Using Underbalanced Drilling to Reduce Invasive

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PElROLEUM SOCffiTY OF CIM PAPER NO. PTS 98-58

D.B. Bennion. F.B. Thomas. A.K.M. Jarnaluddin. .MaHycal Energy Research aboratories td.

increase rates of penetration, reduce invasive fonnationdamage and reduce significant problems with drillingdue to lost circulation and differential sticking. Manysuccessful in1plementation stories of underbalanceddrilling are evident in the literaturel.2; however,underbalanced drilling is not a panacea or all fonnationdamage problems in that inappropriately designedunderbalanced drilling jobs may actually result in moreformation damage than if a weU-designed andcontemplated overbalanced job had been used in thesame situation in some circumstances.

Abstract

UnderbaJanced drilling (UBD) is becomingincreasingly used as a technique to reduce significantinvasive formation damage in vertical and horizontalwells to improve production rates of oil and gas. andenhance njectivity in gas andwater injection situations.UBD may be a technically demanding procedure toexecute in certain reservoirs and much of the benefitwith respect to mitigation offormation damage may becompromised if the underbalanced drilling operation isnot screened; designed and conducted n an appropriatefashion. This paper reviews commonformation dam.agemeclKmisms which may occur in reservoirs and how, incertain situations, these types of damage may bereduced or eliminated through the appropriate use ofunderbalanced drilling technology. Various situationsin which underbaJanced drilling technology may result

in potential significant formation damage are alsodiscussed

This paper reviews some of the current technology inuse at the present time in underbalanced drilling andillustrates some ofdte points which operators should beaware of before embarking on an underbalanced drillingoperation.

What is Underbalanced Drilling (UBD)?

A rigorous definition ofUBD is the situation in whichthe exerted circulating pressure of the drilling fluid incontact with the formation is less than the effective porepressure in the adjacent section of the matrix. Thedesirable course of action is to have this occur along the

Introduction

Underbalanced rilling is utilized worldwide for thedrilling of horizontally and vertically oriented wells to


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although an increased use in coiled tubing in recentyears has been noted, particularly in the westernCanadian industry.

underbalanced rilling is often thought of as apreventative technique are generally defined as follows.

Fines Migration. Fines migration refers to the naturalmotion of pre-existing particulates contained within theformation. In general, fmes will tend to migrate fairlyexclusively with motion of the wetting phase, be this oilor water depending on the given reservoir situation.Significant loss of oil- or water-based drilling fluid. ifthis drilling fluid represents the wetting phase in ahighly overbalanced pressure situation, may result in thephysical migration of fines or, subsequent to this, theattempt to clean up the wellbore under high drawdownconditions to remove significant volumes of invadedloss filtrate may also result in particulate mobilization,blocking and plugging. The overall motivation foroverbalanced drilling in such situations would be toreduce or eliminate fluid losses to the formation toreduce significant concerns with fines migration duringboth drilling and subsequent cleanup operations. The

only potential risk associated with fines mobilizationproblems during underbalanced drilling is if a sufficientunderbalanced pressure condition exists, and themobilized phases from the formation (either oil orwater) represent the wetting phase, in which case,sufficiently high fluid production rates from thereservoir during a UBD operation may result in thepremature nitiation of fines migration. The case may bemade that flow rates of this magnitude would normallyoccur during the normal production operations of thewell subsequent o the drilling operations.

For many Canadian operations, closed surfacesystems, in which a four phase separator is used toseparate gases, liquids and solids, combined with arotating control head and onsite flare stack and tankagefor produced fluids are commonly utilized. In otherareas of the world, non-closed surface control systemswith open separators, sometimes called gas busters ,are utilized to conduct various types ofUBD operations.

What are the Motivations for Conducting anUnderbalanced Drilling Job?

A variety of possible reasons exist for a givenoperating company to conduct an underbalanced drill jngjob. A number of papers are present in the literaturewhich discuss this technology as well as presenting

screening criteria for the appropriate selection of thebest reservoir candidates for underbalanced drillini .

