POOO4-97

By

D. B. Bennion and F. B. ThomasHycal Energy Research Laboratories Ltd.

Presented at the 2M Annual Conference on Emerging Technology - Coiled Tubing - Horizontal Wells - Extended Reach andMultilaterals, June 1-3, 1994, Aberdeen, Scotland.C Petroleum Society of CIM, 1994.

ABSTRACT designed with ultra-low fluid loss in mind as extensive mudlosses to the fonnation are not contemplated. However, the factthat a stable filter cake is not developed during underbalanceddrilling, due to continual inflow from the fonnation face, couldincrease the severity of losses to the formation if underbalancedconditions are not maintained 1000/0 of the time during drillingand completion. Overbalanced pulses may occur due to mudpulsed logging programs, bit trips or mechanical problemswhich may occur during the drilling process of most horizontalwells. This results in the majority of underbalanced horizontalwells being overbalanced for some fraction of the time duringthe drilling and completion process. A discussion ofmechanical damage due to near surface glazing effects is alsopresented as a possible mechanism of damage which can occurduring underbalanced drilling.

Underbalanced drilling has been utilized as a technique tominimize damage due to whole mud, fluid filtrate and solidsinvasion into high permeability and fractured formations andformations which exhibit a high degree of sensitivity to invadedfluids. While it is generally accepted that the process ofunderbalanced drilling reduces the potential for formationdamage, the possibility of invasive formation damage still existsin certain reservoirs where the initial saturation (eitherhydrocarbon or water) is substantially less than the irreduciblesaturation of the phase which is being utilized in the drilling orcompletion process. This situation could commonly occur whendrilling in an underbalanced mode in gas reservoirs that are ina desiccated state and exhibit abnormally low initial watersaturations, or in oil and gas reservoirs in a portion of thereservoir significantly above the oil-water or water-gastransition zone water saturation. Although dynamic fluid flowis occurring from the formation during the drilling process, thecirculating drilling fluids are in continual contact with theformation face. At the low initial liquid saturations which existin these reservoirs, there is an ability for strong countercurrentspontaneous imbibition effects which can, in some situations,counteract pressures that are far greater than the apparentunderbalance pressure which is present during the drilling orcompletion process. If the formation contains potentiallysensitive materials such as deflocculatable fmes or swellingclays, the potential for the formation of in-situ emulsions or thepossibility of permeability reductions due to the establishmentof near wellbore aqueous phase traps, these phenomena couldoccur and cause potential reductions in productivity of the well.

The paper elaborates on potential damage scenarios based onthe results of specific laboratory tests which have beenconducted and reviews a variety of new techniques which havebeen designed to evaluate and pre-screen fluids and proceduresprior to costly and possibly ineffective implementation ofunderbalanced drilling in the field.

INTRODUCTION

Various authors' have documented why concerns withformation damage in horizontal well applications often outweighthose observed in vertical wells. These would include:

I. Greater exposure time2. Greater potential depth of invasion.3. The majority ofhorlzontal wells are open-hole completions,

resulting in potential flow impairment even with verylocalized shallow damage.The majority of under balanced drilling fluid systems are not

D.B. BENNION and F.B. THOMAS POOO4-972

weight of the fluid column between the surface and thedownhole fonnation, plus the physical pump pressure requiredto cause the fluid system to circulate through the annulus.

Most fonnations are commonly drilled in an "overbalancedmode" due to the fact that conventional fluid system densitiesusually create a downhole pressure which is higher than thein-situ fonnation pressure. This overbalance pressure causes anatural tendency, if the exposed fonnation is penneable to anydegree, for circulating fluids (and possibly associated solids) toinvade into the fonnation.

4. Selective cleanup of certain zones due to large exposedfonnation area results in non-effective flow from largeportions of the horizontal section.

s. Anisotropic flow effects caused by adverse kv

POOO4-97 Recent Investigations Into Formation Damage in Horizontal WellsDuring Overbalanced and Underbalanced Drilling and Completion Procedures

3

All these types of reservoirs can be severely damaged bywhole mud. mud filtrate, and mud solids losses. In many casesthe radius of invasion in some higher penneability reservoirs athigh overbalance pressures is large and can generally extendwell beyond the range of conventional chemical stimulationtreabnents.

