CALGARY, ALBERTA, CANADA-Underbalanced drilling (UBD) is increasingly beingused worldwide as an alternative technique to conventional overbalanced drilling to reduceinvasive near-well bore formation damage problems in oil- and gas-producing formations.UBD operations can significantly reduce or eliminate formation damage concerns withrespect to phenomena such as mud or drill solids invasion, lost circulation, fluid entrain-ment and trapping effects, and potential adverse reaction of invaded drilling or completionfluids with the reservoir matrix or in-situ reservoir fluids.

When applied correctly, ~anced drilling and completion can provide a new ap-proach to solving complex reservoir management problems. Even so, UBD is not necessarilya panacea for all formation damage problems. If a UBD operation is poorly designed andexecuted, there is die potential for greater damage than if a properly implemented overbal-anced operation had been used in the same situation.

By rigid definition, an underbalanced condition is generated any time the effective circu-lating downhole pressure of a drilling, completion, stimulation or workover fluid (d1e pressureexerted by die hydrostatic weight of the fluid column plus whatever pump pressure is appliedto the fluid system to circulate or inject it, and the associated frictional pressure 'Mop) isgreater than d1e effective pore pressure in the formation.

Unless abnormally pressured, most formations are naturally placed in an overbalancedstate when water-based fluids of normal density are used in typical oil field operations. Insome abnormally high-pressured formations (and in some normally pressured formations),a naturally underbalanced condition can be generated using conventional oil- or water-based drilling fluids. This condition is termed "flow drilling" if it occurs during a drillingoperation, and it has been used successfully for many years in formations such as die AustinChalk in Texas.

Much oftoday's discussions related to UBD concern a condition where formation pressureis sufficiently low that an effective underbalance pressure cannot be achieved downholewithout entraining some type of non-condensable gas with the circulating drilling fluid tolower fluid density to the point where an underbalanced condition is obtained. This is oftenreferred to as the "artificial" generation of an underbalance condition.

FEBRUARY 1999 48

Emerging Techn<;>logies

Advantages, DbadvantagesThe advantages of UBD include:. Reduced invasive fonnation damage;. Minimal lost circulation and differ-

ential sticking;. R1iminAting the need for costly mudsystems and ~ disposal of exotic muds;. Improved rates of penetration, reduc-

ing drilling costs and increased bit life;. Mitigating extensive and expensive

completion and stimulation operations;. Rapid indication of productive res-ervoir zones during drilling;

. The potential economic benefit fromflush production during drilling; and

. The potential to flow test while drill-

mg

dents" in a UBD operation) may result insevere invasion and damage. This severeinvasion and damage often exceeds thedamage d1at would have been generatedif a conventional, well-designed. low-fluidloss overbalanced system had been used.

SoIv~ DrUUng Problems

Undetbalanced drilling is not a solu-tion to poor reservoir quality; the processdoes not inherently "manufacture" penne-ability. It can, however, yield significantproduction increases if the problem withconventional overbalanced completions isa high degree of fonnation damage orother drilling-related issue (severe lost cir-culation, extreme pelmt;ability zones, sig-nificant pessure depletion, major rock-fluid sensitivity issues Widt reactive clays,emulsions, or differential sticking) thatcannot be overcome by conventional fluidor operational practices.

Many operators attempt to apply UBDtechnology in extremely poor-quality for-mations, expecting high production rateswhen even if a well were drilled and com-pleted in a completely undamaged fash-ion, productivity would be uneconomicbecause of permeability limitations.

There are four main questions opera-tors must answer to decide whether toimplement UBD technology in a givenreservoir situation. First, considering pastexperience, can this particular well bedrilled and completed using overbalancedtechnology (not necessarily the same

FIGURE 1

overbalanced technology that was used inthe past and yielded marginal or disap-pointing ~1S)? Second, based on an en-gineering evaluation of reservoir param-eters, will UBD provide a significant tech-nical or economic advantage to theproject? Third, what is the risk of failurdproblems associated with d1e UBD opera-tion? Finally, does the expected increasein value justify the increased risk com-pared to conventional overbalanced drill-ing and completion practices?

