Properly designed and executed underbalanced drilling operations can eliminate

or significantly reduce fonnation damage, moo or drill solids invasion, lost circulation,.

fluid entrainment and trapping effects, and potential adverse reactions of drilling fluids

with the reservoir matrix or in-situ reservoir fluids. me key to selecting appropriate'\

reservoir candidates is achieving a balance of technical, safety and economic factors.

. nderbalanced drilling is increasingly used worldwide

to ~uce near wellbore for-

mation damage problems in

oil and gas producing fonnations. It is

not necessarily a panacea for all for-

mation damage problems. In fact, if

an undeJi)a)anced drilling operation is

poorly designed and executed, there

may be potential for greater damage

than if a properly implemented over-

1:MIIancedoperation had been used.

If implemented properly, underba.l-

anced drilling can increase oil and gas

well profitability by reducing invasive

formation damage to maximize pro-

duction rates and by increasing ROP

to reduce drilling time and costs. It

can also eliminate various technical

problems associated with overbal-

anced drilling operations such as dif-

ferential sticking and I~t circulation.

There have been many cases in

which underbalanced drilling has been

particularly successful in providing

added value to operators, and the

resulting publicity has contributed to

the technology's rapidly increasing

popularity. Unfortunately, there have

also been many instances where

underbalanced drilling operations

have been less SI_~, not due to a

are in an overbalanced state whenconventional water-based fluids areused in typical oilfield o~tions. Iflower density oil-based drilling Buidsare used or high natUral fonnationpressures are present. an underbal-anced condition may be generated:This condition. sometimes called -flowdrilling.- has been used successfully inTexas for years in the Austin Chalk.This article focuses on the conditionwhere the formation pressure is lowenough that an effective underbalancepressure cannot be achieved downholewithout using a non-condensable gasto lower the eff-ective Buid density ofthe circuJating drilling Buid. This typeof generation of underbalanced condi-tions is often refen'ed to as "artificial-underbalanced drilling.

deficiency in the basic technology, butto the faully execution or inappropri-ate application of the underbalanceddrilling re

DRILLING TECHNOLOGY

Underbalanced drilling can yields~cant increases in ptOOuction ifconventional overbalaIx:ed approach-es have experienced formation dam-age 01" if problems such as ~tiaISticking have gccu1Ted: However, itwill not improve pI-c:;duction frommarginall'eselVOirs that sirriply have10 --"ill. The tWO ~ crite-w }"" ..~ ty;. .ria in deciding whether ~r not toimplement underbalanced drillingtechnology in a given reservoir situa-tion are:.Detennini ng if UIKIerbalanced drill-

ing offers a signifi~t technical oreconomic advan~e

DRILLING TECHNOLOGY

FIg. 2. Pha.. trapping can range from slight to .evere depending on the relativepermeabllty curves.

, .prevent spontaneo~ imbibition 8\ldreduce phase trapping potential aslopg as ~~ancedconditions ~maintained. This ~d revent direct pdispl~~ent and en~pment of the. .~~ fltrlq: in~!J1e fotfua9co~Form~tioD. ~f ~.hly Va~.~le~ty. Highly ~ted foFItlationsor massive sandstone or Carbonate for-mations that exhibit Wide variations inreservou permeability. p6~osity or~ thr()a.t size 4ist1ibUtion~~t.. .. cl1a11 to desi 1 ~ effecii~Jor eDges &,~&g yeOvCrbalanced $y8teIn$, In theSe ~-ips, o~¥~c~ SY$~ms ~ ~y~ed...to prqtect dle be~ q~tyP9~ons of the matrix, from .which ~tlle~jority ~f pioductio~ is ~~\.t9ocCUr. This CcaD resiiIt iq Si8ni~~t

d~ t9.other PO~~ns ofpotenti~1.ly p~uctive fom1ation. 'l1he Qse ofunderoaIanced ~ technolOgy 'insome of these Si~ti'c;i1s Can.'~Ult inmore un~form prOdhctionfrom the~t intelVal.Formations With Low Rep. Fors~me hard:rockforrf{atioJ{; ~tes of,penetration (Rap} up to 1iO: tim~s

~ can be ob~ with.~-anced' drilling compared to ov~~l-:anced drilling, ther:eby r~ducing~titnes and ~a~;c~.Jrla liniited;num~ of~ the p~moti~oq;fur ~~~ceCi '~ghas beep fOr ROP pupposes ~tneI'than formation daroage avoidance. .