DAMAGE DURING UNDERBALANCED DRILLING

A number of potential damage mechanisms exist whenoperating in an underbalanced mode. These include:

The benefit of underbalanced drilling is that, since thepressure in the reservoir is actually higher than the circulatingfluid, the potential for both the invasion of whole mud andsolids is eliminated. This can greatly reduce damage, mudlosses and mud volume required and lower ultimatecompletion/stimulation costs. Also, more conventionalreservoirs which exhibit apparent sensitivity to invading fluids(i.e. high concentrations of swelling or deflocculatableclay, thepresence of highly emulsifiable oils or a severe propensity foraqueous phase trapping, etc.) may often be considered ascandidates for underbalanced drilling.

1. Problems associated with high fluid and solids losses to theforDlation due to the lack of a protective sealing filter cakeif true underbalancedconditions are not maintained 1000/0 ofthe time during drilling and completion.

2. Spontaneous countercurrent imbibition effects which allowthe entrainment of potentially damaging aqueous fluid filtratein 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 fmes.These points will he' elaborated upon in greater detail.

LACK OF A PROTECTIVE SEALING FILTER CAKE

DISADVANTAGES OF UNDERBALANCED DRILLING In truly underbalanced operations, since flow is occurringfrom the fonnation, an external bridging and sealing filter cakeis not established. In addition, fluids utilized in underbalancedoperations often do not contain fluid loss additives or bridgingagents as it is not normally anticipated that any type of effectivefilter cake will be required or established and low fluidviscosity is required to effect adequate solids removal anddisengagement of gas from the circulating drilling fluid.

Underbalanced drilling has several detrimental aspectsassociated with its use, some of these being:

Unfortunately. in most artificially induced underbalanceddrilling operations. truly underbalanced conditions are notmaintained from the commencement of drilling to theconclusion of the completion phase of the well. This could bedue to a number of situations. including:

I. Need to temporarily kill the well for bit trips, 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

horizontal well results in localized pressure depletion of thisarea by the tinle the toe of the well is reached. If thedepletion is significant, as would possibly occur in a lowerpenneability fonnation, the effective underbalance pressurein this section may be greatly reduced, or in some caseseliminated, increasing the propensity for invasive damage.

Figure 2(A) provides a schematic representation of a poorlydesigned conventional overbalanced mud system. Due to highoverbalance pressure and improper selection of fluid loss andbridging agents, extensive flushing of matrix, fracture orvugular permeability systems may occur. Whole mud andsolids losses could be especially damaging to fracture andwgular permeability in this type of a situation.

I. Safety concerns (risk of blowout, fire, explosion, loss ofcontrol, etc.). This is a particular concern in oil or gasreservoirs containing ~S gas and in very high pressuresystems where a blowout could have catastrophic results.

2. Expense - Underbalanced drilling can be much moreexpensive than conventional overbalanced drilling. In manycases, nitrogen is utilized to generate the underbalancedcondition. This greatly increases the cost of underbalanceddrilling due to the volume of nitrogen required to completean extended length horizontal section. Recently, manyoperators have expressed interest in the use of air as analternate media to reduce the density of the circulating fluidcolumn. Air is inexpensive, but has the dual disadvantage ofan increased propensity for corrosion and the significantpossibility of downhole or surface fires or explosions. Highpressure combustible limit tests (as illustrated in Figure 1)allow the definition of the lean and rich limits forcombustion for a given gas-oil-air system at elevatedtemperature and pressure. The application of this type oftechnology, coupled with continuous surface monitoring ofthe composition of the effluent gas stream, has facilitated amuch increased level of safety with respect to theflammability concerns associated with underbaJanceddrillingwith air.

