Under Balanced Drilling of Horiz Wells Does It Really Elimina

8/2/2019 Under Balanced Drilling of Horiz Wells Does It Really Elimina

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SPE 27352Underbalanced Drilling of Horizontal Wells: Does It ReallyEliminate Formation Damage?O.B. ennion and F B. Thomas: Hycal Energy Research Laboratories Ltd.'SPE Member~ 1884,Soci8Iy ~ Engi-., Inc.Th. PIPIf - PIWP8/8dor ~ 8I1heSPE ndo oIm~k8n on FOIIII8IDID8m8ge h8kI I ~ Lou8 7-10 F8bII8IY1884.The ~ - HI8cI8db ~ bv en SPE p~ C ~ ,..,.. d .-.mation ~ iI en 8bItr8d 8iDnl\l8d bv Ihe ~8). ~ d the peper.u ~ ~ notbe8I r8Mw8dbv ie Soci8Iy ~ Engi-. and .. -.IIj8:I to .-8dIon bv he d!Of(8). Them818f81.. ~ -*- ~ -'Y ~ anypoaIIkIn Ihe Society PeIJoI8Im ra,Is ~. ~ _belL PIp-. ~ 81SPE~ .. 81G18o1o ~1c8IDI bv Edk)ri8ICom"*- d fie Soci8Iy P8Iro8I8n P8IIn8HI to CXIPYe ~ to en 8b8b'8d01 mmore -1300 worc8. DMIr8IkxImaynmbe ~ The~ shouldCXIIII8iIonapiQIOU8duXJWtadgemen\MIera n by whom he paper ~. WrMallll8r8n. SPE,P.O.Box833838,RIjIanIIon. TX 7a3-3838. U.S.A.T8i8x.183245SPEUT.ABSTRACT 1. Fluid-ftuld Incompatibilities such as reaction of invaded mud filtrate with

In-situ fluids (oil or formation brine) to form scales, Insoluble precipitates,asphaltic sludges or stable emulsions.ecently, underbalanced drilling has been utiHzed as a technique tominimize invasive formation damage in both horizontal and vertical wells.While It Is widely accepted that underbalanced drilling greatly reduces thepropensity for formation damage, partk:ular1ywith respect to the potentialfor whole mud losses and invasion of mud solids, the potential for formationdamage still exists in situations where the original saturation in the reservoir

(either hydrocarbon or water), is less than the irreducible saturation of thephase utilized in drilling and the capillary pressure and wettabllitycharacteristics can cause spontaneous imbibition. This often occurs whendrilling in an underbalanced mode in low permeability gas reservcjrs thathave abnormaUy k7N initial water saturations. Although fluid flow Isoccurring frOOI the formation, circUating drilling fluds are In dynamiccontinual contact with the formation face. At abnormaly low initialsaturations, there exists the ability for strong, spontaneous imbibition effectswtich can, In some situations, coooteract pressures that are far greaterthan the apparent underbalance pressure occurring during the drillingprocess. If the formation contains potentially sensitive clay materials,deflocculatable fines, the potential for emulsions or permeabiHty eductionsooe to aqueous phase trapping, these phenomena ooUd possibly occur andreduce permeability. The fact that a stable filter cake Is not developedduring underbalanced drillng, due to continual Inflow frOOI he formationface, could increase the severity of these phenomena as a fuay exPosedformation face which remains unshielded by any type of brldsjng or sealingfilter cake is exposed for potential knbibition effects at all times. In addition,if any overbalanced pulses occur during drllHngor completion, the potential

