O.B. BENNION, F.B. THOMAS, D.W. BENNION, A.F. BIETZHycal Energy Research Laboratories Ltd.

IntroductionVarious authors(l) have documented why concerns with fonna-

tion damage in horizontal wen applications often outweigh thoseobserved in vertical wells. These would include:

I. Greater exposure time.2. Greater potential depth of invasion.3. The majority of horizontal wens are open-bole completions.

resulting in potential flow impainnent. even with very local-ized shallow damage.

4. Selective cleanup of certain zones due to large exposed for-mation area results in non-effective flow from large portionsof the horizontal section.

5. Anisotropic flow effects caused by adverse Kv<Khpenneability effects.

6. Difficulty and expense of effective stimulation due to size ofthe damaged zone.

Mechanisms of Formation DamageMechanisms of damage common to both horizontal and vertical

wells could include:1. Fluid-fluid incompatibilities-reaction of invaded mud fil-

trate with in situ fluids (oil or formation brine) to formscales, insoluble precipitates, asphaltic sludges or stableemulsions.

2. Rock-fluid incompatibilities-<:ontact of potentially swelling(i,e., smectitic clay) or deflocculatable (ie., kaolinite clay)minerals by non-equilibrium aqueous phase solutions mayhave the potential to severely reduce near wellborepenneability.

3. Solids invasion-the invasion of artificial solids containedin die drilling fluid (i.e., weighting agents or artificial bridg-ing agents) or the invasion of formation solids (rnicrofines)generated by the milling action of the drill bit on the fonna-tion. The permanent entrainment of these solids in the for-mation can have a severely reducing effect on penneabilityin some situations.

4. Phase trapping/blocking-the invasion and permanententrapment of high oil or water phase saturations in the nearwellbore region can have a substantially reducing effect onoil or gas productivity, particularly for certain types offonnations.

5. Chemical adsorption/wettability alteration-most drillingfluids contain a variety of chemical additives to improvemud performance and character. In some cases these addi-tives may be incompatible with the formation fluids or rock.or exhibit a high propensity for physical adsorption. This canresult in a number of undesirable phenomena such a$ perm~-ability reductions due to physical polymer adsorption, orwettability alterations due to surfactant adsorption.

6. Fines migration-the actual internal movement of fonnation

The Journal of Canad"tan Petroleum Tectv1OIogy

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fines or loosely attached in situ formation particulates can bea concern in certain reservoirs where high, uncontrolled fluidloss is apparent at highly overbalanced drilling conditions.

7. Biological activity-both aerobic and anaerobic bacterialagents can be introduced into the formation by the drillingprocess. Bacteria produce polysaccharide polymer slimes aswaste products which can occlude porosity and reduce per-meability in the near wellbore region. Toxicity and corrosionconcerns may also be present with certain bacterial types.

All of these types of reservoirs can be severely damaged bywhole mud, mud filtrate, and mud solids losses. In many cases theradius of invasion in some higher permeability reservoirs at highoverbalance pressures is large and can generally extend wellbeyond the range of conventional perforating and chemical stimu-lation treatments.

The benefit of underbalanced drilling is that, since the pressurein the reservoir is actually higher than the circulating fluid, thepotential for both the invasion of whole mud and solids is elimi-nated. This can greatly reduce damage, mud losses and mud vol-ume required and lower ultimate completion/stimulation costs.Also, more conventional reservoirs which exhibit apparent sensi-tivity to invading fluids (i.e., high concentrations of swelling ordeflocculatable clay, the presence of highly emusifiable oils or asevere propensity for aqueous phase trapping, etc.) may often beconsidered as candidates for underbaIanced drilling. There is alsoevidence in many underbalanced drilling operations that rate ofperforation is increased significantly, thus reducing drilling timeand costs.

Disadvantages of Underbalanced DrillingUnderbalanced drilling has several detrimental aspects associ-

ated with its use, some of these being:1. Safety concerns (risk of blowout, fire, explosion, loss of

control, etc.)-this is a particular concern in oil or gas reser-voirs containing H~ gas and in very high pressure systemswhere a blowout could have catastrophic results.