3. Damage - Underbalanceddrilling does not eliminate all typesof damage in all reservoir situations. In some cases,underbalanced drilling has its own unique damagemechanisms. This theme of damage during underbalanceddrilling operations will be the subject of the balance of this

paper.

Figure 2(B) illustrates a similar conventional overbalancedsystem with a properly designed fluid loss control/bridging

D.B. BENNION and F .B. THOMAS PO004-974

agent system. In this case solids invasion depth (particularly inthe fracture and vugular system) is minimized and extensivefilb'ate losses are eliminated. The filter cake must be designedto be easily removable by reverse flow or conventionalcompletion/stimulation techniques.

COUNTERCURRENT IMBIBITION

Figure 4 provides an illustration of the mechanism ofcountercurrent imbibition. It is well recognized that imbibitioneffects can draw water hundreds of meters up into formationsfrom water-oil or water-gas contacts, and similar forces cancause equivalent effects in the near wellbore region duringunderbalanced drilling.

Figure 2(C) illustrates the same system in an underbalancedmode. It can be seen that if continuously underbalancedconditions can be maintained, that this likely represents theoptimum scenario for maximizing potential productivity fromthe vug and fracture system. Damage to the mabix due tocountercurrent imbibition could still occur in certain situationsand this phenomena will be discussed shortly.

This problem tends to be the most severe in dehydratedlower penneability water-wet gas reservoirs which exhibitabnormally low, sub-irreducible water saturations. A detaileddiscussion of these types of formations is provided in theliterature.. This problem can also occur, to a lesser extent, inboth water-wet oil and gas reservoirs where the horizontalsection is placed a considerable distance above the water-oil orgas-water contact. The circulating underbalanced drilling fluidbasically regenerates an artificial water-oil or gas-oil contactdirectly adjacent to the wellbore, and the generation of atransition zone, even against prevailing underbalance pressureeffects, can potentially occur.

Figure 2(0) illustrates how the situation in Figure 2(C) canbe degraded if the unprotected matrix/fracture/wg system issuddenly exposed to an overbalanced pulse. This creates thepotential for significant invasion into exposed matrix, fracturesand wgs. Since the invading fluids contain drill solids, severeplugging may occur and the influx of filtrate could also damagethe fonnation by one or more of the mechanisms of fonnationdamage discussed previously.

Spontaneous imbibition effects will cause the mabix in thenear wellbore region to saturate itself with water until theinternal capillary pressure effect balances the pressure exertedby the differential underbalance pressure. The severity of thepotential degree of imbibition will be a function of the initialdifference in saturation between the initial water saturation andthe "irreducible" saturation at the given underbalance pressurelevel. It can be seen that many reservoir systems, particularlyin lower penneability matrix situations, countercurrentimbibition can effectively counteract even very highunderbalance pressures. The absence of any type of sealingfilter cake tends to aggravate the problem as there is no barrierto filtrate access to the formation.

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 bereadily removable, rather than to go to the expense of drillingonly partially underbalanced and possibly creating a zone ofmore severe, deeply invaded and inaccessible damage.

FOAMED MUD SYSTEMS

The use of foamed mud systems has become more popularas interest in underbalanced drilling has increased. Foamedsystems tend to have certain rheological advantages (greaterapparent viscosity) over conventional nitrified systems. Inaddition, certain foamed systems tend to maintain foam integrityfor a fmite period of time after cessation of gas injection. Thisprevents a total collapse of the gasified fluid column duringshort term shutdowns. Also, laboratory tests have indicated thatnear wellbore entrainment of foam laminae in the actual rockmatrix can act as an blocking agent for subsequent fluid loss ifthe fluid contained in the wellbore abruptly flips to anoverbalanced mode. A schematic illustration of this phenomenais illustrated in Figure 3. Depending on the quality and Stabilityof the foam block eStablished, this phenomena can lastanywhere from a few minutes to potentially several hours.Caution must be observed with some of the surfactants utilizedto generate Stable foam systems as they may have the potentialto cause formation wettability shift to a more oil-wet state.This may have a deleterious effect on increasing water cut andreducing 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.

