D R I L L I N G A N D C O M P L E T I O N F L U I D S

36 NOVEMBER 1999 •

Previous rheological studies ofwater-based muds (WBM’s) and oil-based muds (OBM’s) concentratedon fluid viscosities at elevated tem-peratures and pressures. To evaluatethe impact of cold-temperature rhe-ology on the equivalent-circulating-density (ECD) calculations in deep-water wells, three objectives were setfor the present study.

• Extend the available rheologicaldata for the most common drilling-fluid systems to low-temperatureenvironments.

• Examine the rheograms and de-termine the best model to fit the data.

• Evaluate the effect of low-tem-perature drilling-fluid rheology onthe ECD for deepwater applications.

FLUID RHEOLOGYFluids that have a viscosity dependent onshear rate, such as drilling fluids, exhibitnon-Newtonian behavior that is difficultto describe with simple models. De-scription of two different non-Newtonianfluids may require use of two completelydifferent models.

The simplest model often used todescribe drilling-fluid flow properties is theBingham plastic model which describes thebehavior of a fluid that does not flow unlesssubmitted to a minimum stress (the yieldstress). The Bingham plastic model has beenfound to be an unrealistic description ofdrilling-fluid rheograms. More appropriatemodels are the Herschel-Bulkley model andCasson models. The Herschel-Bulkley mod-ifies the power-law model by introducing a

yield stress. The Casson model combines ayield stress with greater shear-thinningbehavior than the Bingham plastic model.These three models have been applied to thedata set reported in the full-length paper.The best data fit was computed for eachrheogram. A linear least-squares methodwas used for the Bingham plastic model,and nonlinear regression was used for theHerschel-Bulkley and Casson models.

EXPERIMENTAL METHODSFluid Selection. Drilling fluids selectedincluded many of the most commonly usedfluids, including OBM’s, synthetic-basedmuds (SBM’s), and WBM’s. Two fluid densi-ties were used for all drilling fluids:unweighted and 1.6 g/cm3. The base oilused for the OBM’s was a low-toxicity min-eral oil while the base for the SBM’s was alinear alpha olefin.

The fluids mixed and tested for each fluidsystem were typical formulations using stan-dard drilling-fluid chemicals. All fluids testedhad simulated drilled solids added. The OBMand SBM formulations included primary andsecondary emulsifiers, calcium chloridebrine, organoclay, gilsonite, and lime, withbarite as the weighting agent. The salt-poly-mer-based WBM used xanthan gum as theviscosifier and either starch or polyanioniccellulose (PAC) for fluid-loss control. Thebentonite-based WBM used bentonite as theviscosifier/fluid-loss additive with eitherstarch or PAC for additional fluid-loss control.

The fluids were formulated to achieve ayield point of 8.6 to 12.9 Pa, measured witha Fann 35 viscometer. After mixing, the mudwas hot-rolled at 79°C for 4 hours to aidhomogenization and additive yielding. Fluidviscosity then was measured by use of theFann 35 viscometer at 49°C to verify that theyield point was close to the specified yieldpoint. A Fann 70 viscometer was used forrheological measurements at the varioustemperature and pressure conditions.

Test Matrix. The conditions at which fluidviscosity was measured were the following.

• −1°C at 1 and 137.9 bar.• 5°C at 68.9 bar.• 20°C at 1 and 137.9 bar.

• 49°C at 1 bar.• 90°C at 344.7 bar.

Viscosity was measured first at −1°C andthen the fluid heated to 90°C and viscositymeasured. The temperature was reduced to−1°C. Viscosity measurements wererepeated at the various temperatures andpressures to determine whether there wasany hysteresis in the fluid behavior. Thistemperature cycling models drilling-fluidcirculation in deepwater wells.