2. Expense-underbalanced drilling can be much more expen-sive than conventional overbalanced drilling. In many cases,nitrogen is utilized to generate the underbalanced condition.This greatly increases the cost of underbalanced drilling dueto the volume of nitrogen required to complete an extendedlength horizontal section. Recently, many operators haveexpressed interest in the use of air as an alternate media toreduce the density of the circulating fluid column. Air isinexpensive, but has the dual disadvantage of an increasedpropensity for corrosion and the significant possibility ofdownhole or surface fires or explosions. High pressure com-bustible limit tests (as illustrated in Figure 1) allow the defi-nition of the lean and rich limits for combustion for a givengas-oil-air system at elevated temperature and pressure. Theapplication of this type of technology, coupled with continu-ous surface monitoring of the composition of the effluentgas stream, has facilitated a much increased level of safety

Overbalanced vs. Underbalanced DrillingOverbalance pressure is defined as the downhole pressure dif-

ferential between the circulating fluid stream (drilling, completionor stimulation fluid) and the in situ pressure in the formationbeing contacted. The circulating fluid pressure is a combination ofthe hydrostatic pressure induced by the physical weight of thefluid column between the surface and the downhole formation,plus the physical pump pressure required to cause the fluid systemto circulate through the annulus.

Most formations are commonly drilled in an overbalancedmode due to the fact that conventional fluid system densities usu-ally create a downhole pressure which is higher than the in situformation pressure. This overbalance pressure causes a naturaltendency, if the exposed formation is permeable to any degree, forcirculating fluids (and possibly associated solids) to invade intothe formation.

Underbalanced drilling occurs when the effective downholecirculating pressure of the fluid system in contact with the fonna-tion is less than the existing formation pressure. Underbalancedconditions occur naturally in some reservoirs when unweightedfluids are utilized if the reservoir is geostatically overpressured forits depth. In other situations, underbalanced flow can be obtainedthrough the use of lower density hydrocarbon based fluids in lieuof denser water based systems.

In many cases, particularly when considering pressure depletedformations, it is necessary to artificially reduce the apparent densi-ty and hydrostatic pressure of the applied fluid system in order togenerate an underbalanced condition. This is commonly conduct-ed by entraining a low density gas (either nitrogen, air or naturalgas) in the circulating fluid stream. In some cases, special surfac-tants are utilized to generate stable foam systems which have highapparent viscosity and low superficial density. In others, the gas ismerely injected either into the entire circulating fluid stream, orpart way into the vertical or build section by the use of a parasitetubing string or special concentric drill string configuration toallow single phase flow in the horizontal section to facilitate cut-tings transport. but still retain the advantage of underbalanceddrilling by reducing the density of the majority of the verticalfluid column. When the entire circulating fluid stream is not gasi-fied, pump pressure and full hydrostatic fluid pressure is stillexerted directly at the bit-rock interface while the previouslydrilled sections are maintained in an underbalanced mode by thegasification of a portion of the returning recirculating fluid stream.

Why Underbalance Drill?Underbalanced drilling has particular advantages in situations

where the potential for severe fluid loss or total lost circulationexists. This would include reservoir situations such as:

I. Highly fractured sandstone or carbonate fonnations wherethe majority of penneability is contained in the fracturesystem.

2. Heterogeneous high permeability vugular carbonates3. High permeability unconsolidated or consolidated sands.4. Lower matrix permeability sands or carbonates in pressure

depleted formations where the potential for fluid loss underextreme overbalance conditions is high.

5. Formations exhibiting extreme sensitivity to chemicallyinduced formation damage (i.e., high swelling clay content).

35NOYenUf 1995. VoIlm9 34. No.9

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FIGURE 2: Schematicrepresentation or nuld andsoUds Joss In overbalanceduDderbalaDCed operations.

with respect to the flammability concerns associated withunderbalanced drilling with air.

