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Many oil and gas reservoirs in theUnited States are mature andbecoming increasingly depleted of

hydrocarbons, which makes for more costlydrilling. While the formations above andbelow these producing zones typically havemuch higher pore pressures and require highfluid density to stabilize them, exposure of adepleted zone to this high-density fluid canresult in significant loss of whole drillingfluid and differential sticking. Uncontrollabledrilling fluid losses are at times unavoidablein the often-large fractures characteristic ofthese formations. Furthermore, pressured

shales often are found interbedded withdepleted sands, thus requiring stabilizationof multiple pressure sequences with a singledrilling fluid. Drilling such zones safely andinexpensively is difficult with conventionalrig equipment. Preventive measures withnormal- or high-density fluids generallyentail use of a low concentration of a plug-ging agent in the circulating system or reme-diation when the rate of loss of drilling fluidexceeds some threshold level. The latterrequires injection downhole of a pill—a 50-bbl to 100-bbl slug of fluid—that contains ahigh concentration of a plugging agent or a

settable/cross-linkable fluid. None of thesemeasures is completely satisfactory.

An increasingly popular alternative fordrilling depleted or multiple pressure zonesis the use of a fluid that has a density lowenough to balance the pore pressure in thelowest-pressure zone. However, this resultsin drilling the zones above and below thedepleted zone “underbalanced,” a conditionthat risks wellbore collapse and blowouts. Anew drilling fluid technology was developedrecently that does not entail drilling under-balanced, yet is designed to mitigate loss offluid and differential sticking. This novel

By Fred Growcock,MASI Technologies

LLCEnhanced Wellbore Stabilizationand Reservoir Productivity withAphron Drilling Fluid TechnologyLaboratory research is required to provide understanding and validation of aphron drilling fluid technol-ogy as a viable and cost-effective alternative to underbalanced drilling of depleted oil and gas reservoirs.

Figure 1. Aphrons can survive pressurization to at least 3,000 psig. Note that images 2 through 4 are magnified to 40x.

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technology is based, in part, on the use ofuniquely structured micro-bubbles of aircalled “aphrons.”

Because of concerns about corrosion andwell control, drillers generally discourageentrainment of air in drilling fluids; indeed,they often go to substantial lengths to elim-inate air altogether from drilling fluids.Consequently, the purposeful incorporationof air, as in aphron drilling fluids, is lookedon with some apprehension. Furthermore,there are no controlled studies that havebeen able to shed much light on the mecha-nism or the effectiveness of the aphrons atdownhole pressures. This project was devel-oped to provide the much-needed validationdata that would put such issues to rest,thereby leading to greater application ofaphron drilling fluid technology. Withwider use of aphron technology, clients willreduce their drilling costs significantly andbe able to pass some of those savings on toAmerican consumers.

MASI Technologies, funded in part by theU. S. Department of Energy’s Office of FossilEnergy, is carrying out a study with threeobjectives: develop a comprehensive under-standing of how aphrons behave at elevatedpressures and temperatures; measure the abil-ity of aphron drilling fluids to seal permeableand fractured formations under simulateddownhole conditions and establish how thefluids control fluid invasion with minimal for-mation damage; and determine the role playedby each component of the drilling fluid.

BackgroundAphron drilling fluids have been used suc-cessfully to drill depleted reservoirs andother low-pressure formations in a largenumber of wells, particularly in North andSouth America. These novel fluids have twochief attributes that serve to minimize fluidinvasion and damage of producing forma-tions. First, the base fluid is very shear thin-ning and exhibits an extraordinarily highlow-shear-rate viscosity (LSRV); the unique

viscosity profile is thought to reduce the flowrate of the fluid dramatically upon entering aloss zone. Second, very tough and flexiblemicrobubbles are incorporated into the bulkfluid with conventional drilling fluid mixingequipment. These highly stabilized bubblesor aphrons are essential to sealing the prob-lem area and are thought to form a sealwithin the permeable or fractured formation.