2. Rock-Fluid ncompatibUities contact of potentiany sweUing i.e.,smectiticclay) or deflocculatablei.e., kaoliniteclay) mineralsby nonequilibrium queous hasesautlons mayhave he potential o severelyreducenear wellborepermeability.3. Solids Invasion - The invasion of artificial solids contained in tt1edrillingfluid (i.e., weighting agents or artificial bridging agents) or tt1e nvasionof formation solids (mk:roflnes) generated by the milling action of thedrill bit 00 the formatioo. The permanent entrainment of these solids Inthe formation can have a severely reducing effect 00 permeability in

some situations.4. Phase trapping/blockingi.3 This phenomena s the Invasion andentrapment f high 011 r water phasesaturationsn the near wellboreregion and can have a substantially educing effect on 011 r gasproductivity, articularlyor certain ypes of formations.5. Chemical Adsorption/Wettability Alteration - Most drilling fluids contain

a variety of chemical additives to improve mud performance andcharacter. In some cases these additives may be incompat~e with theformation fluids or rock. or exhibit a high propensity tor physicaladsorption. This can result In a number of oodesirable phenomenasuch as permeability reductions due to physical p~ymer adsorption. or


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UNDERBALANCED DRILLING OF HORIZONTAL WELLSDOES IT REALLY ELIMINATE FORMATION DAMAGE? SPE 273522reservoir is actually ~gher than the circulating fluid, the potential for boththe invasioo of whole mud and soHds s eliminated. This can greatly reducedamage, mud costs and volume required and ultimatecornpletionlstimulatloo costs.

WH AT I S U NDE RB AL ANC ED D RI LLI NG- -

Overbalance pressure is defined as the downhole pressure differentialbetween the circulating fluid stream (driDing.completion or stimuatlon fluid)and the in-situ pressure in the formation being contacted. The circulatingfluid pressure is a combination of the hydrostatic pressure induced by thephysical weight of the fluid column between the surface and the doWTi)oieformation and the physical pump pressure required to cause the fluidsystem to circulate through the well.

DisadvantaGes f UnderbalancedDrillingUnderbaJanceddrilling has several detrimental aspects associated with

Its use, some of these being:1. Safetyconcerns risk of bk7Nout.ire, explosion, oss of control,etc.).

Ttis is a particlJar orw:efnn oil or gas reservoirs ontainingHIS gas.Most formations are commonly drilled In an "overbalanced mode" due

to tle fact that conventional flud system densities usually create adownhole pressure whk:h is higher than the in-situ formation pressure. Thiscauses a natural tendency, if the exposed formation is permeable, forcirculating fluids (and solids) to Invade into the formation.

2. Exoense Underbalanced riHing an be much more expensive hanconventional verbalanced rilling. When nlb"ogens used o reducemudweight. ib'ogenoostscan be high.particularlyor extended eachhorizontalwells or low penetration ates due to hard formationsortecmical problems. Air has been consideredas a cheaperalternategasificationmedun. but cara must be taken o determineand monitorthe combustibleenvelop between air and the flowing hydrocarbonstreamunderconsiderationo eHmlnate xplosionand fire hazards.

Underbalanced drilling occurs when the effective downhole circulatingpressure of the fluid system in contact with the formation is less than theexisting formation pressure. Underbalanced oorxJitions occur naturally insome reservoirs when unwelghted fluids are utilized if the reservoir isgeostatically overpressured for Its depth. In other situations. underbalan:edflow can be obtained through the use of lower density hydrocarbon basedfluids n lieu of denserwater basedsystems. 3. Damage - Underbalanced drilling does not eliminate all types of damagein all reservoir situations and In some cases has its own unique damagemechanisms. This theme of damage during underbalanced drillingoperations will be the subject of the balance of this paper.In many cases, partioolarty when considering pressure depletedformations, it is necessary to artificially reduce the apparent density andhydrostatic pressure of the applied fluid system in order to generate anunderbaJarx:ed ondition. This is commonly conducted by entraining a lowdensity gas (either nitrogen, air or naUa! gas) in the circulating fluidstream. In some cases special surfactents are utlUzed o generate stablefoam systems wNm have high apparent viscosity. In others, the gas Ismerely injeded either Into the entire circulating ftlid stream. or part way Intothe vertical section or build section by the use of a parasite tubing string orspecial catcentric drill string configuration to allow single phase flow In thehorizontal section to facilitate cuttings transport, but still retain theadvantage of underbalanced drilUng by reducing the density of the majorityof the vertical fluid column.