Figure 2 is a schematic of a typicalUBD process using a closed surface con-trol system. The vast majority of the UBDoperations perf<Xmed to date have utilizedjointed drill pipe and injecting the basedrilling fluid and some type of non-con-densible gas directly down the drill string.This is not necessarily because this is thebest technology, but because it is the leastexpensive and easiest to execute with thenumber of conventional jointed pipe r0-tary rigs available.

Most of these through-string injectionUBD projects have used water-based flu-ids (often produced water) and nitrogen.The average gas injection rates have beenin the range of 1.5 million-2.0 millioncubic feet a day, although this is highlydependent on reservoir pressure and theamount of reservoir gas that can be pr0-duced to assist in the UBD operation oncethe producing zone has been penetrated.

Liquid circulation rates vary widelydepending on the bottom-bole pressure

Well- versus Poorly-Designed Under- andOverbalanced Drilling Operations

Ca88 I . Poorly Designed Ca.. II . Well DesignedOverbalanced Operation Overbalanced Operation

I ~ 0f8na F\JkI:::':; >- I ~ 0f8na F\JkI::> .

Case III - Well DesignedUnderbalanced Operation

c... IV - Overbalanced Pulse During.n Underbelanced OperatIon

Many studies have investigated someof d1e potential disadvantages of d1e UBDprocess, particularly if poorly designedand executed. They include:

. Well bore stability and consolidation

concerns;. Safety and well conb'ol concerns in

high-pesswe or sour environments;. Increased drilling costs (and complex

completions, if required, because of theessential nature of completing the wellsin a "live" condition);

. The inability to use conventionalmeasurement while drilling technologyfor through-string injection techniques;

. Spontaneous counter-current imbibi-tion effects;

. Gravity drainage effects in high-per-meability zones, even under constantunderbalanced flow conditions;

. Condensate drop out and gas libera-tion effects;

. Glazing or mashing (near-well boremechanical damage);

. An increased propensity for corro-sion problems if air- or oxygen-contentreduced air is used as the non-condens-able gas medium for generating an under-balanced condition; and

. Failure to maintain a continuous un-derbalanced condition, resulting in sig-nificant invasive damage because of pe-ricxtic overbalanced IX'es5U1'e pulses dueto pipe connections, bit trips, conventionalMWD signal transmissions, localizeddepletion effects, frictional flow effects,hydrostatic column effects, multiple dif-ferent pressured zo~, variable presswein a common zone, poor knowledge oforiginal reservoir pressure, or operationalor supply problems.

Figure 1 illustrates the effect of a peri-cxtic loss of an underbalanced pressurecondition. By failing to establish a pr0-tective filter cake on the formation face,unconb'olled and severe fluid losses (thatmay occur during overbalanced "inci-

FEBRUARY 1999 51

Special Report: Emerging Technologies

limited pressure range of 1.500-2.500 psioffe~ by the majority of rotating con-trol heads used in jointed pipe UBD 0p-erations.

Coiled tubing technology bas provenviable and practical for effectively drill-ing horizontal and vertical wells-particu-larly in re-entry work. out of smaller cas-ing sizes, cieepenings, and sidettacks wherethe objective is to barely enter the zone toget flow back to ~, and re-enteringexisting horizontal wells to drill exten-sions and additional lateral legs under-balanced.

The disadvantages of coiled tubing arethe time and commitment involved in us-ing a newer technology, the availabilityof ~t, coocans over accW'8te ~control and safety concerns in high-pres-sure wells with large volumes of pressu-rized gases at surface in the exposed. non-injected cr string. Also of concern aredepth control and reach limitations forbcxizontal drilling applications. A majorchallenge today for any cr operation isobtaining the experienced personnel andmobilizing dIe equipment on site to makecr technology a cost-effective contenderwith more readily available jointed pipesystems.

The operational system that is best fora particular application is determined bythe technical, safety and economic pointsof view. Technically, the preferred systemwill yield the best bottom-hole pressure

FIGURE 2

control with a continuously underbalancedcondition and allow accurate and continu-ous real-time bottom-hole location andpressure ~ments. This heavily favorscoiled tubing-particularly for small-diam-eter holes and re-entries-but cost, avail-ability and required horizontal reach mayeliminate cr. Alternate parasite sbing orannular injection techniques may also beconsidered, although these techniques areoften costly, and from an abandonmentperspective, may be more problematic forthe parasite string approach.

