Improved drilling fluids advances operationsU N D E R S T A N D I N G A P H R O N S

APHRON DRILLING FLUIDS arebeing used globally to drill throughdepleted reservoirs and other underpres-sured zones. The primary features ofthese fluids are their unique low shearrheology and aphrons (speciallydesigned pressure-resistant microbub-bles of air). However, how aphron drillingfluids work is not well understood, whichlimits acceptance of this technology.Recently a study was undertaken underthe auspices of the US Department ofEnergy to gain understanding of aphrondrilling fluids. This presentation willshowcase results of the study.

Various laboratory techniques wereapplied to determine the physicochemi-cal properties of aphrons and other com-ponents in the fluid and how they affectflow through permeable and fracturedmedia. These included wettability andsurface tension, bubble stability, radialand dynamic flow visualization and fluiddisplacement tests.

One key discovery was that aphrons cansurvive compression to at least 4,000 psi,whereas conventional bubbles do notsurvive long past a few hundred psi.

When drilling fluid is lost into a loss zoneunder the drill bit, aphrons move fasterthan the surrounding liquid phase andquickly form a layer of bubbles at thefluid front. At the same time, the shearrate of the fluid is continually decreasingand the viscosity is rapidly rising. Thecombination of the bubble layer and therapidly increasing viscosity of the liquidseverely curtails fluid invasion.

Another key finding of the study is thataphrons show little affinity for eachother or for the mineral surfaces of thepore or fracture. Consequently, the sealthey form is soft, and their lack of adhe-sion enables them to be flushed out easi-ly during production.

Depleted wells, which are very expensiveto drill underbalanced or with otherremediation techniques, can now bedrilled overbalanced. This study has pro-vided a sound technical basis for the suc-cess of aphron drilling fluids and is pro-viding guidance on the way to run thesefluids in the field to optimize their per-formance.

Recent Advances in Aphron DrillingFluids (IADC/SPE 97982) FB Growcock,

M-I Swaco; A Belkin, MASI TechnologiesLLC; M Irving, B O’Connor, M Fosdick,M-I Swaco and MASI Technologies LLC;TL Hoff, MASI Technologies LLC.

C O N T I N U O U S C I R C U L A T I O N

A Continuous Circulation Valve (CCV)was developed for enabling drilling indepleted reservoirs at HP/HT fields inthe Norwegian sector of the North Sea.By utilizing a system to obtain circula-tion through the whole drilling operation,the downhole pressure will remain con-stant even during drillpipe connections.By balancing this downhole pressurebetween a maximum pore pressure and aminimum fracture pressure, drilling canbe performed properly even through nar-row drilling windows.

The valve is a two-position, three-wayball valve. It is possible to circulatethrough the valve from the top drivedown the drill string or through a sideport and down the drill string. Such avalve must be installed at the top of eachdrill pipe stand before the continuous cir-culation operation starts. When a con-nection is to be performed, a hose mustbe connected to the side inlet of thevalve, the flow from the mud pumps willthen be switched from the top inlet to theside inlet, and top drive can then be dis-connected and a new stand installed. Tocontinue drilling, the operation isreversed. The valve is designed to with-stand HP/HT pressures, including gas-filled casings, bull heading and maxi-

mum pressure dur-ing standard drillingoperation.

A prototype of thevalve has been pro-duced and operationtested on the full-scale research drill-ing rig Ullrigg in Sta-vanger, Norway. Thecurrent paper pres-ents the advantagesgained by using theCCVs in a drillingoperation. The for-mation types mostvulnerable for drill-ing problems whiledrilling depleted res-ervoirs are described.

Continuous Circulation DuringDrilling Utilizing a Drillstring Inte-grated Valve — The Continuous Circu-lation Valve (IADC/SPE 98947) ATorsvoll, K Bekkeheien, I Kjellevoll, Sta-toil; N Reimers, Sensor Link; P Horsrud,DH Breivik, Statoil.

M A N A G I N G B A R I T E S A G

Barite sag is a well-recognized but poor-ly understood phenomenon in the drillingindustry. Industry experts have offered avariety of measuring parameters basedon empirical data that only partially cor-relate with the occurrence of barite sag.

The development and introduction ofstandardized well-site techniques to pre-dict the onset of barite sag in dynamicflow has evaded the industry.

The effect of shear rate on dynamicbarite sag has been studied and quanti-fied using new and advanced technology.Changes in mud weight as a function ofshear rate, hole angle, annular velocityand eccentricity correlate with ultra-lowshear rate viscosity. Field-proven tech-nology has been developed to predict thepotential for barite sag and to provideremedial measures through ultra-lowshear rate viscosity modification.

This technology was originally developedand validated in the field with invertemulsion drilling fluids. Subsequent ver-ification and validation work showed thetechnology was equally valid for water-based drilling fluids. This innovativetechnology is well suited for use with a

IADC/SPE DRILLING CONFERENCE: FLUIDS I

97982: Although aphron drilling fluids are being used around the world,how they work is not well understood in the industry.

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variety of commercially available fieldviscometers and therefore lends itself towidespread industry use.

The paper will discuss the theoreticalbasis for this barite sag managementtechnology and present the viscosity lev-els with corresponding shear ratesrequired to manage barite sag. Case his-tories also will be presented.

Field Proven Technology To ManageDynamic Barite Sag (IADC/SPE 98167)WM Dye, GA Mullen, WJ Gusler, BakerHughes.

