04_4- Spacer Design

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    Spacer DesignConsiderations andLaboratory Testing

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    General Observations

    Spacers are difficult systems to design. Theymust be compatible with two other fluids, muds andcement slurries, which are in general very incompatible.

    Spacers need to be designed to chemically work with

    muds and cement slurries. Since every mud and cement slurry is different, spacers

    should be essentially custom designed for every job. It should NEVER be assumed that a spacer

    formulation which worked with a previous fieldmud will also work with the present field mud,even if similar mud additives are being used.This is pa

    rticularly important when designingspacers for oil-based mud systems . Test in thelab!

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    Design Criteria

    The spacer must be compatible with both the drillingmud and the cement slurry.

    The spacer must be non-settling (use the suggested

    procedures to test for static and dynamic settling). The spacer should have (if possible) a density between

    that of the drilling mud and the cement slurry. If possible, the spacer should have a consistency

    between that of the drilling mud and the cement slurry.But it is MUCH better to design for pressure drophierarchy.

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    Design Criteria (cont.) When using oil-based muds, the spacer must contain surfactants for water-wetting the pipe

    and formation face, to enable the cement to bondeffectively to those surfaces.

    If needed for formation compatibility , the spacermay need the addition of KCl to minimizeformation damage, help stabilize shales, etc. 2 -3% by weight is normally used in these cases.

    For best results, the spacer should be pumped atthe rates indicated by the erodibility technology .

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    Design Criteria (cont.)

    Enough spacer volume should be pumped toachieve a minimum of 10 min contact time at thetop of the pay, or to fill 800 to 1,000 ft of annulus,

    whichever produces the greater volume. The spacer should possess good fluid loss control.

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    Design Criteria (cont.)

    Without a positive barrier (plug) between thefluids, mixing takes place in the casing particularlybetween the preflush and the spacer (big densitydifference). This is worst for long and large IDcasing. Worst for vertical holes.

    By the way, in cases when only one bottom plug isused, it must always be placed in front of theslurry, regardless of what fluids are beingpumped ahead of the slurry. That is the fluidwe must protect from contamination. I knowyou have been told other things. They are wrong!

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    Design Criteria in Highly Inclinedand /or Horizontal Holes

    In the annulus, thinning of the mud by thepreflush may cause settling. This is particularlybad in horizontal holes since the solid particlesonly have to travel a few inches to get to the lowside of the hole.

    I do not like to use preflushes in highly deviated orhorizontal holes for that reason.

    In this situations the preflush may damage thesolids suspending properties of the mud and thespacer.

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    Compatibility

    Two fluids are considered to be compatible if no significantchange in desired properties occurs as a result of mixingthe fluids.

    Compatibility testing is time consuming! Compatibility tests must be performed between the spacer

    and the field mud, and the spacer and the cement slurry tobe used in the job. It is also necessary to investigate thecompatibility of the three fluids together, becauseproblems may develop downhole if the three fluids come incontact (channeling, bypassing).

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    Compatibility (cont.)

    Appendix P of API SPEC 10, entitled Investigation ofPreflushes and Spacers for Cementing, provides detailson tests which can be performed to determinecompatibilities.

    Tests are also outlined to check the effect of spacers onthe cement slurry thickening time, compressive strength,fluid loss and settling characteristics. Approximately 5-10% spacer contamination into the cement slurry isnormally used for those tests.

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    APIRecommended

    Mixtures toTest for

    Compatibility

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    Other Tests Use the selected mixtures detected during the

    Screening Procedure Thickening time - Look for no acceleration

    and/or excessive gelation (high BC readings)

    Compressive Strength - Look for excessivenegative effect on CS development Settling - Look for excessive settling at the

    interfaces . Use static or dynamic procedureas applicable

    Fluid Loss - Look for possible severeloss of control at the interfaces.

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    Impact of Spacers and Preflusheson Cement Slurry Properties

    Regarding compressive strength of the cement slurry,spacers are expected to negatively affect it because ofdilution and chemical effects.

    This is not considered a serious problem since the

    contaminated interface spacer-cement slurry will bepumped above the zone of interest. Some retardation of the slurry by the spacer is also

    acceptable based on the same argument. However, the spacers should not accelerate the cement

    slurry for obvious reasons. In addition, the spacer should not severely affect the fluid

    loss control properties of the mud and the cement slurry,particularly across high permeability zones.

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    Other Compatibility Tests Some additional testing may be required for special

    situations. When the BHCT is above 250 F, it isrecommended that a contaminated thickening time test berun.

    Test the proposed cement slurry with 5-10% spacercontamination to see if the thickening time is shortened.

    If it is shortened, then add retarder to the spacer toprevent the reduction on thickening time.

    Normally, when cement slurry is diluted/contaminated withanother fluid, it will lengthen the thickening time.

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    Other Compatibility Tests

    (cont.) At elevated temperatures, however, with cement slurriescontain large amounts of retarder, contamination with afluid with no retarder in the water phase may dilute the

    retarder concentration sufficiently to compensate for thedilution effect of the slurry and actually shorten thethickening time.

    If retarder is added, check the spacer again to ensure that

    it has sufficient suspension properties because cementretarders typically have a thinning effect on spacer fluids.

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    A Simple CompatibilityScreening Test

    Conduct these tests first: Condition approximately 300 ml mud and 300 ml spacer to

    BHCT or 190 F, whichever is less, in an atmosphericconsistometer for 20 minutes.

    Transfer 50 ml each of the heated mud and spacer to 100ml beakers.

    Slowly add the spacer into the beaker containing the mudwhile stirring the mud by hand with a glass rod.

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    A Simple CompatibilityScreening Test

    If at any point during the mixing test, the mixture of thetwo fluids does not drip from the rod on its own (pick upthe rod occasionally while stirring), the two fluids shouldbe considered incompatible and the test should beterminated.

    If the mixture remains fluid (passes the drip test), then

    look (visually) for any significant viscosity change whichindicates possible incompatibility. If a significant viscosityincrease is noted, record the volume of the spacer used.

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    A Simple CompatibilityScreening Test

    Repeat the procedure by slowly pouring the mud intothe beaker containing the spacer.

    If the mixture passes the drip test, again note thevolumes of mud and spacer at which significantviscosity changes occurred. If no significant viscositychanges were noticed, the two fluids are compatible,

    and the test is terminated. If significant viscosity changes were noticed,

    measure the rheology at BHCT conditions or 190 F ofthe mud/spacer mixtures that showed the highest

    viscosities.

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    A Simple CompatibilityScreening Test

    Compare the measured rheology of the mixtures to therheologies of th

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