based fl~i4s, sc:ale for:niatio~ an4solids precipitation by mixing incom-patible wa~ m~ filtrate wittlin-situ fonnationbrine: "' ;

r ~

Appropriate geochemical ~~and compatibWtY testing can eliJP;i-nate this prob.em fQr: most conven-tional oveI1balanced operations.-".However, in ~xtl'eme situatiops,unde~ced ~ ~y be ~to avoid. intrQdqcing potentially reac-tive material into the formation in thefirst place.Formations With Potential ForSpoa~ ImIXbltbL The mcech-anism of aqueous and hydrocarbonphase trapping has been discussed indetail in the ijterature.II.'J ~ermanententrapment of water or hydJ'ocarbonsin the near ~ region can resultin reduced fonnation produCtivity dueto adverse relative permeability effects(FIg. 2). If the ~Qg base 8uid is used,unden.Janced drilling may aggravatethis problem due to spontaneouscountercurrent imbibition effects.'However, appropriately designedunderbalanced drilling technology canbe an efficient means of mitigatingpotential problems with 8uid retentionand trapping effects. Using a non-wet-ting fluid as the base fluid for anunderba1anced drilling operation can

Contraindications For .Underbalanced DrillingCombination Of High Pressure ~lligh Permeability. Although deep,high-pressure, high-perm~abilitYzones represent one of the best po~n-tial applications for underbalanceddrilling from a fonnation damage per-spective. safety and well contrortsSUcsat the sUrlace may arise. Downholepressures above 4,300 psi. particularlyin gas 1"eseIVOirs. are more suited toconventional, rotary drilling equipmentand rotating diverter heads. The use ofcoiled tubing drllling in such situa-tions may be preferable, as surfacepressure ratings are much higher.Conversely, if high surface injectionpressures are ~ a large volumeof pressurized fluid at the swface inthe uninjected cr string may also pre-~t a safety hazard

can be caused by the adverse reactionof inc'ompatible watel'-based filtrateswith in-situ clays or other reactivemateclal~. Many fo~ations con~nhydioatable clays such as smectite ormixed-la~ reactive cla'ys' 'l1hese ~yswill'~ on ' contact with non-inhib-ited~ Water~f 8uids and can~-candy affect productivity and; in somecases, neal'"wellbore consolidation.Some formations may also containdeflocculatible clays and fines. or~table materials, such as kaoliniteclay, denital rocks ~ents, pyrobi-turnen !1nd anhydrite. Many of theseproblems can be addressed througheither underbalanced drilling or theapp~te use of 0Yerbalanced tech-nology with oil-based or inhibitedwater-based 8uids.F~~tio~ With FIuid.Fhdd SeDai.~ Und erl.1anced drilling can ~vent the invasion of incompatibledrilling 8uid filtrate into the forma-tion, which can mitigate adverse reac-tions of these invading 8uids with thefonnation brine or oil. One such reac-tion is the formation of highly viscousoil-in-water emulsions which maybecome trapped in the near wellboreregion. Other reactions include reduc-tion in penneability due to deasphalt-ing of reservoir crude by invading oil-

FEBRUARY 1997 35Hart's Petroleum Engineer International

DRILLING TECHNCLCJGY

used, p~ pulses ,d~ connec-tionS Can re mwmized by using dou-ble pip'e stands, rapid connections andappropriate circuJation practices priorto breaking for connections to main-tain "more uniform undel'balanced.conditions (Fig. 3). Top dri~ rlgsofferthe advan~e of drilling with niplepipe stands, which further ~uces thenumber of conn~tions required;

Underbalanced drilling should beavojded in reservoirs where any typeof hydrostatic kill on the fonnationwo~ld be required for specializedcompletions, bit trips, etc.Reservoir Preuure Conatraints.Asa general ~Ie,reservoirs havingmultiple zones of different pre~-sures-Or significant areal pressurc:variations in a given targe"t zone-are undesirable candidates forunderbalanced drilling.Normal Formations" MoSt fonna-tions would benefit from a perfectlydesigned, and executed underbalancoodrilling ope!:'ation, but underbalanceddrilling operations can be morcexpensive than conventional overbal-anced drilling. and by their natul"emay have certain risks and Pl'Oblems.For normal formations, such ashomogeneous intercrystalline fonna-tions with penneability l~ than 500md. and fonnations with low rock-fluid andOuid-Ouid sensitivity. a,prop-erly designed overbalanced operationmay yield comparable or superiorresults to a more ~ive and riskyunderbaJanced operatioti.