Jointed pipe operations will likely con-tinue to be used in the majority of sys-tems for the ~xt few years until cr costsand availability, plus improvements intechnology, allow it to assume a moredominant position in the UBD market-place.

Prime ApplicationsPotential prime applications for UBD

include:. Reservoirs with significant circula-

tion loss or fluid invasion potential (fnk:-tures, vugs, extreme permeability consoli-dated/unconsolidated intercrystalline for-mations, and zones of extreme JX'CSSuredepletion);

. Formations that exhibit extreme

rock-fluid sensitivity;. Formations that exhibit significant

fluid-fluid sensitivity;. Formations widl subirreducible oil or

Typical Closed System UBD Operation

required and the amount of fluid producedfrom the formation. The liquid circulationrate is a dominant factor in conb'O1lingeffective bottom-hole pressure. 1Ypicalliquid flow rates in many UBD operationshave ranged between 2.400 and 6,500ban-els a day. Detailed calculations andmonitoring are required to ensure that sig-nificant annular slug flow (which cancause large bottom-hole pressure swings)does not occur, but that sufficient blrbu-lence is maintained for adequate holecleaning and cuttings transport to mini-mize pipe sticking problems.

ReaI- 'DIne Data AcquisitionReal-time data recording and gather-

ing systems-both down hole and at the~ essential to ensuring successwith any UBD operation, allowing theperformance of the entire operation to bemonitored in order to maintain an opti-mum underbalanced condition.

Many early UBD operations did nothave the ability to condI.x:t real-time <k1wn-hole pressure measurements. The resultwas many supposed underbalanced wellsactually drilled in a complete or periodicoverbalanced state. Later improvementsused memory pressure gauges to acquiredownhole pressure profiles, but only af-ter it was ascertained that a continuouslyunderbalanced condition had not beenmaintained and damage had already beendone. Finding out after the fact that anoperation was not underbalanced is littleC<?nsolation for a failed job.

The development of electromagnetictelemetry (EMT) and extended-rangeEMf downhole tools capable of transmit-ting downhole pressure and survey datadirectly to d1e surface using an electro-magnetic pulse, or coiled tubing applica-tions with an internal wireline for directdownhole data transmission, have greatlyenhanced d1e technology for UBD by al-lowing real-time acquisition of pressureand location data while drilling. Wet-con-nect systems have also been used withvarying degrees of success.

Coiled TubingIn many aspects, coiled tubing is cus-

tom designed for an effective UBD proc-ess. The advantages of coiled tubing areobvious in that the single unjointed pipefacilitates the operation of the UBD proc-ess in a more continuously underbalancedmode. Using an internal wireline allowsaccurate and real-time MWD measure-ments, which can be less problematic thanEMT tools. Surface pressure operatingconstraints with coiled tubing are muchgreater (IO,<XX> psi) in comparison to the~

FEBRUARY 1999 53

water saturations;. Formations of highly-variable qual-

ity; and. Formations where low rates of pen-

etration are problematic.Reservoirs witl1 significant lost circu-

lation or fluid invasion potential wouldinclude zones of extreme intercrystallinepermeability (1,000 milliDarcies andgreater); large macroscopic open frac-tures; heterogeneous carbonates withmassive, interconnected and high-perme-ability vugularporosity; zones of extremepressure depletion (resulting in extremeoverbalance pressures of 1,000 psi orgreater); or tl1e worst-case scenario-acombination of one of tl1ese high-perme-ability features in a significantly pressure-depleted situation.

These reservoir candidates are primeapplications for UBD because of tl1e dif-ficulty (particularly in fractured and het-erogeneous carbonates) of designing ef-fective overbalanced fluid systems tl1atwill generate uniform and stable filtercakes to prevent significant invasion in tl1enear-well bore region by potentially dam-aging mud filtrate and solids, yet still bereadily removable to allow unrestrictedformation flow. When filter cakes doform, tl1ey often tend to result in prob-lems witl1 differential sticking, which canlead to expensive and sometimes termi-nal stuck pipe problems.