The primary motivations most often quoted forconducting underbalanced rilling operations nclude:

. Reduction n invasive onnation damage.

. Increase n effective rates of penetration nd overalldrilling rates and drilling time/cost eduction.

. Reduction n significant ost circulation problems.

. Reduce drilling problems in highly penneable orheterogeneous eservoirs containing fractures orvugular porosity. External Solid\ Entrainment. During conventional

overbalanced drilling operations, a net overbalancedpressure differential exists which has a tendency topotentially drive fluids and associated solids into thefonnation. Conventional drilling fluids contain a widevariety of potential suspended solids material such as aweighting agents, (barite, hematite, calcium carbonate,etc.), fluid loss agents (organic clays, bentonite, etc.) ora variety of granular and solid loss circulation materials(sized carbonates, sized salts, cellulosic fibers, oilsoluble resins, gas sublatable crystals) and other moreinteresting LCM materials, such as chicken feathers,walnut hulls, bamboo fibers, cardboard, golf balls, etc.In a normal hydrostatic overbalanced condition, there isan impetus for these fluids and associated solids to bedisplaced into the fonnation. In most relativelyhomogeneous mabix situations, the physical depth ofsolids invasion tends to be rather shallow ( I - cm intothe formation). The result is that this mechanism ofdamage becomes elatively inconsequential in situations

Formation Damage and Underbalanced Drilling

In many situations, the prime motivation forconducting an underbalanced drilling operation may beassociated with severe fonnation damage hat may havebeen observed in horizontal or vertical wells drilled inthe area. It must be emphasized that underbalanceddrilling is not a solution for poor reservoir quality. It isnot a stimulation technique and it does not manufacturepermeability. It only allows one to maximize the useand potential of the already existing permeability.Fonnation damage is a complex phenomenon havingvarious root causes and mechanisms, depending onparticular reservoir situations and lithology and ondrilling, completion and production practices utilized ina given reservoir situation. Many detailed discussionson formation damage are present n the literature6.7.' utthe primary mechanisms of formation damage or which

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where cased, emented nd perforated completions recontemplated, s the damage adius will typically notextend beyond the radius of a normal perforationcharge. However, he vast majority of underbaIanceddrilling operations re completed n an openhole mode,as are most horizontal wells. For this reason. hisparticular damage mechanism s one of significantimportance and one of the prime motivations for theapplication of UBD technology n some situations.

materials such as kaolinite, kaolin, halite, etc. whichmay be partially solubilized or softened by the contactof water-based luids. The use ofunderbalanced drillingattempts to minimize the contact of the formation withtbesepotentially solubilizing fluids, resulting in minimalinvasion and release of fines and on gauge hoteapplications.

Formation of Stable Emulsions. In certain situations,invading drilling fluids may combine with insitu fluidsto form highly viscous stable water in oil emulsions(Figure 4). These emulsions may have high apparentviscosity, and result in the formation of what is knownas an emulsion block in the near wellbore region whichmay significantly impair oil or gas production rates inthe affected zone. The use of underbalanced drillingavoids the inclusion or entrainment of these potentiallyemulsifying fluids in the near wellbore region, althoughsignificant emulsion issues may exist between producedfluids and circulating drilling fluids (which will need tobe addressed n the design of the basic underbalanceddrilling operation used in a given situation).

Phase Trapping. Phase b'apping refers to the pennanententrainment of an increased trapped water orhydrocarbon saturation in a porous media causingadverse and deleterious relative penneability effects(Figure 2). The blockage of the entrained fluid causesa reduction in the effective relative permeability to oilor gas resulting in a roDe of potentially significantdamage surrounding the wellbore region. The primemotivation of underbalanced drilling in such a situationis to prevent the significant loss of potentially damagingand trapping water or oil based filtrates into theformation. thereby minimizing and mitigating thepotential severity of damage associated with phasetrapping effects. Scales. lncompauole waters may result in the formation

of carbonate or sulphate based scales which may behighly damaging to injectivity or productivity in the nearwellbore region. The use of appropriately designedunderbalanced drilling minimizes the potential for fluidcontact (in the fonnation) and the associatedpenneability impairment.