U T I L I Z I N G F O R M A T E B R I N E S

Formate brines have been successfullyused as essentially solids-free reservoirdrilling in and completion fluids in morethan 100 HP/HT wells over the past 10years. These have included 70 cases inwhich the high downhole pressures havenecessitated the use of cesium formatebrines for well-control purposes.

Out of these 70 cases of cesium formatebrine use in the field to date, 10 havebeen HP/HT reservoir drilling in andcompletion operations in high-anglewells where operators considered thatthe use of traditional fluids might com-promise project economics, well safetyand well integrity.

This paper looks in detail at the driversthat encouraged BP and Statoil toembark on the use of cesium formatebrines as their preferred HTHP drillingand completion fluids in the Devenick,Huldra, Kristin and Kvitebjorn fields.

The paper then reviews the technical andeconomic performance of the cesium for-mate brines in these drilling applicationsand evaluates whether the benefitspromised some 15 years ago have beenrealized.

Drilling and Completing DifficultHP/HT Wells With the Aid of CesiumFormate Brines — A PerformanceReview (IADC/SPE 99068) JD Downs,Cabot Corp.

S A N D - C O N T R O L T E C H N I Q U E S

A large majority of the recent deepwa-ter/subsea developments in West Africarequire sand-control applications. Open-hole gravel packing is the preferredsand-control technique adopted by mostoperators in this region. Since most ofthese reservoirs contain reactive shalestreaks, they require synthetic/oil-baseddrilling fluids.

Significant progress has been made inrecent years toward overcoming thechallenges associated with gravel-pack-ing of such wells, through new develop-ments in fluids, tools and techniques.These developments have resulted insuccessful gravel-packing of wells drilledwith oil-based fluids, which have yieldedwell productivities, which, in manyinstances, exceeded those completed inwater-based drilling fluid environments.Currently, techniques and fluid systemsare available for gravel-packing witheither a water-based or an oil-basedgravel-packing fluid.

Both approaches have been practiced invarious regions in the world. A commonand critical element of both approachesis the proper assessment of potentialplugging of the sand-control screens dur-ing installation and subsequent displace-ment processes, since the screen instal-lation often must be accomplished withsolids laden fluids (conditioned OBM) inthe wellbore, due to logistical challengesand economics.

An oil-based carrier fluid was recentlyused for openhole gravel packing of awell in Okpoho field, offshore Nigeria.The application resulted in the best pro-ducer in the field. The paper details thesteps taken through the design and exe-cution stages of the treatment, incorpo-rating the lessons learned from the twoprevious applications of the oil-basedcarrier fluid.

A detailed description of the yard testsconducted in Nigeria will be thoroughlydiscussed. A critical discussion of theonsite QA/QC procedures with screensamples is also included.

A simple approach is proposed for quan-tification of the impact of screen plug-ging test results on field practice. Thisallows the operators to evaluate howmuch, if any, screen plugging can beexpected with the particular conditionedOBM as the screens are run in hole andduring subsequent displacementprocesses, so that the operator candecide whether further conditioning(additional rig time) is worth the associ-ated cost.

Open Hole Gravel Packing with OilBased Fluids: Implementation of theLessons Learned from Past Experi-ences Leads to the Best Producer inOkpoho Field, Nigeria (IADC/SPE98151) EP Ofoh, ME Wariboko, NigerianPetroleum Development Co; F Uwaifo, MParlar, Schlumberger.

O V E R C O M I N G D E N S I T Y L I M I T S

Displacing drilling mud with clear solids-free completion brine is a critical stepduring well completion. Conventionalcleaning fluids use fresh water or seawa-ter treated with surfactants to removewellbore solids and water wet tubulars.Using low-density cleaning fluids createsa negative differential pressure betweenthe working kill weight fluid and the for-mation, casing and cement liners. Inmany situations, the negative differentialpressure cannot be tolerated and therisk of failure at the liner top, etc, isincreased, especially if the wellbore hasnot been pressure-integrity tested.

In order to overcome the density limita-tion of these cleaning fluids, convention-al techniques such as additionalhydraulic horsepower, back pressureschedules, the addition of solids tolighter cleaning fluids (water, seawater),or balancing the weight of the low densi-ty cleaning fluid with a matching higherdensity fluid is utilized. However, each ofthese “fixes” has inherent limitations.

New brine-compatible surfactant chem-istry and the corresponding balanceddisplacement engineering design weredeveloped to overcome limitation of con-ventional displacement technology.

This paper describes the field applica-tions of new brine-based, high-density,solids-free cleaning fluids in balanceddisplacements in both deepwater and off-shore shelf wells. The new high-densityfluids were based on new surfactanttechnology developed to ensure effectivewellbore cleaning, wellbore designparameters and displacement modeling.In addition, weighted spacers aid inreducing pump pressures with high dif-ferentials.

In one case history, maximum pumpingpressure of more than 9,000 psi isexpected for a conventional water-baseddisplacement while it was reduced to alittle over 3,000 psi with the new design.High-density cleaning fluids, with densi-ties up to and greater than 17.5 ppg, havebeen produced and utilized successfullywithout compromising cleanup efficiencyand significantly reducing differentialpressures. Results from laboratorydevelopment and field applications willbe presented.

Effective High Density Wellbore Clean-ing Fluids: Brine Based and SolidsFree (IADC/SPE 99158) PH Javora, GDBaccigalopi, Q Qu, BM Franklin, BJ Ser-vices Co. �

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