Rock-fluid sensitivity is a concern be-cause considerable formation damage canbe caused by tl1e adverse reaction of in-compatible water-based filtrates witl1 in-situ clays or otl1er reactive materials.Many formations contain hydratable clayssuch as smectite or mixed-layer reactive

clays that expand on contact with non-inhibited water-based systems and cansignificantly affect productivity, and insome cases, near-well bore consolidation.Some formations may also contain de-flocculatible clays and fines or velocitymigratable materials (kaolinite clay, de-trital rocks fragments, pyrobitumen, anhy-drite, etc). Many of these problems canbe addressed by appropriately using over-balanced technology with hydrocarbon-or inhibitive water-based fluids.

The invasion of incompatible drillingfluid filtrates into a formation can resultin potential incompatibility between theinvading fluids and the in-situ formationbrine or oil. Reactions could include theformation of extremely viscous oil-in-water emulsions that can become en-trapped in the near-well bore region andcause reduced permeability, the "de-as-phalting" of in-situ reservoir crude (causedby contact with incompatible invadingforeign hydrocarbon-based fluids), or theformation of scales and precipitations(caused by the reaction with water-basedfiltrates and in-situ formation brines).

Appropriate geochemical and compat-ibility testing can generally eliminate thisproblem for most conventional overbal-anced operations. Conversely, in somesituations where the potential for damageis extreme, UBD may be contemplated toavoid introducing the potentially reactivematerial into the formation in the firstplace.

Permanent entrapment of an increasedsaturation of water or hydrocarbons in thenear-well bore region can significantlyreduce the productivity of the formationas a result of adverse relative permeabil-ity effects. If the wrong base fluid is used.UBD may aggravate this problem becauseof spontaneous counter-current imbibitioneffects. But if appropriately designed,UBD technology can be an efficientmeans of mitigating potential problemswith adverse fluid retention and trappingeffects. Using a non-wetting fluid as thebase fluid for a UBD operation generallynegates the potential for spontaneousimbibition and reduces the potential forphase trapping as long as a continuouslyunderbalanced condition is maintained.

Formations of highly-variable quality-including highly-laminated formations ormore massive sandstone, or carbonate for-mations that exhibit a wide variation inreservoir permeability and porosity (alarge variation in pore throat size distri-bution)-represent major challenges todesigning effective overbalanced fluidsystems that effectively bridge and pr0-tect the wide range of pore features. In

D. BRANTBENNION

D. Brant Bennion is president ofHycaJ Energy Research LaboratoriesLtd., an international reservoir opti-mization, laboratory and engineeringconsulting company based in Calgary,Alberta, Canada. He has more than20 years of experience in formationdamage, driUing, enhanced recovery,flow in porous media, and reservoir

optimization.

general, overbalanced systems are de-signed in dtese situations to protect dtebetter-quality portions of dte matrix, sincedtis is dte zone from which dte majorityof production will occur. In some cases,this approach can significantly damageportions of potentially productive forma-tions. Using UBD can result in more uni-form production from dte target interval.

For some hard rock formations, sig-nificantly greater rates of penetration canbe obtained widt UBD, ~ignificantly re-ducing drilling time and associated costs.In fact, dte primary motivation for UBDin some operations has been improvedROP.

Non-Applicable ReservoirsMajor criteria that make UBD unad-

visable for a reservoir include combina-tions of extreme pressure and permeabil-ity, situations where a continuous under-balance condition is likely to be compro-mised, reservoir pressure constraints, andnormally pressured intercrystalline for-mations of less than 500 roD permeabil-ity and limited rock-fluid and fluid-fluidsensitivity.

AldlOugh deep, high-pressure and high-permeability zones represent perhaps oneof the best potential applications for UBD,from a formation damage perspective,safety and control issues with respect towell control at dte surface may becomeproblematic with high downhole pres-sures (particularly in gas reservoir appli-cations) using conventional rotary drill-ing equipment and rotating diverter heads.Using coiled tubing drilling in such situ-ations is preferable since surface pressureratings are much higher. Conversely, ifhigh surface injection pressures are re-quired, dten a large volume of pressuredfluid is present at dte surface in dte un-injected CT string, which also presentspotential safer;y hazards. ,

A situation where a continuous under-balance condition is likely to be compro-mised is one of dte single greatest flagsto a UBD operation. Much ofUBD's ben-efit is lost and dte operator may actuallybe in a greater damage scenario if under-balanced conditions are not maintained atall times during bodt drilling and com-pletion. There is little advantage to drill-ing in an underbalanced mode (unless in-creased ROP is dte only motivation), andthen using normal overbalanced practicesto complete a well.

