As time passes, the drilling fluid and cake dehydrateand shrink due to gas flow, resulting in a highly permeable pathway for gasmigration.

The second suspected cause is the cement separating from the casing after it hasset. One reason for this debonding is that the casing diameter changes after thecement has set because of pressure or temperature changes during workovers orstimulation treatments. The resulting long-term gas migration occurs through thediscontinuity in the cement sheath either through (1) micro-flow channels indrilling fluid or (2) through microannuli between the pipe and cement or betweenthe formation and cement.

This problem can be prevented by the following two methods: (1) focusingimproving on drilling fluid displacement principles and/or (2) using expandedcement compositions (such as ettringite-base cement systems: Type K, M and Scements Fig. 4.8).

0 30252015105

0.05

0.10

0.15

Line

ar E

xpan

sion

(%)

0.25

0.20

Neat PortlandCement (15.8 lb/gal)

Ettringite-Base ExpansiveCement System (14.8 lb/gal)

Time (days)

Fig. 4.8 - Comparison of expansion between neat Portland cementand ettringite-base expansive cement system.

Often, improving fluid displacement efficiency has helped to stop gas migration.Drilling fluid conditioning, pipe centralization and the correct application ofspacers/flushses will achieve better displacement efficiency. Pipe movement withscratchers/wall-cleaners attached and high pump rates are also helpful.

If long-term migration is still known to occur after all these methods are used,expansive cement compositions can help correct the conditions that cause long-term gas migration. Expansive cements have been used successfully to producebetter cement bonding. The two general types of cement expansions are plasticstate expansion, which occurs before the cement completes its initial set; andchemical expansion, which occurs after initial set.

Plastic state expansion additives such as Halliburtons SUPER CBL blend provideexpansion by internal generating a chemical reaction that creates a gaseousdispersion throughout the cement matrix before the cement sets.



4.8 Cement Job SimulationThere are commercial software, developed by service companies, which can simulatethe effects of various cement job design parameters prior to the actual cement job.This helps identify potential problems before pumping starts and allows operators tomake appropriate modifications.

An example of cement simulator is the Halliburtons CJOBSIM (Figs. 4.9-4.11).CJOBSIM can be used to improve pump rates for maximum mud displacement bydesigning the highest allowable pump rates, without exceeding the fracture gradient.It can also predict circulating pressures at any specific time during the job, evenduring free-fall, when the well is on vacuum and the surface pressure is zero.

Fig. 4.9 - CJOBSIM well schematic1.

Fig. 4.10 - CJOBSIM rates graph1.



Fig. 4.11 - CJOBSIM pressure and density graph1.

The use of a simulator allows operators to evaluate job results by comparing the pre-job simulation to onsite recorded job data. This helps operators improve futuredesigns, or analyze and pinpoint the probable cause of a problem job.

4.9 Waiting-on-Cement TimeOnce the cement slurry has been placed downhole, it is important to know how longto wait before performing additional work on the well. To determine this waiting-on-cement (WOC) time, we must understand the strength requirements needed for thecement to perform specific tasks. The following strength recommendations can beused to help make this decision:

Pipe support and zonal isolation: 100 psiDrilling out: 500 psiPerforating: bullets 500 psi

hollow carrier or expendable jets > 2000 psiWhipstock plug: > 2500 psi (or harder than formation)

WOC time is determined by measuring the compressive strength development of theslurry. During this test, the slurry sample should be cured at the temperature adjacentto the zone of interest. For example, on a liner job, it is important that the cementplaced across the liner lap is allowed to set before drilling operations proceeds. As aresult, a laboratory sample of the cement should be cured at static temperatureadjacent to the liner top.

Since the dehydrated filter cake will develop more strength than a slurry that has notlost fluid under pressure, compressive strength tests are not applicable to squeezejobs. Commonly, dehydrated filter cake (nodes in perforations) will develop strength



of several thousand psi in the first 8 hours. Therefore, a waiting period of 4 to 12hours is generally recommended on squeeze jobs. Washing or flushing between stagescan damage squeezed zones if they are agitated or disturbed less than 4 hours aftersqueezing.

REFERENCES

1. Offshore Systems High Pressure/Temperature Cementing. Halliburton.



REVIEW QUESTIONS

1. Why accurate knowledge of wellbore temperatures is important to cementing?

2. When and why do you use fluid-loss additives?

3. What is compressive strength retrogression? What effect does this phenomenon have oncement?

4. In deviated and horizontal wells, how do you prevent solid segregation in cement?

5. Explain the cause of short-term migration.

6. How do you tell when long-term gas migration has occurred? How do you locate thesource of the problem?

7. What are the cement strength requirements for pipe support and zonal isolation, drillingout, perforating, and whipstock plug?

4 slurry design

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