3. Damage-underbalanced drilling does not eliminate alltypes of damage in all reservoir situations. In some cases.underbaIanced drilling has its own unique damage mecha-nisms. This theme of damage during underbalanced drillingoperations wiJ) be the subject of the balance of this paper.

Damage During Underbalanced Drilling

established. In addition, fluids utilized in underbalanced 0pera-tions often do not contain fluid loss additives or bridging agents,as it is not nomlally anticipated that any type of effective filtercake will be required or established and low fluid viscosity isrequired to effect adequate solids removal aDd disengagement ofgas from the circulating drilling fluid.

Unfortunately, in many artificially induced underbalanceddrilling operations, truly uDderbaIanced conditions are Dot main-tained from the commencement of drilling to the conclusion of thecompletion phase of the well. This could be due to a number ofsituations, including:

I. Need to temporarily kill the weD for bit nips, mud pulsedlogging programs, or other operating considerations.

2. Penetration of unexpected underpressured zones.3. Mechanical, technical or supply problems resulting in a

shutdown of gas injection.4. Sustained flow from the initial (heel) portion of the horizon-

tal well results in localized pressure depletion of this area bythe time the toe of the well is reached. If the depletion is sig-nificant, as would possibly occur in a lower penneabilityformation, the effective underbalance pressure in this sectionmay be greatly reduced, or in some cases eliminated,increasing the propensity for invasive damage.

Figure 2a provides a schematic representation of a poorlydesigned conventional overbalanced mud system. Due to highoverbalance pressure and improper selection of fluid loss andbridging agents, extensive nushing or matrix, fracture or vugutarpermeability systems may occur. Whole mud and solids lossescould be especially damaging to fracture and vugular permeability

Lack of a Protective Sealing Filter CakeIn truly underbalanced operations, since flow is occurring from

the fomlation, an external bridging and sealing filter cake is no(

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A number of potential damage m«banisms exist when operat-ing in an underbalanced mode. These include:

1. Problems associated with high fluid and solids losses to theformation d~ to the lack of a protective sealing tilter cake iftrue underbalanced conditions are not maintained 100% ofthe time during drilling and completion.

2. Spontaneous countercurrent imbibition effects which allowthe enttainment of potentially damaging aqueous fluid til-b'ate in the reservoir matrix in the near wellbore region.

3. Glazing and surface damage effects caused by the low heatcapacity of circulating fluids and insufficient lubricating andturbulence to effectively remove all drill cuttings and fines.

These points will be elaborated upon in greater detail.

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HFIGURE 3: Establishmentor roam laminae as a nearweUbore fluid lossmechanism.

cosity) over conventional nitrified systems. In addition, cenainfoamed systems tend to maintain foam integrity for a finite periodof time after cessation of gas injection. This prevents a total col-lapse of the gasified fluid column during shon term shutdowns.Also, laboratory tests have indicated that near wellbore entrain-ment of foam laminae in the actual rock matrix can act as anblocking agent for subsequent fluid loss if the fluid contained inthe wellbore abruptly flips to an overbalanced mode. A schematicillustration of this phenomena is illustrated in Figure 3. Dependingon the quality and stability of the foam block established, this phe-nomena can last anywhere from a few minutes to potentially sev-eral hours. Caution must be observed with some of the surfactantsutilized to generate stable foam systems as they may have thepotential to cause formation wettability shift to a more oil-wetstate. This may have a deliterious effect on increasing water cutand reducing oil rates due to preferential relative permeabilityeffects associated with the transition of the near wellbore regionfrom a water-wet state to an oil-wet state.

Countercurrent ImbibitionFigure 4 provides an illustration of the mechanism of counter-

current imbibition. It is well recognized that imbibition effects candraw water hundreds of metres up into formations from water-oilor water-gas contacts, and similar forces can cause equivalenteffects in the near wellbore region during underbalanced drilling.

