Download pdf - 04_4- Spacer Design Considerations.ppt

8/11/2019 04_4- Spacer Design Considerations.ppt

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Spacer DesignConsiderations andLaboratory Testing


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Beirute Consulting, L.L.C. 2

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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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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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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Compare the measured rheology of the mixtures to therheologies of the spacer and the mud.

If the rheology of the mixtures is substantially higher thanthat of any of the two fluids, the two fluids should beconsidered incompatible.

Repeat all of the previous steps with the spacer and the

cement slurry.


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Wettability of Spacers and Flushes

When an oil-based drilling fluid is being used, whendisplacing the oil-based drilling fluid, thepipe/formation must be changed from an oil-wet to a water-wet condition so that the cementcan bond properly.

With oil (or synthetic oil) based drilling fluids, awater-based spacer with water-wetter surfactantsneeds to be used.

Some oil-based spacer/flush may be used ahead ofthe water-based spacer (oil-based spacers arenaturally compatible with oil-based drilling fluids).When a water-based drilling fluid is being used, thespacer system should also be water-based.


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Wettability Test

Spacers/Flushes (cont.) A simple test can be performed to determine if the

surfactant concentration is adequate for completewater-wetting of the pipe and formation:

Condition and heat the fluids as for thecompatibility screening test.

Insert a clean metal rod into the beaker of oil-

based mud (heated to BHCT or 190 F) and agitatefor 10 min to ensure that the rod surface becomesoil-wet.


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Wettability TestSpacers/Flushes (cont.)

Place the rod under a slowly dripping water tap to observethe beading associated with oil -wet surfaces.

Insert the rod into the beaker of spacer (heated to BHCTor 190 F) and agitate for 5 min.

Again place the rod under a slowly dripping water tap andobserve for adhesion of the water to the surface of the rod(no beading) which indicates that it has been renderedwater-wet by the spacer.

The spacer or flush must pass this test if the mud is oil-based.


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Spacers Erodibility andPressure Drop Hierarchy

Computer simulation:

Given mud erodibility, design spacer propertiesfor PDG removal Design for positive pressure drop hierarchy


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Spacer and Preflush

Volume Optimization Go to that spreadsheet