Aphrons are made of a spherical gas core anda protective outer shell. Contrasting with aconventional air bubble, which is stabilizedby a surfactant monolayer, the outer shell ofthe aphron is thought to consist of a muchmore robust surfactant tri-layer. This tri-layer is envisioned as consisting of an innersurfactant film enveloped by a viscous waterlayer; overlaying this is a bi-layer of surfac-

Figure 2. Large aphrons surove pressurization from 500 psig to 2,000 psig, butsmaller aphrons (50 µm to 100 µm) do not.

Figure 3. Oxygen in aphron drilling fluids depletes rapidly.

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tants that provides rigid-ity and low permeabilityto the structure whileimparting some hydro-philic character to it.Under quiescent condi-tions, the structure iscompatible with theaqueous bulk fluid; how-ever, it is thought thatwhen enough shear orcompression is appliedto the aphron, for exam-ple, when bridging apore network, theaphron may shed its out-ermost shell layer, ren-dering the bubblehydrophobic.

Aphrons are claimedto act as a unique bridg-ing material, forming amicro-environment in apore network or fracture that appears tobehave in some ways like foam and in otherways like a solid, yet flexible, bridging mate-rial. As is the case with any bridging mater-ial, concentration and size of the aphrons arecritical to the drilling fluid’s ability to sealthief zones. Drilling fluid aphrons have coresof air and are constructed by entraining air inthe bulk fluid with standard drilling fluidmixing equipment; this reduces the safetyconcerns and costs associated with high-pressure hoses and compressors commonlyutilized in underbalanced air or foamdrilling. Although each application is cus-tomized to the individual operator’s needs,the drilling fluid system is generally designedto contain between 12 vol % and 15 vol % airat the surface (ambiant temperature andpressure), and the aphrons generated arethought to be sized or polished at the drillbitto less than 200 µm diameter, which is typi-cal of many bridging materials.

The project is divided into two phases.Phase I (year 1) dealt with developing evi-

dence for the ways in which aphrons behavedifferently from ordinary surfactant-stabi-lized bubbles, particularly how they seal per-meable and micro-fractured formations dur-ing drilling operations. Various methodswere evaluated for characterizing the proper-ties of aphrons, including acoustic bubblespectrometry, optical and electronic micro-imaging and interfacial tension. Key proper-ties investigated included the effects of pres-sure on bubble size, the influence of environ-mental parameters on aphron stability, theaffinity of aphrons for each other and themineral surfaces in rock pores and micro-fractures, and the nature of aphron seals inpermeable and micro-fractured rock. Initialsealing and formation damage tests were car-ried out, using lab-scale apparati designed tosimulate permeable and micro-fracturedenvironments. Phase II (year 2) is focusingon optimization of the structure of aphronsand composition of aphron drilling fluids,quantifying the flow properties of the fluids(radial vs. linear flow, shear and extensional

viscosity effects and bubblyflow phenomena), andunderstanding formationsealing and damage undersimulated downhole condi-tions (including scale-uptests), so as to furnishirrefutable evidence for thistechnology and providefield-usable data.

Much of the scenariodescribed above about the roleof aphrons in reducing fluidlosses downhole is conjecturethat has not been confirmedunder stringent laboratoryconditions. Furthermore, theoverall manner in which thedrilling fluid is able to reducefluid losses downhole hasbeen brought into question.As a result, there has beenconsiderable resistance in

some places to acceptance of the technology.

Aphron propertiesIn contrast to conventional bubbles, whichdo not survive long past a few hundred psi,aphrons have been found to survive com-pression to at least 27.3 MPa (4,000 psig)for significant periods of time. When afluid containing bubbles is subjected to asudden increase in pressure above a fewhundred psi, the bubbles initially shrink inaccordance with the modified Ideal GasLaw. Aphrons are no exception. However,conventional bubbles begin to lose airrapidly via diffusion through the bubblemembrane, and the air dissolves in the sur-rounding aqueous medium. Aphrons alsolose air, but they do so very slowly, shrink-ing at a rate that depends on fluid composi-tion, bubble size, and rate of pressurizationand depressurization.

Compression will reduce a bubble of 200-µm diameter at atmospheric pressure to 76µm when subjected to a pressure of 250 psi,

Figure 4. Aphrons undergo bubbly flow during displacement of water from20/40 sand pack by an aphron drilling fluid. Note that the fluid front (at thefluid/water interface) is predominantly aphrons (white.)