This problem tends to be the most severe in dehydrated lowerpermeability water-wet gas reservoirs which exhibit abnormallylow, sub-irreducible water saturations. A detailed discussion ofthese types of formations is provided in the literature<2-'). Thisproblem can also occur, to a lesser extent, in both water-wet oiland gas reservoirs where the horizontal section is placed a consid-erable distance above the water-oil or gas-water contact. The cir-culating underbalanced drilling fluid basically regenerates an arti-ficial water-oil or gas-oil contact directly adjacent to the wellbore,and the generation of a transition zone, even against prevailing

in this type of a situation.Figure 2b illustrates a similar conventional overbalanced sys-

tem with a properly designed fluid loss control/bridging agent sys-tem. In this case solids invasion depth (particularly in the fractureand vugular system) is minimized and extensive filtrate losses areeliminated. The filter cake must be designed to be easily remov-able by reverse flow or conventional completion/stimulationtechniques.

Figure 2c illustrates the same system in an underbalancedmode. It can be seen that if continuously underbalanced condi-tions can be maintained. that this likely represents the optimumscenario for maximizing potential productivity from the vug andfracture system. Damage to the matrix due to counte~nt imbi-bition could still occur in certain situations and this phenomenawill be discussed shortly.

Figure 2d illustrates how the situation in Figure 2c can bedegraded if the unprotected matrix/fracture/vug system is sudden-ly exposed to an overbalanced pulse. This creates the potential forsignificant invasion into exposed matrix, fractures and vugs. Sincethe invading fluids contain drill solids, severe plugging may occurand the influx of filtrate could also damage the formation by oneor more of the mechanisms of formation damage discussedpreviously.

Therefore, it can be seen from evaluation of Figure 2 that insome cases it may be more advantageous to drill and completewith a conventional system where we can rapidly establish animpermeable and sealing filter cake which is designed to be readi-ly removable, rather than to go to the expense of drilling only par-tially underbalanced and possibly creating a zone of more severe,deeply invaded and inaccessible damage.

Foamed Mud SystemsThe use of foamed mud systems has become more popular as

interest in underbalanced drilling has increased. Foamed systemstend to have certain rheological advantages (greater apparent vis-

37November 1995. Volume 34. No. 9

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Glazing and Surface DamageGlazing and surface damage can occur if the circulating fluid

has insufficient heat capacity to cool and lubricate the bit-rockinterface. This results in a high localized temperature and thepotential fonnation of a relatively thin (I - 5 mm), but often veryimpermeable zone. This may be a problem in some underbalanceddrilling operations where high gas rates are utilized or penetrationrates are very low. This problem generally does not occur if para-site or concentric strings are utilized to generate the underbal-anced condition, as the bit-rock interface is totally liquid contact-ed in these situations. Glazing tends to generally be a problem intighter homogeneous formations. If the fonnation exhibits frac-tures or open vugs, glazing does not appear to significantlyocclude these features in most situations.

Table 1 provides a summary of core air permeability measure-ments conducted on dry cut carbonate cores. Examination of thisdata indicates that the dry cut samples did not experience anyappreciable reduction in permeability, although the magnitude ofthe reduction did slightly increase with a reduction in samplequality. These samples were totally dry when cut, which does notlikely provide a good approximation to the paste generated whencutting water and hydrocarbon saturated samples in conjunctionwith circulating drilling fluids. Ongoing testing continues to beconducted in this area to better quantify the effect of these morerigorously simulated drilling conditions.

Spontaneous Imbibition ExperimentsA series of five spontaneous imbibition experiments were con-

ducted to note the effect of countercurrent imbibition in a water-wet. gas-water system as a function of:

1. Reservoir quality2. Underbalance pressure3. Initial pre-existing water saturation

underbalance pressure effects, can potentially occur.