Reactive Clays. Many sandstone fonnations containreactive clay minerals such as smectite ormontmorilinite, or potentially detlocculatible clays suchas kaolinite. These clays can be destabilized by contactwidt fresh or low salinity brines or, in some cases,associated widt rapid changes towards a caustic pHsituation. The motivation of underbalanced drilling is,of course, to minimize dIe potential losses of potentially

damaging water-based filtrates to dIe formation whichmay result in dIe creation of a wne of significantlyimpaired penneability and productivity in dIe nearwellbore region.

Wettability Alterations. Many drilling fluids contain avariety of polar surfactants and additives which are used

for corrosion, imbibition. torque reduction. emulsioncontrol, etc. Some of these components may have apropensity to be adsorbed on both carbonate andsandstone surfaces and cause a wettability alteration ortransition which may significantly alter the water-oilrelative penneability characteristics in the near wellboreregion. In situations where a mobile water saturation isapparent, this may increase effective water-oilproducing ratios of a given well and adversely affectwell economics (Figure 5). The use of properunderbalanced drilling technology prevents the contactand entrainment of the near wellbore matrix volumewith potential wettability altering fluids and reduces the

potential for damage.

Polymer Ad\'orplion. Many drilling fluids containrheology enhancing agents and polymers to improvefluid viscosity and reduce fluid losses in anoverbalanced situation to the formation. Thesepolymers may have a physical tendency to be adsorbedon the face of the formation and, particularly in lowpermeability rock (Figure 3), may result in significantpermeability impairment (localized in general o the near

wellbore region). The use of underbalanced drillingoperations eliminates both the inclusion of thesepotentially absorptive materials and the loss of thesefiltrates to the formation.

Bacterial Damage. Water-based luids may contain, fimproperly subjected to biological control, viablepopulations of aerobic and anaerobic bacteria whichmay be entrained in the near wellbore region andineral Dissolution. Certain fomlations contain soluble

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subsequently resuh in bacterial growth and propagation.This may result in die creation of polysaccharide richbacterial biofilrns, possibly reducing productivity orinjectivity, downhole and surface corrosion issues and,if the bacteria are sulphate-reducing in nature, thepotential reduction of elemental sulphate (that may bepresent in insitu formations or injected fluids) into toxichydrogen sulphide gas. The appropriate use of UBDtechnology prevents die long-tenD losses of potentialwater-based fluids which may contain viable bacteriacolonies into die formation. However, in mostsituations, if water-based fluids are contemplated forany UBD operation, the appropriate bacterial andbiological control is recommended as a low costinsurance should, for some, reason, the underbalancedcondition be compromised and fluids be imbibed ordisplaced into the formation.

fractured fonnations wiili high vertical penneability,ilie effect ofilie damage s relatively insignificant and,even in situations of high damage, relativelyproductive horizontal wells may be obtained.However, in situations wiili adverse vertical tohorizontal penneability ratios d1at are oftenencountered in many carbonate and clastic formationapplications, ilie damage extends o the greatest extentin ilie streamline of greatest permeability, greatlycompromising ilie productive nature of the horizontalwell. Even moderate amounts of near wellboreformation damage may therefore become radtersignificant in very adverse vertical to horizontalpermeability ratio situations.

Potential Issues Associated With the Use ofUnderbalanced Drilling

Underbalanced Drilling and Horizontal Wells A number of potential problems can be associatedwith the implementation of under balanced drilling with

the primary motivation to reduce invasive fonnationdamage. The single largest issue, in most situationswhere underbalanced drilling yielded poor results, havebeen cases where there is clearly documented evidencethat it has been difficult or impossible to maintain aneffectively underbalanced condition 100% of the timeduring the drilling and completion operation. Much ofthe benefit of the UBD operation may be compromised,and it is possible to be in an even worse situation (if theUBD pressure condition is periodically compromised)than if a conventional drilling operation had been used(Figures 6-9).