DamaGe DurinGUnderbalancedDrillingA number of potentialdamagemechanisms xist when operating n an

underbalanced mode. These nclude:1. Lackof a protective ealing ilter cake f true underbalanced onditions

are not maintained 00%of the time duringdrillingand completion.2. Spontaneous countercurrent imbibition effects which allow theentrainment of potentially damaging fluid filtrate into the reservoir matrix

in the near well bore region.WHY UNDERBALANCE DRILL? 3. Glazi1g aoo surface damage effects caused by Insufficient heatconductive apacityof circulating luids.nderbalanced drilling has particUar advantages in situations where thepotential for severe fluid loss or total lost circulation exists. This wouldinclude reservoir situations such as: These points wil l be elaborated upon in greater detail.

Lack of A Protective SealinGFilter Cake. Highly fractured sarxtstone or carbonate formatk>nswhere the majorityof permeability is cootalned In the fracture system. In truly underbal~ed ~erations, since flow is occurring from theformation, an external bridging and sealing filter cake is not established.Fluids utilized in underbalanced operations typically do not contain fluid lossadditives or bridfjng agents as it is not normally anticipated that any typeof effective filter cake wi. be requirad or established.2. Heterogeneoos igh permeability ugularcarbonates.3. High penneabllity uncoosolidated or oonsolidated sands.


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imbi)1t!on an effectivelycomteract even IV~ underbalance ressures.The absenceof any type of sealing filter cake tends to aggravate heproblemas there s no barrier o filtrate access o the formation.

3. Mecharical, tecmlcaJ or supply problems res~tlng in a shukk>wnof gasinjection.Recent improvements In underbalanced ted\noIogy, su:h as ~e use of

coDed tubing, parasite strings, concentric strings and oon-mud pusedelectromagnetic measurement while drilling ted1nlques have been useful Inreducing some of ~e periodic overbalance pressure phases occurringduring some drinlng operations.

TNs water filtrate based imbibition can cause reductions in nearwellbore productivity due to water blocking effects. In addition, if theformation contains potentially sensitive clays or Incompatible fluids, adversereactions with the imbibed fluid may occur causing additional reductions inpenneability. Since the majority of horizontal weNs are open holecompletions, even a relatively shallow damaged zone, which might normallybe perforated through in a vertical wen, coud have a significant detrimentaleffect on productivity.

Agure 1 A) providesa sd1ematic epresentation f a POOfIyesignedconventional overbalanced mud system. Due to high overbalaree pressureand improper selection of fluid loss and bridging agents, extensive flushingof matrix, fracture or vug~ar permeability systems can occur. Whole mudand solids losses could be espeaally damaging to fracture and vugularpermeability in this type of a situation. Agure 1(8) illustretes a similarconventional overbalaooed system with a properly designed fI~d lossoontrol/bridging agent system. In this case solids invask>n depth(particularly in the fracture and wgular system) is minknlzed and extensivefiltrate penetration is eliminated. The filter cake is designed to be easilyremovable by reverse flow or conventional completion/stimulationtechriques.

Oil-wet systems do not typically tend to spontaneously imbibe waterbased fluids (uriess tttey exhibit a mixed or spotted wettability concltlon),but tttey can spontaneously imbibe 011 esed fluids in a manner analogousto tttat described for water-wet systems. This typically does not pose aprOOlem or 011 eservoirs (as ttte matrix is already highly saturated withhydrocarbon at a level which minimizes or eliminates imbibition effects) butmay be of coocem in some gas reservoirs which contain a low, immobileliquid hydrocarbon saturation and exhibit oil-wetting tendencies (i.e.,sub-dewpoint depleted retrograde coodensate systems). The presence ofan Initial low saturation hydrocarbon phase In an oil-wet system can act asimbibition causing .sites. for additional hydrocarbon inbibitlon and trapping.This phenomena has been documented by McCaffer/.