Coiled tubing drilling represents whatmany feel is dte future in UBD becauseof dte ability to maintain a relatively con-~

SEE UBD PAGE 84M THE AMERICAN OIL & GAS REPORTER

.

monitored correctly. Failing to carefullyplan and design an operation can resultin improperly applying UBD technologyin a potentially viable situation, resultingin significant losses of capital and pro-duction potential. LJ

erly designed and executed, however,UBD provides a whole new approach tocomplex reservoir management problemsand may facilitate the economic exploi-tation of reserves unobtainable using anyodler technology.

The detailed study and design of theUBD process by a multifaceted reservoirteam is required, along with acquiring thenecessary data to ensure that the opera-tion is viable for the reservoir under con-sideration.IfUBD is considered, the proc-ess must be designed, implemented and

Editor's Note: The author acknowl-edges Vivian Whiting for her assistancein preparirig this article and the manage-ment of Hycal Energy Research Labora-tories for funding this project.

UBD .FROM PAGE 54tinuous underbalanced condition and de-ploy MWD using internal wireline. Pres-sure pulses during connections with aconventional jointed pipe can be mini-mized by using double or triple pipe stands,rapid connections and appropriate circu-lation practices prior to breaking for con-nections to minimize dte degradation ofunderbalance pressure that occurs duringor after a connection is made. Large-driverigs offer the advantage of drilling withtriple pipe stands, further reducing thenumber of connections required. Otherfactors include reservoirs where any typeof hydrostatic kill would be required forspecialized completions, bit trips, etc.

Reservoirs that may exhibit zones ofmultiple different pressures or a signifi-cant areal variation of pressure in a giventarget zone may be difficult candidates forUBD.

The case can be made that most for-mations can benefit from a perfectly de-signed and executed UBD operations.While this is probably true, the unfortu-nate fact is that UBD operations tend tobe considerably more expensive dtan con-ventional overbalanced drilling, and bytheir nature, may be fraught with risks andproblems. So, for normal formations, itis likely that a well-designed conventionaloverbalanced operation can yield compa-rable or superior results to a more expen-sive UBD operation.

. Underbalanced drilling is a very com-

plex process that should not be designedand implemented on a "gut feel" basis orbecause it appears to be the trendy ap-proach to a difficult problem. When prop-84 THE AMERICAN OIL & GAS REPORTER

FERC. .. .. .. . . .FROM PAGE 27

collected a 4-cent surcharge from the cus-tomers that were not obligated to take gasat Carleton in order to compensate theother customers for the higher gas pricesthere. Northern wanted to be able to dis-count this surcharge before it discountedits main transportation rate, but FERCwanted the opposite result. Some smallcustomers wanted exemptions, but FERCdid not grant any.

On appeaL the D.C. Circuit affinnedFERC. The court writes, "If the global

Williams Discovers Gas In Leon County, TexasTULSA-Williams says it has discov- rate of 25.8 million cubic feet of gas a

ered natural gas at its Nash No.2 well day at a flowing tubing pressure of 5,360located in the Kenwood Field of Leon psi on a 241M-inch choke while continu-County in East Texas. The discovery is ing to unload treatment fluids.the third for WIlliams' exploration and Williams says it owns 170,<XX> grossproduction unit since July, the company leasehold acres and 52,<XX> net leaseholdindicates. acres throughout the East Texas pinnacle

The Nash No.2 wcll has tested at a reef trend. !j

' settlement (of the Order 636 restructur-

ing case) covered any and all receipt pointcapacity allocations related to or arisingout of Order 636 restructuring, it wouldbind in perpetuity all parties with respectto problems and situations unknown andunforeseen when the agreement was cre-ated."

The bottom line is that the decisionsmade in the Order 636 restructurings inthe early 19908 don't necessarily applyforever. D