Horizontal wells have ong been ecognized s moresevere candidates or formation damage han theirvertical well cotmterparts. These phenomena ave beendiscussed n detail in the literature9. A number ofreasons xist why formation damage s known to be amore significant problem in horizontal wells than invertical wells. A brief synopsis of these damagemechanisms nclude:

Figure 6 illustrates a poorly designed and executedconventional overbalanced drilling operation where highamounts of both filtrate and potentially damaging solidsmay be invading mabix, fractures or interconnectedvugs in the near wellbore region, resulting in a zone ofsignificant near wellbore damage.

Figure 7 represents he better designed and conceivedoverbalanced drilling operation where appropriaterheology and bridging/LCM materials have been utilizedto create a stable and external filter cake which acts asan impenneable barrier to prevent significant loss ofdamaging solids to the matrix, fractures or wgs in the

near wellbore region. With an appropriately designedoverbalanced drilling fluid, the objective is that thisstable filter cake may be easily removed by directmechanical backflow of the formation or, due to itslocalized condition directly adjacent to the weUboreformation interface, subsequently emoved by some ype

. Longer duration of time required to drill horizontalwells resulting in potential deeper invasion ofdamaging filtrates and solids.

. The majority of horizontal wells are completed in an

openhole fashion resulting in relatively shallowinvasive fonnation damage. In a typically cased.cemented and perforated completion, this would notbe considered significant enough to result insignificant near wellbore penneability impairment ina horizontal completion.

. Localized drawdown effects resulting in partialcleanup of only a relatively small portion of thehorizontal section.

. Due to the large exposed inflow area of horizontalwells, the vast majority of stimulation treabnentstends to be fairly superficial in nature, in comparisonto highly penetrating stimulation treatments which canbe conducted relatively economically on equivalentvertical damaged wells.

. Anisotropic flow effects - In situations where thevertical permeability is not equal to the horizontalpermeability, damage is preferentially created out inthe streamline of greatest permeability. In vertically

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of a localized chemical or mechanical stimulationtreatment which may be used during the completionphase of the well.

reduce some of these problems). This being die case,the nitrogen injection must be stopped at each pipeconnection resulting in possible oscillation of dieeffective bottomhole pressure nd periodic loading ofthe fluid system with slugs of fluid which may result npotential overbalanced ulses Figure 10).

Figure 8 provides a schematic illustration of theoptimally designed and executed UBD operation. Sincethe fonnabon pressure in this situation is greater thanthe net circulating fluid pressure, here is a net inflow offluids from the fonnation. This eliminates the potentialinvasion of damaging filtrates or solids materials intothe matrix, fractures or vugs in the near wellbore region.However, it must also be emphasized that there is alsoan absence of potentially sealing and protective filtercake. This, therefore, may be disadvantageous when, asseen n Figure 9, if the underbaIanced pressure conditionis compromised, very rapid invasion of whole drillingfluids into the fractures, vugs or near wellbore matrixmay physically occur even f the underl>alanced ressurecondition is compromised even for a relatively shortperiod of time. To further exacerbate his phenomena,typical underbalanced drilling operations rely primarilyon turbulent flow to clean the hole and gravitysegregation to disengage gas and solids from thecirculating drilling fluids at surface. This being thecase, in general, very low viscosity drilling fluids(commonly produced water or low viscosityhydrocarbons) tend to be utilized as the base drillingfluids for most artificial UBD operations. If this is thecase, and the underbalanced pressure condition iscompromised, in addition to having no protective filtercake, very low viscosity fluid with high API fluid losscharacteristics which typically contains a relatively highconcentration of very fine powered solids (due to the

poor hole cleaning capabilities of many UBDoperations) is contained in the annulus. This fluid maybe displaced rapidly into the formation, resulting in aconsiderable degree of invasive formation damage n analmost instantaneous fashion, negating much or all ofthe benefit associated with respect to the use ofUBD toreduce fonnation damage in a given wellbore situation.