Agure 1 C) Rlustrates he same system in an underbalanced mode. Itcan be seen that If continuously underbaJanced conditions can bemaintained, that this likely represents the optimum scenario for maxlmizi1gpotential pro


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UNDERBALANCED DRILLING OF HORIZONTAL WELLS.DOES IT REALLY ELIMINATE FORMATION DAMAGE? SPE 273524uniformlydispersedprior to testing. The final core (Test 5) had a 38%Initialwater saturation ntroduced nd dispersedprior o testing.saturation) was utilized as a displacing ftud to simulate underbalanced gasftow from the formatloo and neutral pH 5% KCI solution to simulate thepotentially mbming filb'ate phase. Press~ measurements were condlx:tedusing capacitance transducers having a range of 0 to 35 kPa or 0 to 350kPa with an accuracy of 0.5%. Tables 1 and 2 provide a summary of core

physical parameters and test parameters for the experimental program.Complications in the analysis of the results from these three testsoccurred due to the low permeability of the core and the resulting inf\\Jenceof KDnkenbergslippage effects on the gas permeabilities at the Increasingunderbalance pressure. Higher ~erbalance pressures (corresponding toa higher effective net pressure drops and flow rate across the individualcore samples) resulted in lower permeabilities. Therefore, to eliminate theeffect of this flow rate inciJced artifact on the measured laboratory data therelative peroentage of reduction in permeability before and after exposure

to water In an underbalanced mode, at a given ooderbalance pressure,provided a more accurate evaluation of what was actually occurring In thetests. This data is also contained as a portion of Table 5 and has beenplotted and appears as Agure 6. Examk1atlon of this data indcates that:

Test #1 (5.. . .0. k... ~ 1083 mD)T able 3 provides the reBUts of the test conducted on core #1. This corewas the ti~9St permeabiUty tested and was Initially ~ a "dry" state (0%

Sw.) to simulate the worst possible scenario for spontaneous Imbibition.A set of permeability ersus ime measurements on~cted over a 96tX>Ur eriod Inclcated hat permeabilitydeclined o the greatest degreeclrectlyadjacent o the slmtjated wellboreas wotjd be expected, 0t thatsubstantialpermeability mpairmentextendedup to 25 cm into the core.The core was run In a vertical orientatkJn, roviding he most optimisticscenarioas the imbibing luids had to counteractgravitational s well asunderbalancedlow effects.

1. Imial penneabilityat a given underbalancepressure prior to waterexposure s reckJceds a functionof initial rappedwater saturationaswoud be classicallyexpected.2. The severity of the observed reduction in penneabllity is a direct~ion of decreasing underbalance pressure, consistent with the

results of ttle earlier tests. Although underbalance pressures evaluatedin ttlese tests were fairty low (a maximum of 50 psi). other tests haveIndicated that spontaneous countercurrent imbibition can occur, giventhe app~rlate conditions, at underbaJancepressures of up to 1000 psi.

The data of Table 3 also illustrates the effect of reducing overbalancepressure on equHibrkJm pontaneous imbibition. As would be expected, thelower the overbalance pressure. the greater the degree of spontaneousimbibition and damage. The relative increase in damage with reduction inunderbalance pressure was relatively smaO or this test, likely due to thetigh Inherent reservoir quality. The permeability reduction profiles havebeen plotted as a function of time aoo underbalance pressure for Test '1and appear as Figure #4.

3. The relative severity of the observed reduction in permeability isreduced as the initial water saturation increases. This is we to thedecreased propensity for water imbibition in a higNy water saturatedmedum due to the "initial" water saturation being closer to the true"irreducible" value. If additional tests had been conducted at evenhigher initial saturation levels that approached or equalled the trueirreducible level, the data indicates that imbibition damage effects wouldlikely be negll~ble.

Test #2 (S.. - 0.0. k... = 390 mD)Table 4 sunmarlzes the results of Test 12. This test was conducted on

a lower quality core, but only two hours were allowed at each Imbibitionpoint to observe the rate effect of Imbibition. This core was slighUy ongerthan that used in core #1 and had ttvee Internal pressure taps Instead oftwo. CONCLUSIONS

Underbalanced drilling has specific application h1 ractured. vugular orextremely high permeability systems where high losses of bothpotentially damaging fluids and solids to the formation have the potentialto severely impair ultimate oil or gas prodootivity.