Water based filtrate imbibition can cause reductions in nearwellbore productivity due to water blocking effects. Inaddition. if the formation contains potentially sensitive clays orincompatible fluids, adverse reactions with the imbibed fluidsmay occur causing additional reductions in permeability. Sincethe majority of horizontal wells are open hole completions, evena relatively shallow damaged zone, which might normally beperforated through in a vertical well, could have a significantdetrimental effect on productivity.

Oil-wet systems do not typically tend to spontaneouslyimbibe water based fluids (unless they exhibit a mixed orspotted wettability condition), but they can spontaneouslyimbibe oil based fluids in a manner analogous to that describedfor water-wet systems. This typically does not pose a problemfor oil reservoirs (as the matrix is already highly saturated withhydrocarbon at a level which minimizes or eliminates imbibitioneffects) but may be of concern in some gas reservoirs whichcontain a low, immobile liquid hydrocarbon saturation andexhibit oil-wetting tendencies (i.e., sub-dewpoint depletedretrograde condensate systems). The presence of an initial lowsaturation hydrocarbon phase in an oil-wet system can act as

POO04-97 Recent Investigations Into Formation Damage in Horizontal WellsDuring Overbalanced and Underbalanced Drilling and Completion Procedures

5

imbibition causing "sites" for additional hydrocarbon imbibitionand trapping. This phenomena has been documented byMcCatIery5 .

imbibed water saturation) was utilized as a displacing fluid tosimulate underbalancedgas flow from the formation and neutralpH 5% KCI solution to simulate the potentially imbibing filtratephase. Pressure measurements were conducted usingcapacitance transducers having a range of 0 - 35 kPa or 0 - 350kPa with an accuracy of 0.5%. Tables 2 and 3 provide asummary of core physical parameters and test parameters forthe experimental program.

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

GLAZING AND SURFACE DAMAGETEST #1 (SWI = 0.0, k.r = 1083 mD)

Glazing and surface damage can occur if the circulating fluidhas insufficient heat capacity to cool and lubricate the bit-rockinterface. This results in a high localized temperature and thepotential formation of a relatively thin (1-5 mm), but often veryimpermeable zone. This may be a problem in someunderbalanced drilling operations where high gas rates areutilized or penetration rates are very low. This problemgenerally does not occur if parasite or concentric strings areutilized to generate the underbalanced condition, as the bit-rockinterface is totally liquid contacted in these situations. Glazingtends to generally be a problem in tighter homogeneousformations. If the formation exhibits fractures or open vugs,glazing does not appear to significantly occlude these featuresin most situations.

Table 4 provides the results of the test conducted on core # 1.This core was the highest penneability tested and was initiallyin a "dry" state (00/0 SwJ to simulate the worst possible scenariofor spontaneous imbibition.

A set of permeability vs time measurements conducted overa 96 hour period indicated that permeability declined to thegreatest degree directly adjacent to the simulated wellbore aswould be expected, but that substantial permeability impairmentextended up to 25 cm into the core. The core was run in avertical orientation, providing the most optimistic scenario asthe imbibing fluids had to counteract gravitational as well asunderbalanced flow effects.

Table I provides a summary of core air permeabilitymeasurements conducted on "dry" cut carbonate cores.Examination of this data indicates that the "dry" cut samples didnot experience any appreciable reduction in permeability,although the magnitude of the reduction did slightly increasewith a reduction in sample quality. These samples were totallydry when cut. which does not likely provide a goodapproximation to the "paste" generated when cutting water andhydrocarbon saturated samples in conjunction with circulatingdrilling fluids. Ongoing testing continues to be conducted inthis area to better quantify the effect of these more rigorouslysimulated drilling conditions.