Close Up

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and 36 µm at 2,500 psi. But the biggesteffect of pressure by far on the fate of a bub-ble is increased gas solubility. Henry’s Lawstates that the solubility of a gas is roughlyproportional to the pressure. When a fluidcontaining 15% v/v entrained air at ambientpressure is compressed to just 250 psi, all ofthe air becomes soluble. If the stabilizingmembrane surrounding the bubble is perme-able, the air will diffuse into the surroundingmedium and go into solution. This is whathappens with ordinary bubbles, and it occurswithin a matter of seconds after compres-sion. Aphrons have a much less permeablemembrane, so they do not lose their air asreadily; indeed, when subjected to a pressureof 250 psi, air is prevented almost indefi-nitely from diffusing out of the aphrons andinto the aqueous medium.

Aphrons will survive compression anddecompression for short periods of time. Asshown in Figure 1, rapid compression of anaphron drilling fluid from 0 psig to 3000 psigfollowed by decompression back to 0 psigresults in essentially full regeneration of theaphrons. Large aphrons appear to be able tosurvive better than small aphrons. Figure 2shows the effect of the size of an aphron on itssurvivability. Aphrons of different sizes arepressurized from 500 psig to 2000 psig insteps of 500 psi every 10 seconds. Aphrons(larger than 200 µm diameter) decrease in sizewith increasing pressure as expected byBoyle’s Law (modified Ideal Gas Law); thesmall deviation from Boyle’s Law is because of

loss of air via slow diffusion into the sur-rounding fluid. When aphrons reach a criticalminimum size (50 µm to 100 µm diameter),they undergo a structural change that leads totheir rapid demise, with the expelled air againdissolving in the surrounding fluid. Upondecompression to a pressure sufficiently lowfor the aqueous medium to become supersat-urated with air, the air is released; most of theair goes into existing aphrons, though it mayalso create new aphrons.

Another important finding is that the oxy-gen in aphrons—even the oxygen dissolvedin the base fluid—is lost via chemical reac-tion with various components in the fluid, aprocess that usually takes minutes and resultsin nitrogen-filled aphrons. Thus, corrosionof tubulars and other hardware by aphrons isnegligible. Figure 3 shows that even at ambi-ent temperature and pressure, the oxygen insolution in an aphron drilling fluid disap-pears within hours after preparing the fluid.By contrast, in a typical clay-based or poly-mer-based fluid, the concentration of oxygenin solution remains relatively constant.

Fluid dynamicsThe base fluid in aphron drilling fluids yieldsa significantly larger pressure loss (or, for afixed pressure drop, lower flow rate) in longconduits than any conventional high-viscos-ity drilling fluid. Similarly, if flow isrestricted or stopped, aphron drilling fluids(at a fixed wellbore pressure) generate signif-icantly lower downstream pressures than do

other drilling fluids. In permeable sands, thesame phenomena are evident. In addition, inpermeable sands of moderate permeability(up to at least 8 Darcy) and low pressures,aphrons themselves slow the rate of fluidinvasion and increase the pressure dropacross the sands. Lastly, and most impor-tantly, aphrons move more rapidly throughthe sands than the base fluid. Figure 4 showsa transparent version of an aphron drillingfluid (dyed blue) displacing water from a bedof 20/40 sand under the influence of a 100-psi pressure gradient. The drilling fluid frontis populated with a high concentration ofbubbles that turn the fluid nearly white. Thisphenomenon, called “bubbly flow,” appearsto follow conventional Navier-Stokes theory.High-density particles such as barite (a den-sifying material) or drilled cuttings tend tobe left behind the base fluid. Low-densityinternal phases, such as bubbles, flow morerapidly than the base fluid. For a rigid spherein a fluid under the influence of a one-dimensional pressure gradient, ∆P/L, therelative velocity of the bubble in an infinitelywide conduit is:

V = 0.23 r2/µm * ∆P/L

where r is the bubble radius and µm is thefluid viscosity. For flow through permeablemedia, the expression is modified to incor-porate Darcy flow.

Wettability tests indicate aphrons havevery little affinity for each other or for the

Figure 5. In time, conjoined aphrons separate and show little affinity for each other.