Spontaneous imbibition effects will cause the matrix in the nearwellbore region to saturate itself with water until the internal cap-illary pressure effect balances the pressure exerted by the differen-tial underbalance pressure. The severity of the potential degree ofimbibition will be a function of the initial difference in saturationbetween the initial water saturation and the "irreducible" satura-tion at the given underbalance pressure level. It can be seen that inmany reservoir systems, particularly lower permeability matrixsituations, countercurrent imbibition can effectively counteracteven very high underbalance pressures. The absence of any typeof sealing filter cake tends to aggravate the problem as there is nobarrier to filtrate access to the formation.

Water-based filtrate imbibition can cause reductions in nearwellbore productivity due to water blocking effects. In addition, ifthe formation contains potentially sensitive clays or incompatiblefluids, adverse reactions with the imbibed fluids may occur caus-ing additional reductions in permeability. Since the majority ofhorizontal wells are open hole completions, even a relatively shal-low damaged zone, which might normally be perforated throughin a vertical well, could have a significant detrimental effect onproductivity.

Oil-wet systems do not typically tend to spontaneously imbibewater based fluids (unless they exhibit a mixed or spotted wetta-bility condition), but they can spontaneously imbibe oil based flu-ids in a manner analogous to that described for water-wet systems.This typically does not pose a problem for oil reservoirs (as thematrix is already highly saturated with hydrocarbon at a levelwhich minimizes or eliminates imbibition effects) but may be ofconcern in some gas reservoirs which contain a low, immobile liq-uid hydrocarbon saturation and exhibit oil-wetting tendencies (i.e.,sub-dewpoint depleted retrograde condensate systems). The pres-ence of an initial low saturation hydrocarbon phase in an oil-wetsystem can act as imbibition causing sites for additional hydrocar-bon imbibition and trapping. This phenomena has been document-ed by McCaffery<S).

The phenomena of spontaneous water imbibition against adynamic underbalanced condition is illustrated in a series ofexperiments documented later in the paper.

The Journal of Canadian Petroleum T~:m

the degree of spontaneous imbibition and damage. The relativeincrease in damage with reduction in underbalance pressure wasrelatively small for this test, likely due to the high inherent reser-voir quality. The permeability reduction profiles have been plottedas a function of time and underbalance pressure for test I andappear as Figure 6.

Test 2 (SWj = 0.0, kair = 390 mD)

Test 3, 4, 5 (Variable SWi = 0 - 38%,kair= 22 - 27 mb)

The fmal three tests were conducted on samples of relativelylow penneability. Three dolomite samples as similar as possiblewith permeability ranges from 22 - 27 rnD and porosities from8.8 - 9.9CIJ were selected for use in this part of the test. One sam-ple (test 3) was tested dry. The next (test 4) had a 12CIJ initialwater saturation introduced and unifornlly dispersed prior to test-ing. The final core (test 5) had a 38CIJ initial water saturation inbO-duced and dispersed prior to testing.

Complications in the analysis of the results from these three

~1~.V~34.No.9 s

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Figure 5 provides a schematic of the equipment utilized forthese tests. Pressure tapped core samples were utilized for two ofthe tests to investigate permeability impairment as a function ofdistance from the flee water contact and three tests were conduct-ed using non-pressure tapped core to investigate the effect of themagnitude of a pre-existing water saturation prior to water contacton countercurrent imbibition.

Humidified nitrogen (to avoid desiccation of any in situ orimbibed water saturation) was utilized as a displacing fluid tosimulate underbalanced gas flow from the formation and neutralpH 5% Ka solution to simulate the potentially imbibing filtratephase. Pressure measurements were conducted using capacitancetransducers having a range of 0 - 35 kPa or 0 - 350 kPa with anaccuracy of 0.5%. Tables 2 and 3 provide a summary of corephysical parameten and test parameters for the experimentalprogram.

Test 1 (SWj = 0.0, kair = 1083 mD)Table 4 provides the results of the test conducted on core I.

This core was the highest ~bility tested and was initially in adry state (0% SwJ to simulate the worst possible scenario forspontaneous imbibition.