RESULTSOBM/SBM. The data is best described by theHerschel-Bulkley and Casson models. Bothmodels show very good correlation for OBM’sand SBM’s at both densities. Low-temperatureeffects are quite pronounced on OBM andSBM viscosities. All the test fluids thickenconsiderably at low temperatures. The OBMexhibits the most pronounced increase in vis-cosity at low-temperature. In all cases, theincrease in pressure at various temperaturesincreased OBM and SBM viscosity, especiallyat higher shear rates. Pressure effects do notappear to be very dependent on temperature.A similar magnitude increase in viscosityoccurred at −1°C and 20°C for a pressureincrease from 1 to 137.9 bar.

WBM. The Herschel-Bulkley model provid-ed the best fit for the rheograms of bothunweighted and weighted salt-polymer-based fluids. There was no significant evi-dence of hysteresis in the salt-polymer-basedfluids when circulated from cold to hot andback to cold. For bentonite-based fluids, theHerschel-Bulkley model fit the rheogramsvery well, but the Casson model was a betterfit for the weighted fluids. Bentonite-basedmuds showed some viscosity hysteresis.Although the plastic viscosity wasn’t greatlydifferent, yield stresses were greater meas-ured on the cooling-down cycle after thefluid had been heated to 90°C. Viscosityincreases at cold temperatures were mostprominent at high shear rates. All WBM’sshowed a relatively larger viscosity increasefor unweighted muds than for weightedmuds. For the salt-polymer-based fluids, the

DRILLING-FLUID RHEOLOGY UNDERDEEPWATER DRILLING CONDITIONS

This article is a synopsis of paper SPE56632, “Rheology of Various Drilling-Fluid Systems Under Deepwater DrillingConditions and the Importance ofAccurate Predictions of Downhole FluidHydraulics,” by J.M. Davison, SPE, andS. Clary, SPE, Dowell; A. Saasen, SPE,Statoil; and M. Allouche, SPE, D.Bodin, SPE, and V-A. Nguyen, SPE,Dowell, originally presented at the1999 SPE Annual Technical Conferenceand Exhibition, Houston, 3–6 October. (To Page 39)

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3-cm core section was extruded and alsoplaced in a precleaned, prelabeled glass jar.The remainder of each core was discarded.Core samples for biological analysis weretreated similarly, except only the top 2 cm ofthe core was used. All glass jars were placedinside clean plastic bags and frozen. The full-length paper describes sample handling indetail. When sampling was complete, sam-ples were transported to the Geochemicaland Energy Research Group analytical labo-ratories at Texas A&M U. for analysis.

FIELD OBSERVATIONSVideo Inspection. No cuttings piles wereobserved during either field survey. There wasa thin veneer of cuttings dispersed over muchof the seafloor in a patch fashion. In someareas, the cuttings were as thick as 2 to 25 cm.The largest deposits of large chunk-like cut-tings were along the southwest transect.These likely would have been seafloor drilledduring the riserless-drilling phase. In the1997 survey, the sediment appeared dark,interspersed by whitish mats and small patch-es of an orange, mat-like gelatinous materialthat was dislodged easily from the bottom.During the 1998 survey, the seafloor wasmore uniform in color and texture, which was

not surprising because the last well had beendrilled just days before the survey.

In 1997, the videotransects covered 360m2, representing 90 m in each of the fourdirections. A total of 53 fish were seen, rep-resenting six taxa. Estimated fish densitiesalong each transect ranged from 1,333 to1,888 fish/ha. Crabs and eels also were seen.

In 1998, the videotransects covered a418-m2 area representing between 80 and122 m in each of the four directions. A totalof 62 fish were seen, representing seven dif-ferent taxa. Estimated fish density rangedfrom 875 fish/ha on the southwest transect to2,037 fish/ha on the northeast transect. Othermegafauna observed in the 1998 surveyincluded several unidentified shrimp and 9crabs. Eight of the nine crabs were found onthe southwest transect where seafloor topog-raphy was the most rugose.