Fig. 3. Bo~ L" ~ spices that OCOIr when bt-eaking for connections ~ bereduced by cIn:uIatIng nitrogen prior to ,;u tions. This wil help maintain a contklUOtaund.b8I8Iceconcition. .

Non-continuous UnderbalanceConditions. This is the greatest indi-cation that an underbalance

DRILLING TECHNOLOGY

equipment~to ex~te the underbal-anced drllli~ operation.Step 6. Conduct a prespud mc:e~.Step 7. Frocure. transport, set up.and test the eqUipmenLStep ~,

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References1. Lunan. S.: "Surface Control Systems for

Underbal8rtc8d On1l1ng.. Journal of c.nedlan~ T8dvw*)f1I. ~T~ 1995).

2. TIadwob. R. . 81.: -u.. of a ~ 011 s.WIg~ for Und8Ib8I8nc8d Completion. " 01/ and

G8.,bma1.(1~3. S~ponJa, j;. et al.: "Considerations f!lr

U-~~-~~~ ~"~~ at die n.~ U-~~-~~ ,Conference and exhibition, The Hague.t~~ ,.:., ~ 2-4, 1995).

4. ~ P.L;. ~ "088gIq 81d FI8kj T~ of~ ~ ~ 10 ~ FormeIion ID~~e: ~~.. From the WesteroH Field.c.nada," ~ at the Ant Intemationel I~~~81d~.The H8gI8. :"-=-.~.-~ ~ 2-4.19951-

s. Deis. P,V.~ et al.: "The Development of anU~IancedDrilling Process: An OperatofS I.,J.u iI w..m C81Mi8.. ~ at h~Irst Int!rnatlonal Underb.lanced DrillingConference and Exhibition. The Hague,

r'!I84h8rt.1ds. (Oct. 2-4. 1995).6. a..v.Ion. 0.8.: "Und8Ib8lanced Drilling Ofers

~ ~Mn- . PIIt I.. 01 and G8 .bJm8/.tj8\. 2. 1 ~

7. Benniol1, 0.8.: "Underbala"ced On111ng Ofers~81diMn-.PartIl: CWandG8~~9.1~

8. Bennion 0;8. 8t al.: "Underbalanced Drilling ofHorizonuIW.Is. ItItRNIIy N~ng?-Journllof Cana!ilan Petroleum T,chnology,~19951-

9. a..IIon, O.B.: -U-~iI8Ii.-~~. ~ 81dP.iI. " ~ NIl. SPE 3S242 ~ . the SPE

PwIri8I B88I Oi 81d G8 ~ CCXIt...I(8!.MkI81d. T-. (MardI 27.29, 1996).

10. B8Mi0n, 0.8.. 8t II.: "Adv8nc.. in L8b«8tofyCoref1owEvlluatlon to Minimize ~ormltlonc.,wge~~~ ~~~ " ~ No. 95-1~ ~. die

~~~~~C8'IcII, ~ 19-21. 1995). .

11. 8!Mion. 0:8., at II.: -Water and HydrocarbonPha~ Tra~ inPocvuI;Medla . DIagnosis,~ 81dT~" Paper No. OM 95-69~.h~Arn8T~ ~ofdie ~ Society of aM-, B81tf, C8'IcII, 0I.-y

14-17.19951-12. ~ O.B.. .81.: -R8ciIdkx. ilh ~

d Oi 81d LON :' IIIy ~ ~ ~ m~ "- T~ " :JOI.naI of' ~

~ ~. ~ 1994).

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ABOUT THE AUTHOR. Brant Bennion joined HycalEnergy Research Laboratories,Ltd. in 1979 and is ~dy prS-dent. He is reSponsible for re-search and development in multi-phase flow in porous media andformation damage. He graduatedwith distinction &'Om the Upiver-sity of Calgary with a BS in chemi-cal engineering. Author of morethan 80 technical papers, he pre-sents lectures aiound the world.Bennion is a member of AGEGGAand SPE and serVes as a directoron the national bOard of the Petr0-leum Society of CIM.

97-Clrd8119

Hart's Petroleum Engineer InternationalFEBRUARY 1997 39