Conventional Mud Pulsed MWD Operatiom. Forhorizontal applications, it is often necessary o ttansmitgeo-steering infonnation back to surface toappropriately guide the trajectory of the horizontal wellto ensure that drilling operations proceed in the zone ofrnaximwn quality in the pay interval desired. The mosteconomic classical technology for use is theincompressible drilling fluid and a downhole pulse unitto ttansmit survey data back to surface via thetransmission of incompressible pressure waves throughthe fluid contained in the central portion of the drillstring. The use of artificial UBD technology, wheregases are often injected in the drill string, negates theuse of conventional mud pulsed MWD technology dueto the presence of a highly compressib1e gas phase n thedrilling fluid. In some situations, conventional mudpulsed telemetry has been uti1ized in UBD operationsby circulating the hole to a pure incompressible fluidprior to transmitting a survey back to surface on aperiodic basis. This, of course, results in die applicationof a full hydrostatic head of f luid to the hole, resultingin periodic overbalanced pulses of pressure beingapplied to the formation, which may negate much of theresults of the underbalanced drilling operation withrespect to reducing fonnation damage.

Kill Jobs/Bit Trips. Due to mechanical problems andthe necessity of bit trips in may situations, wells whichhave been drilled underbalanced have beenhydrostatically killed to facilitate these operations. Thisresults in the physical loss of the underbalancedpressure condition with the adverse effects mentioned

previously.

How is the Underbalanced Pressure ConditionCompromised Resulting in Aggravated FormationDamage Effects?

Localized Depletion Effects. The phenomena oflocalized depletion is schematically illustrated as Figure11. It can be seen, particularly for a horizontal wellapplication, that an extended period of time may berequired to drill the entire length of the horizontal

section. Portions of the formation intersected early onin the life of the underbalanced drilling operation willtherefore be exposed to an extended flow situation.This will result in the formation face directly adjacent tothe wellbore being subjected to localized drawdowneffects and, after a period of flowing time, the pressure

Pipe Connectiom. Most UBD operations are currently

conducted using conventional jointed pipe technology.When the underbalanced condition must be artificiallygenerated through the use of injection of a non-condensible gas. this gas is most often injected directlydown the standpipe (although some variance which willbe discussed later may also be utilized which tends to

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inclusion of a small amount of liquid (i.e. mist drilling)generally tends to mitigate this problem for mostapplications. Mashing is caused by sliding drill pipe orpoor drill string centralization, and refers to the workingof small drill cuttings and fmes into die formation facedirectly adjacent to the wellbore, causing a phenomenasimilar in extent and severity to glazing.

the refinement of these techniques with actual real timedetennination of the effects with surface operations andthe ability to maintain in a much closer fashion arelatively uniform effective bottomhole pressurecondition.

Parasite or Concentric Strings. Another variation forjointed pipe drilling technology is the use of amicro annular or concentric string or a cemented parasitestring to continuously inject nitrogen in the verticalannular section of ilie wellbore. This allows a 100%compressive fluid stream to be injected in ilie centralportion of ilie drill string and allows for conventionalMWD operations and limits problems with connections,while maintaining a relatively uniform bottomholepressure condition for an UBD operation. Typicalparasite and concentric string configurations for UBDare illustrated in Figures 16 and 17. These techniquestypically tend to be more utilized for new wellapplications and are often not utilized due to expense

and greater gas consumption, but have been successfullyapplied in a number ofUBD applications in the WesternCanadian Basin and elsewhere.