Resultsonce again Indicate he most severe mbibltlon/penneabilityredtx:tion effect at the simulated wellbore face. Damage ncreasedsubstantiallywith reciJctk>nsn underbalance ressureand likely wouldhavebeenevenmoresevere f extended ime periodshadbeenallowed oreqlilibriwn as In Test 11. Depthof propagation f the damageappears obe rate dependentwith exposure ime with a much shallowerdamageprofile being observed n this test than in Test #1, even though corapenneabilitywas lower and one ~d expect stronger countercurrentimbibition effects.

2. UnderbalancedriDing an be dama~ng n certainsituations ue o thelack of fonnatlon of an impenneablesealing filter cake to preventinvasive osses f underbalanced onditionsare not maintainedat aDtimes. Countercurrent pontaneousmbibitionof water based iltrates(in water-wet and low SWI media) and 011based filtrates (in oil-wet, lowSo,media)have also been llustrated o be potentiallydamagingeveniftotanyunderbalancedonditions recontinuouslymaintained.Surfaceglazing effects can also contribute o reduced productivity n certain

This indk:ates that degree of imbibition InOOceddamage wiD be, asexpected, not only a fw1dk)n of the relative underbaJancepressure, but alsoof the length of exposure time. The greater the length of exposure, the


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5.B. ENNION and F.B. THOMASSPE 273524. One must evaluate the relative damage expected to be incurred during

a conventional deslooed overbalanced drilling operation against thepotential risk for damage from the poorly designed or executedunderbalanced driling operation. In many homogeneous formationswhere the potential for significant fluid Invasion is not significant. a well-designed overbalanced driRing program may provide limited fluid lossand Invasion and mW\imai damage, provide comparable or superiorresults to a more costly and risky underbalanced drilling approach.

5. A well-designed,engineered and executed underbalanceddrillingprogramcan eliminateor minimize ormation damage and increasepro


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TABLE 1SPONTANEOUS IMBIBITION TESTSCORE PARAMETERS

TABLE 2SPONTANEOUS IMBIBITION TESTS -FLUID ANDTEST PARAMETERS

TABLE 3SPONTANEOUS IMBIBITION TEST RESULTSCORE #1


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TABLE 4SPONTANEOUS IMBIBITION TEST RESULTSCORE #2Permeability o Humidified Nitrogen (mD)[% of original]Core Section ~ #4~ #2

45313241488[,410 r

456 [100.0]328 [100.0]465 [95.3]247 [60.2]

452 [100.0]326 [100.0]422 [86.4]216 [52.7]

462 [100.0]322 [99.3]386 (79.1]156 [38.1]Top8.3anNext 11.0 emNext 5.6 cmBottom 3.3 em11121314

Initial dry core, no in-situ water saturationEquilibrium after water contact at 82.7 kPa (12 psI) underbaJalx:eEquilibrium after water contact at 55.1 kPa (8 psi) underbalanceEquilibrium after water contact at 27.6 kPa (4 psi) underbalance-

TABLE 5SPONTANEOUS IMBIBITION TEST RESULTS -VARIABLEINITIAL WATER SATURATION TESTS, CORES #3, #4 and #5

Core #412'1. SwCore #538% Sw.Core #30% SWIUnderbalancePressurePostWater(mD)

PostWater(mD)

Initial(mD) '/.ReductionInitial(mD)%ReductionInitial(mD)

%ReductionkP8) (psi)36.848.257.1

2.313.696.09

1.791.992.7922.546.154.2

PostWater(mD)5.325.245.14

60.365.177.37.0110.5113.94

4.435.445.9834517268.9

50251013.4215.0222.62

100.0]100.0]100.0]100.0]


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Under Balanced Drilling of Horiz Wells Does It Really Elimina