The data of Table 4 also illustrates the effect of reducingoverbalance pressure on equilibrium spontaneous imbibition.As would be expected, the lower the overbalance pressure, thegreater the degree of spontaneous imbibition and damage. Therelative increase in damage with reduction in underbalancepressure was relatively small for this test, likely due to the highinherent reservoir quality. The permeability reduction profileshave been plotted as a function of time and underbalancepressure for Test # I and appear as Figure 6.

TEST #2 (SWI - 0.0, k., ;; 390 mD

Table 5 summarizes the results of Test #2. This test wasconducted on a lower quality core, but only two hours wereallowed at each imbibition point to observe the rate effect ofimbibition. This core was slightly longer than that used in core# 1 and had three internal pressure taps instead of two.

SPONTANEOUS IMBIBITION EXPERIMENTS

A series of five spontaneous imbibition experiments wereconducted to note the effect of countercurrent imbibition in awater-wet, gas-water system as a function of:

Results once again indicate the most severeimbibition/permeability reduction effect at the simulatedwellbore face. Damage increased substantially with reductionsin underbalance pressure and likely would have been even moresevere if extended time periods had been allowed forequilibrium as in Test # I. Depth of propagation of the damageappears to be rate dependent with exposure time with a muchshallower damage profile being observed in this test than inTest #1, even though core permeability was lower and onewould expect stronger countercurrent imbibition effects.

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

Figure 5 provides a schematic of the equipment utilized forthese tests. Pressure tapped core samples were utilized for twoof the tests to investigate permeability impairment as a functionof distance from the free water contact and three tests wereconducted using non-pressure tapped core to investigate theeffect of the magnitude of a pre-existing water saturation priorto water contact on countercurrent imbibition. This indicates that degree of imbibition induced damage will

be, as expected, not only a function of the relative underbalancepressure, but also of the length of exposure time. The greaterHumidified nitrogen (to avoid desiccation of any in-situ or

6 D.B. BENNION and F.B. THOMAS POOO4-97

the length of exposure, the greater the severity of the damagein the near wellbore region and potential extent of invasion.The percentage of penneability retained profiles of Table 5 havebeen plotted and appear as Figure 7.

are 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 Soj media)have also been illustrated to be potentially damaging even iftotally underbalancedconditions are continuously maintained.Foamed systems may be advantageous in certain situations toimpair fluid losses during periodic overbalanced pulses.

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 ofpermeability impairment due to imbibition was found todecrease with increasing initial water saturation (for dIe caseof a water-wet rock), although research tends to indicate thatthe ~ of imbibition may be increased by the presence of apre-existing saturation of the imbibing phase, even thoughthe ultimate severity (i.e., total magnitude of the damage)may be reduced.

TESTS 3, 4, 5 (Variable SWI - 0 . 38%,k..,= 22 .27 mD)

The fmal three tests were conducted on samples of relativelylow penneability. Three dolomite samples as similar aspossible with penneability ranges from 22 - 27 mD andporosities from 8.8 - 9.90/0 were selected for use in this part ofthe test. One sample (Test #3) was tested dry. The next (Test#4) had a 12% initial water saturation introduced and unifonnlydispersed prior to testing. The final core (Test #5) had a 38%initial water saturation introduced and dispersed prior to testing.

Complications in the analysis of the results from these threetests occurred due to the low penneability of the core and theresulting influence of Klinkenberg slippage effects on the gaspenneabilities at the increasing underbalance pressure. Higherunderbalance pressures (corresponding to a higher effective netpressure drops and flow rate across the individual core samples)resulted in lower penneabilities. Therefore, to eliminate theeffect of this flow rate induced artifact on the measuredlaboratory data the relative percentage of reduction inpenneability before and after exposure to water in anunderbalancedmode, at a given underbalancepressure, provideda more accurate evaluation of what was actually occurring inthe tests. This data is also contained as a portion of Table 6and has been plotted and appears as Figure 8. Examination ofthis data indicates that:

ACKNOWLEDGEMENTS

The authors express appreciation to the management of HycalEnergy Research Laboratories Ltd. for the funding of thisproject and for permission to publish the data. This paper hasappeared previously as SPE/CIM/CANMET Paper HWC 94- 7S,presented at the Canadian SPF/CIM/CANMET InternationalConference on Recent Advances in Horizontal WellApplications, March 20-23, 1994, Calgary, Canada.