Techniques or More Continuous Maintenance of anEffective Underbalanced Pressure Condition

Increased Use ofCoi/ed Tubing (C1). The advantagesof coiled tubing are obvious with respect to die morecontinuous maintenance of an underbalanced pressurecondition. Since no pipe connections are required, amore uniform underbalanced pressure condition may begenerated and maintained. The use of an internalwireline associated widi cr also eliminates some of dieproblems mentioned widi conventional mud pulsedMWD techniques. A detailed discussion of die use of

CT for underbalanced drilling has been contained in anumber of literature articlesl2.13 and this technology isoccupying an increasing portion of die westernCanadian underbalanced drilling market. The majordisadvantages of CT are associated with restrictedhorizontal outreach. depdilimitations, potential safetyconcerns associated widi critical sour wells widilargevolumes of CT present on die surface, and frictionalflow and hole size limitations which result in relativelyslim hole applications in most situations.

Foamed Fluids. Foamed fluids appear to have naturalapplication for UBD operations due to their loweffective density, good rheology and hole cleaningeffectiveness. In addition, because most stable foamsystems retain apparent viscosity and rheology evenwhen circulation is ceased, cuttings, transportation andsignificant invasion of these fluids into the formation,even if the underbalance pressure condition iscompromised, tends to be reduced. In the past,

significant problems associated with foam-based drillingfluids have been associated with the stability of the foamand the ability to disengage gas and cuttinp at surfaceand significant handling problems. The development ofa new generation of pH regenerable foams which havehigh resistance to oil contamination has lead to anincrease in the potential application of these fluids forUBD technology. It is expected that foamed drillingfluids will occupy an increasing fraction of the UBDmarket in the years to come. The high apparentviscosity rheology of foam systems may be a limitationwith respect to high frictional backpressure effectswhich may, in some cases, make it difficult to maintain

an effective underbalance pressure condition in somehighly pressure depleted formations.

Use of Electromagnetic Telemetry Techniques forMWD. Some of the major limitations of UBD with

respect to mud pulsed MWD have been overcomethrough the use of electromagnetic telemetry technologywhich uses an electromagnetic pulse to transmit MWDdata back through the formation to the surface. EMTtechnology has been successfully utilized in many UBDapplications but may have problems in very highlyresistive fonnations when using oil-based fluids or atdepths below approximately 2500 meters.

Rapid Connections. Jointed pipe underbalanced drillingstill occupies approximately 80 - 85% of the UBDmarket. Recent refinements in techniques widt respectto the appropriate use of float shoes in die drill string

and very rapid connection times and specific circulationtechniques (which are utilized during connections) hasgreatly improved the stability of underbalance pressureoperations during conventional jointed pipe operationsand artificially generated UBD operations. The use ofreal time bottomhole pressure measurement has allowed

Glass Spheres. The idea of using gas filled silicaspheroids s a means of reducing he density of fluidsis not a new one, and has been utilized for a number of

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9. BENNION, O.B., THOMAS, F B., BIETZ, R.F.,JAMALUDOIN, A.K.M., Recent InvestigationsInto Formation Damage and RemediationTechnology or Horizontal Well Applications ,Paper presented at the 9111 nternationalConference on Horizontal Well technology andApplications, Houston, TX, Aug. 25-27, 1997.

10. BENNION, D.B., THOMAS, F B., Underbalanced Drilling of Horizontal Wells:Does t Really Eliminate Formation Damage? ,Paper presented t the SPE nti. Symposium nFonnation Damage Control, Lafayette, LA, Feb.7-10, 1994.

11 BENNION, D.B., THOMAS, F B., BIETz, R.F.,BENNION, D. W., Underbalanced Drilling,Praises and Perils: Lab and Field Experience ,Paper presented at the Sda nnual Conference onHorizontal Well Technology, Calgary, AB, Nov.

21, 1995.

12 BENNION, D.B., rnOMAS, F B., BIETZ, R.F JAMALUDDIN, A.K.M., Coiled Tubing - TheFuture of Underbalanced Drilling? , Paperpresented at d1e Sib nternational Conference onCoiled Tubing Technologies, Dallas, TX, Jan. 8-10,1997.

13, BENNION, D.B., UBD wid1 CT has Pluses,Mi

uses,

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Using Underbalanced Drilling to Reduce Invasive