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mineral surfaces in rock formations encoun-tered during drilling. This is demonstratedin Figure 5, which shows bubbles purposelyjoined by creating them via air injection.The bond between the bubbles is thought tobe the result of imperfect development ofthe aphron shell. Within a few seconds, thebubbles separate from each other, ratherthan coalesce. This lack of affinity of bub-bles for one another and for silica and lime-stone surfaces does not result from sheddingsurfactant layers, as was thought before, butis an intrinsic characteristic of the wholeaphron structure. Thus, aphrons resistagglomeration and coalescence and areexpected to be pushed back out of a perme-able formation easily by reversing the pres-sure differential, thus minimizing formationdamage and cleanup.

Finally, leak-off tests demonstrate thataphron drilling fluids are capable of sealingrock as permeable as 80 Darcy. Figure 6shows test results in synthetic Aloxite coresof 0.75 Darcy to 10 Darcy permeability. Thebase fluid in aphron drilling fluids is primar-ily responsible for slowing or stopping theinvasion, but properly designed aphrons canreduce these losses even further.

Future effortsPhase II of this Project is continuing with astudy of the effects of individual componentsin the fluid on the properties of aphrons; adetailed investigation of the surface chem-istry involved in the interactions of thedrilling fluid with reservoir rock and pro-duced fluids; visualization of the flow ofaphron drilling fluids in a wellbore/reservoirsimulator; and extension of the fluid flowmodel to include bubbly flow. ✧

AcknowledgementsThis project is partially funded by the U. S.Department of Energy’s (DOE) Office of FossilEnergy, National Energy TechnologyLaboratory contract DE-FC26-03NT42000.The Aphron Technology Team of MASI

Technologies LLC responsible for gathering thedata reported here consists of Arkadiy Belkin,Miranda Fosdick, Tatiana Hoff, MaribellaIrving and Bob O’Connor; my sincerest grati-tude to all of them. I thank Gary Covatch, theDOE project manager, for his guidancethroughout the course of this project, and JohnRogers of DOE for his advice and valuable andinformative discussions. In addition, manythanks to the following individuals for theircontinuing contributions: Tony Rea and TomBrookey of MASI Technologies LLC for theiressential advice and direction; Dr. Peter Popovof Texas A&M University for the all-importantbubbly flow modelling work; GeorgeMcMennamy of M-I Swaco for some criticalanalytical work on the composition of aphrondrilling fluids, and Dr. Ergun Kuru and histeam at the University of Alberta, who are car-rying out some surface chemistry work in paral-lel with our own effort.

References1) Montilva, J., Ivan, C.D., Friedheim, J. andBayter, R.: “Aphron Drilling Fluid: Field LessonsFrom Successful Application in Drilling Depleted

Reservoirs in Lake Maracaibo,” OTC 14278,presented at the 2002 Offshore TechnologyConference, Houston, May 6-9, 2002.2) Growcock, F.B., Simon, G.A., Rea, A.B.,Leonard, R.S., Noello, E. and Castellan, R.:“Alternative Aphron-Based Drilling Fluid,”IADC/SPE 87134, presented at the 2004IADC/SPE Drilling Conference, Dallas, Mar.2-4, 2004.3) Brookey, T., Rea, A. and Roe, T.: “UBD andBeyond: Aphron Drilling Fluids for DepletedZones,” presented at IADC World DrillingConference, Vienna, Austria, Jun. 25-26, 2003.4) Growcock, F.B., Simon, G.A., Guzman, J.,and Paiuk, B.: “Applications of Novel AphronDrilling Fluids,” AADE-04-DF-HO-18,presented at the AADE 2004 Drilling FluidsConference, Houston, TX, Apr. 6-7, 2004.5) Sebba, F.: Foams and Biliquid Foams—Aphrons, John Wiley & Sons Ltd, Chichester(1987).6) White, C.C., Chesters, A.P., Ivan, C.D.,Maikranz, S. and Nouris, R.: “Aphron-BasedDrilling Fluid: Novel Technology for DrillingDepleted Formations,” World Oil, vol. 224, no.10 (Oct. 2003).

Figure 6. Static linear leak-off tests of aphron drilling fluids in permeable Aloxite coresdemonstrate the sealing performance of the fluids and the role played by aphrons (air).