REFERENCES

1. Initial penneability at a given underbalance pressure prior towater 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 adirect function of decreasing underbalance pressure,consistent with the results of the earlier tests.

3. The relative severity of the observed reduction inpenneability is reduced as the initial water saturationincreases. This is due to the decreased propensity for waterimbibition in a highly water saturated medium due to the"initial" water saturation being closer to the true "irreducible"value. If additional tests had been conducted at even higherinitial saturation levels that approached or equalled the b'Ueirreducible level, the data indicates that imbibition damageeffects would likely be negligible.

I. Bennion, D.B., Thomas, F .B. and Bennion, D. W.: "EffectiveLaboratory Coreflood Tests To Evaluate and MinimizeFormation Damage in Horizontal Wells," presented at theThird International Conference on Horizontal WellTechnology, November 12-14, 1991, Houston. Texas.

2. Bennion, D.B., Cimolai, M.P., Bietz, R.F. and Thomas, F .B.:"Reductions in the Productivity of Oil & Gas Reservoirs Dueto Aqueous Phase Trapping," Presented at the 44th AnnualGeneral Meeting of the Petrolewn Society of CIM, May 9-12, 1993, Calgary, Alberta, Canada.

3. Cimolai, M.P., Gies, R.M., Bennion, D.B. and Myers, O.L:"Mitigating Horizontal Well Formation Damage in a LowPermeability Conglomerate Gas Reservoir," presented at theSPE Gas Technology Symposiwn held in Calgary, Alberta,Canada, June 28-30, 1993.

4. Katz, D.L. and Lundy, C.L.: "Absence of Connate Water inMichigan Reef Gas Reservoirs - An Analysis," MPGBulletin, Vol. 66, No. I (January 1982), pp 91-98.

S. McCaffery, F.G.: "The Effect of Wettability, RelativePermeability and Imbibition in Porous Media, " Ph.D. Thesis,

University of Calgary, September 1973.

CONCLUSIONS

I. Underbalanced drilling has specific application in fractured,vugular or extremely high permeability systems where highlosses of both potentially damaging fluids and solids to theformation have the potential to severely impair ultimate oilor gas productivity.

2. Underbalanceddrilling can be damaging in certain situationsdue to the lack of formation of an impermeable sealing filtercake to prevent invasive losses if underbalanced conditions

TABLE 1"DRY" CORING STUDY TEST RESULTS IN

UNIFORM (CORE #1) ANDVUGULAR (CORES #2 and #3) LIMESTONES

TABLE 2SPONTANEOUS IMBffiITION TESTS

CORE PARAMETERS

TABLE 3SPONTANEOUS IMBmITION TESTS - FLUW AND

TEST PARAMETERS

TABLE 4SPONTANEOUS IMBIBITION TEST RESULTS

CORE #1

TABLE 5SPONTANEOUS IMBIBITION TEST RESULTS

CORE #2

TABLE 6SPONTANEOUS IMBIBmON TEST RESULTS - VARIABLE

INITIAL WATER SA TURA TION TESTs, CORES #3, #4 and #5

Underbalance1".-_ure

Core #30%5w,

Core H11% Sw.

Con #538% Sw.

Initial(mD)

Post Water

(mO)

Initial(DID)

POlt Watcr(IUD)

Initial(!DO)

Post Water

(mD).1.

Reductio!!(kPa) (.-I) % Red_do. "I. Reductior

345In68.9

SO2510

13.4215.0222.62

5315.24!,14

60365.177.3

7.0110.5113.94

36.848.257.1

4.435.445.91

2.313.696.09

1.7g1.992.79

22.S46.154.2

FQJREaEST~ OF ~ LAl8NAE AS A

tEAR ~ FWIO LOSS MS:~

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I\

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