Sediment Analysis. The 1997 data indicat-ed that, regardless of the deepwater currentsrunning toward the southwest, most of theSBF was observed on the northeast transect.This is explained by the stronger surface andmidlevel currents that flow primarily towardthe northeast. As the SBF was dispersedthrough 565 m of water, it was affected by

these higher-velocity, higher-level currentscausing the depositional pattern observed.The highest SBF concentration appears to bealong the northeast transect. Lower SBF con-centrations are seen along the other threetransects. Numbers and densities of bothmega- and macrofauna do not appear to beaffected negatively by the elevated SBF con-centration along the northeast transect.

In the 1998 survey, average SBF concentra-tion ranged from 49,000 ppm in the 2-cmcore sections of the northeast transect to2,000 ppm in the southwest transect samples.Average SBF concentration in the 2- to 5-cmcore sections ranged from 30,000 ppm in thenortheast transect to 1,000 ppm in the south-west-transect samples. There were 1,761macrofauna representing 8 taxa counted inthe northeast-transect core samples and 339macrofauna representing 12 taxa counted inthe southwest-transect core samples. Detaileddata analysis was not complete at the time thefull-length paper was written.

Please read the full-length paper foradditional detail, illustrations, and ref-erences. The paper from which thesynopsis has been taken has not beenpeer reviewed.

potassium formate and sodium silicate fluidsexhibited the most significant viscosityincrease. Fluids containing polyglycolshowed excellent tolerance of different tem-peratures. Increasing pressure at low temper-atures does not affect the salt-polymer-basedfluids significantly except for the weightedsodium silicate and potassium formate fluids,which exhibit a fluid-shear-stress increase.Bentonite-based fluids were not affected bypressure increases at low temperatures.

DEEPWATER ECDReliable ECD prediction in wells with sig-nificant changes in temperature and pres-sure regimes requires use of pressure- andtemperature-dependent rheology and den-sity. Rheologies measured during the exper-imental phase were incorporated into asoftware model that can predict ECD’s. Anaccurate transient-temperature profile mustbe computed to model heat transferbetween the wellbore and rock formationand the riser and sea. The advanced numer-ical simulator incorporated in the softwaremodel has been validated in a field applica-tion. The transient-temperature simulator

can be used to compute changing ECDwith time for a given fluid and to predictfluid temperature at any point in the wellduring circulation and noncirculation.

Three cases were examined. For Case 1,measured mud rheology representing tem-perature and pressure regimes in the wellprofile were used as model input. For Case2, mud rheology varied with temperatureand pressure but was based on laboratorymeasurements at 49°C and 1 bar. In Case 3,mud rheology was independent of temper-ature and pressure.

DEEPWATER APPLICATIONSSolids Control. The temperature simulatorcan predict fluid temperature as the drillingfluid exits the riser. This is an importantrequirement if OBM’s or SBM’s are to beprocessed correctly through shale shakersand other solids-control equipment.

Hydrate Stability. A temperature simula-tion was run with the salt/polyacrylamide/polyglycol fluid as an example. After 1,440minutes of circulation, a period of noncircu-lation was simulated to determine fluidcooling at the riser base. After a 3.5-dayperiod, temperature was 3°C, very close toseabed temperature. Predicted temperaturescan be used with mudline pressures to per-

mit correct fluid-chemistry engineering toinhibit gas-hydrate formation.

CONCLUSIONS1. The Herschel-Bulkley and Casson

models both accurately describe OBM andSBM rheology for temperature and pressureconditions used in this study.

2. The Casson model fit the weighted-WBM rheograms best.

3. In simulations of a deepwater-wellprofile for an SBM, ECD determined by amodel where the mud rheology is indepen-dent of temperature and pressure can beunderestimated by as much as 6.1% whencompared with the ECD determined by ahydraulic model that varies mud viscositywith pressure and temperature.

4. For a WBM, ECD determined by amodel where the mud rheology is indepen-dent of temperature and pressure can beoverestimated by as much as 3.1% whencompared with the ECD determined by ahydraulic model that allows mud viscosity tovary with pressure and temperature.

Please read the full-length paper foradditional detail, illustrations, and ref-erences. The paper from which thesynopsis has been taken has not beenpeer reviewed.

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