A set of permeability vs. time measurements conducted over a96 hour period indicated that permeability declined to the greatestdegree directly adjacent to the simulated wellbore as would beexpected, but that substantial permeability impairment extendedup to 25 cm into the core. The core was run in a vertical orienta-tion, providing the most optimistic scenario as the imbibing fluidshad to counteract gravitational as well as underbalanced floweffects.

The data of Table 4 also illustrates the effect of rMucing over-balance pressure on equilibrium spontaneous imbibition. Aswould be expected, the lower the overbalance pressure, the greater

Table 5 summarizes the results of test 2. This test was conduct-ed on a lower quality core. but only two iK)urs were allowed ateach imbibition point to observe the rate effect of imbibition. Thiscore was slightly longer than that used in core 1 and had threeinternal pressure taps instead of two.

Results once again indicate the most severe imbibition/penDe-ability reduction effect at the simulated weUbore face. Damageincreased substantially with reductions in underbalance pressureand likely would have been even more severe if extended timeperiods had been allowed for equilibrium as in test 1. Depth ofpropagation of the damage appears to be rate dependent withexposure time with a much shallower damage profile beingobserved in this test than in test I, even though core permeabilitywas lower and one would expect stronger countercurrent imbibi-tion effects.

This indicates that degree of imbibition induced damage willbe, as expected, not only a function of the relative underbalancepressure, but also of the length of exposure time. The greater thelength of exposure, the greater the severity of the damage in thenear wellbore region and potential extent of invasion. The per-centage of penneability retained profiles of Table 5 have beenplotted and 8AJe8r as Figure 7.

direct function of decreasing underbalance pressure, consis-tent with the results of the earlier tests.

3. The relative severity of the observed reduction in permeabil-ity is reduced as the initial water saturation increases. This isdue to the decreased propensity for water imbibition in ahighly water saturated medium due to the initial water satu-ration being closer to the true irreducible value. If additionaltests had been conducted at even higher initial saturationlevels that approached or equalled the true irreducible level,the data indicates that imbibition damage effects would like-ly be negligible.

tests occurred due to the low permeability of the core and theresulting influence of Klinkenberg slippage and turbulent floweffects on the gas penneabilities at the increasing underbalancepressure. Higher underbalance pressures (corresponding to highereffective net pressure drops and flow rate across the individualcore samp)es) resulted in lower permeabilities. Therefore, to elim-inate the effect of this flow rate induced artifact on the measuredlaboratory data the relative percentage of reduction in permeabili-ty before and after exposure to water in an underbalanced mode,at a given underbalance pressure, provided a more accurate evalu-ation of what was actually occurring in the tests. This data is alsocontained as a portion of Table 6 and has been plotted and appearsas Figure 8.

Examination of this data indicates that:1. Initial permeability at a given underbalance pressure prior to

water exposure is reduced as a function of initial trappedwater saturation, as would be classically expected.

2. The severity of the observed reduction in penneability is a

Conclusions

1. Underbalanced drilling has specific application in fractun:d,vugular or extremely high penneability systems where highlosses of both potentially damaging fluids and solids to the

40 The Journal of Canacian Petroleum Tedmologyå

Michigan Reef Gas Reservoirs-An Analysis; AAPG Bul~ Vol.66. No. /, January /982, pp. 9/-98.

S. McCAFFERY. F.G., The Effect of Wettability. RelativePermeability and Imbibition in Porous Media; Ph.D. Thesis,Uniwrsiry of Calgary. September /973.

Provenance-Original Petroleum Society manuscript,UnderbaJanced Drilling and Fonnation Damage-Is it . TotalSolution (94-17), first presented at the 45th Annual TechnicalMeeting, June 12-15, 1994 in Calgary. Abstract submitted forreview November 16, 1993; editorial comments sent to theauthor(s) November 7, 1994; revised manuscript receivedDecember 6, 1994; paper approved for pre-press December 7,1994; final approval June 20, 1995.&

Authors' Biographies

Brant Bennion is president of HycalEnergy Research Laboratories Ltd. and isresponsible for research and developmentin multiphase flow in porous media andformation damage. He has worked at Hycalsince 1979. He graduated with distinctionfrom the University of Calgary in 1984with a B.S. in chemical engineering. He hasauthored more than ro technical papers andhas lectured in North and South America.

Asia, Europe, Africa, and Australia. Brant is a member ofAPEGGA and serves as a director in the Calgary Section of ThePetroleum Society.

Brent Thomas holds a doctorate in chemi-cal engineering. He has worked onenhanced oil recovery applications for thelast ten years, including gas injection,chemical flooding, solids precipitation andtbennal applications. He is presently vicepresident of Hycal Energy ResearchLaboratories Ltd.

fOmlation have the potential to severely impair ultimate oilor gas productivity.

2. Underbalanced drilling can be damaging in certain situationsdue to the lack of formation of an impenneable sealing filtercake to prevent invasive losses if underbalanced conditionsare not maintained at all times. Countercurrent spontaneousimbibition of water based filtrates (in water-wet and low SWjmedia) and oil based filtrates (in oil-wet, low So; media)have also been illustrated to be potentially damaging even iftotally underbalanced conditions are continuously main-tained. Foamed systems may be advantageous in certain sit-uations to impair fluid losses during periodic overbalancedpulses.

3. Spontaneous countercurrent imbibition was found to be rateand underbalance pressure dependent with longer exposuretimes and lower underbalance pressures both contributing toseverity of damage and depth of invasion. Degree of perme-ability impainnent due to imbibition was found to decreasewith increasing initial water saturation (for the case of awater-wet rock), although research tends to indicate that therate of imbibition may be increased by the presence of a pre-existing saturation of the imbibing phase, even though theultimate severity (i.e., total magnitude of the damage) maybe reduced.

Doug Bennion attended the University ofOklahoma. obtaining a Bachelor's degreein petroleum engin~ng. Following gradu-ation he went to work for Mobil Oil. wherehe worked in their production and reservoirengineering departments for six years, andthen returned to do graduate work atPennsylvania State University. He obtained

, a Master's and Ph.D. degree in petroleumand natural gas engineering from this

school. After obtaining his Ph.D., he joined the chelnical engi-neering staff at the University of Calgary, where he taught for 21years. Since 1988, he has been chief executive officer of HycalEnergy Research Laboratories.

Ronald Bietz is the manager of engineer-ing at Hycal Energy Research LaboratoriesLtd. Ron graduated from me University ofWyoming in 1987 with a B.Sc. in petrole-um engineering. His interests include for-mation damage and multi phase flow inporous media. Ron has authored/co-authored five technical papers.

AcknowledgementsThe authon express appreciation to the management of Hycal

Energy Research Laboratories Ltd for the funding of this projectand for permission to publish the data.

REFERENCESI. BENNION, D.B., mOMAS, F.B. and BENNION, D.W., Effective

Laboratory Coreflood Tests to Evaluate and Minimize FonnationDamage in Horizontal Wells; presented at the Third InternationalConference on Horizontal Well Technology, November 12 - 14,1991, Houston. TeXDS.

2. BENNION, D.D., CIMOLAI, M.P., BIETZ, R.F., mOMAS, F.B.,Reductions in the Productivity of Oil and Gas Reservoirs Due toAqueous Phase Trapping; Presented at the 44t/r Annual GeneralMeeting of The Petroleum Society, May 9 - /2. 1993, Calgary,Albel1a.

3. CIMOLAI. M.P., GIES, R.M., BENNION, D.B., and MYERS, 0.1.,Mitigating Horizontal Well Formation Damage in a LowPemabilily Conglomerate Gas Reservoir; pnsented at the SPE GasTec/rnology Symposium /reM in Calgary, Albel1a. June 28 - 3D,

1993.4. KATZ, D.L. and LUNDY, C.L., Absence of Connate Water in

41November 1995, V~ 34. No.9