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  UNDERBALANCED DRILLING hasemerged as a critical technology thatcould bring significant gains in drilling efficiency.IADC members recognize the potentialof underbalanced drilling technology and have formed the UnderbalancedOperations Committee to promote thesafe and efficient use of the technique. A number of projects are underway by the Committee, including the develop-ment of a standard well classificationsystem and supporting nomenclature. A session, “Underbalanced Well Opera-tions and Engineering,” is a part of the2001 SPE/IADC Drilling Conference.Papers prepared for this session, to bechaired by D Heeman , Tesco Drilling Technology  and J Boyle ,  Weatherford ,focus on safety, well planning, reservoircharacterization, pressure prediction,and other critical areas. MOVING UBD OFFSHORE In 1996, a multidisciplinary team in Shell Expro ’s Gas Supply Group begana project to adapt existing onshoreunderbalanced drilling technology to theNorth Sea offshore environment. Using an evolutionary stepwise implementa-tion path, the technology was maturedfrom low-head drilling operations in1997 to two fully underbalanced drilling strings at present.In SPE/IADC paper 67688 prepared forthe Conference, J Kohnert , Shell UK Exploration and Production highlightsthe safety critical learning that emergedduring the project. The paper, “Safety Critical Learning in Underbalanced WellOperations in Shell Expro,” focuses on:ãThe team structure and implementa-tion plan;ãMud and nitrogen system;ãBOP and snubbing unit;ãPressured flow line and separatorpackage;ãUBD well design. OFFSHORE INDONESIA Repsol-YPF-MAXUS SoutheastSumatra B V decided to implementunderbalanced drilling technology in thedevelopment drilling campaign in the Kris-na Field offshoreIndonesia. The targetproduction zone isfractured carbonate.Previous wellsdrilled in this conces-sion have had mas-sive losses of drilling fluids once the pro-ductive zone isencountered. Thezone also has a very high clay content, which may be proneto fluid damage.In SPE/IADC paper67689 prepared forthe Drilling Confer-ence, G Soerdasono , YPF-Maxus South-east Sumatra; P RBrand , Blade Ener-gy Partners ; and B Allison , NorthlandEnergy Services present the pre-engi-neering, project man-agement and fieldimplementationresults for thedrilling of the KrisnaD12. “Planning andImplementation of the Repsol-YPF-Maxus Krisna Under-balanced Drilling Project” describes thefirst use of the technology offshore inthe region.In an effort to eliminate lost return,eliminate stuck pipe, identify fracturesand eliminate formation damage, Maxusdecided to proceed with the Krisnaunderbalanced drilling project.Since it was the first attempt of under-balanced drilling in Southeast Asia froman offshore unit, significant planning and engineering were required.The Krisna D12 production interval wassuccessfully drilled underbalanced from10-20 June, 2000 from the R & B Falcon  jackup Roger W Mowell. A total of 1,159 ft of formation wasdrilled underbalanced without any sig-nificant drilling related problems.The well was TD’d before reaching theproposed total depth because the base-ment high was encountered. There wereno instances of lost returns or stuckpipe once the well was in an underbal-anced condition. Although the project encountered prob-lems during the initial start up, modifi-cations to the system and changing thefluid system to a diesel mist eliminatedthe majority of the issues.By the end of the well, the underbal-anced system was working well, theauthors report. PROFILING PRESSURES Understanding reservoir characteristicsis a key to successful underbalanceddrilling operations.In SPE/IADC paper 67690 prepared forthe Drilling Conference, author  W  22  D R I L L I N G  C O N T R A C T O R  January/February 2001 Use of underbalanced drilling technology expands This underbalanced equipment layout on the R&B Falcon Roger Mow-ell was used for the first UBD project from a jackup in South East Asia.  Kneissl , Schlumberger CambridgeResearch , describes a method for deriv-ing both pore pressure and permeability profiles of a reservoir in real time—on afoot-by-foot basis—during underbal-anced drilling operations.Such information about near wellborecharacteristics of the reservoir offerinvaluable input into drilling and com-pletion decisions.The algorithm described in the paper,“Reservoir Characterisation WhilstUnderbalanced Drilling,” can be run atthe well site with a laptop computer andhas been extensively tested against realUBD data, according to the author.The mathematical methodology present-ed is an extension of that described inearlier papers, but the new algorithmalso determines local pore pressure val-ues, which allows for a more accuratedescription of the local drawdown, andthereby a more accurate derivation of the permeability than has previously been possible.Indeed, says the author, previous workalong these lines would misinterprethigher pore pressure regions of thereservoir as higher permeability zones.The author also discusses the implica-tions of compressibility in the mudsused during underbalanced drilling forthe extrapolation of flow rates meas-ured at surface to production ratesdownhole.This is a critical factor in accurate char-acterization of the reservoir, since it isthe local production rates downhole thatare used to characterize the response of the reservoir to the drawdown condi-tion. FOAM SYSTEM  An extensive experimental programdeveloped at Petrobras R&D Center aspart of the joint industry project (JIP)named “Lightweight Fluid Drilling fromFloating Platforms,” is summarized inSPE/IADC paper 67691. “Foam Rheology Characterization as a Tool for Predict-ing Wellbore Pressures While Drilling Offshore Wells in Underbalanced Condi-tions,” prepared by  A L Martins ,  A M FLourenço , and V Silva Jr , Petrobras,discusses procedures and results.The lab work steps included foaming agent selection (considering optimumconcentration and salt/oil contami-nants), rheology lab equipment develop-ment or adaptation and a definition of test procedure and matrix.Based on a set of real scale tests underfoam circulation conditions and numeri-cal simulations, the authors propose acomprehensive model for predicting borehole pressures while drilling under-balanced or near balance with foams.The use of foam is a highly attractivealternative for drilling depleted, lost cir-culation or gas zones. Due to its com-plexity, however, a foamed systemrequires special attention while design-ing field operations.The main goal of the project is to makeit possible to drill with foam from float-ing platforms. January/February 2001  D R I L L I N G  C O N T R A C T O R  23   After about 60 rheology tests performedin lab and pilot scale equipment, corre-lations were proposed to predict rheo-logical parameters as functions of foamquality.Based on the experimental data, a set of correlations was developed to predictfoam rheological properties as functionsof base fluid rheological properties andfoam quality. The next step was to eval-uate the lab results by comparing pres-sure predictions with experimental dataobtained by circulating different quality foams in a real scale vertical well locat-ed at the Petrobras Training Center.The proper fitting of the proposed modeland the experimental results supportedits implementation in Petrobras light- weight fluid drilling software.The authors suggest the possibility of applying the technique in deepwaterconditions will open a new strategy forthe development of depleted areas in off-shore fields worldwide.Proper knowledge of downhole pres-sures is important for adequate systemdesign and operation. AERATED MUD Predicting flow patterns in directional wells is always important. But the use of aerated muds poses special challenges.  Alternate SPE/IADC paper 67189, “New Developments in Aerated MudHydraulics for Drilling in Inclined Wells,” describes a study to determinethe hydraulics of aerated mud in aninclined annulus.The paper was prepared for the Drilling Conference by  Ashwin Sunthankar , Ergun Kuru and Stefan Z. Miska  , The University of Tulsa  ; and  Arjan Kamp , PDVSA/Intevep . When drilling with aerated drilling flu-ids, bottomhole pressure is a strong function of gas-liquid flow patterns. Theexisting methods of predicting flow pat-terns are mostly based on the extrapo-lation of results from pipe flow to flow inannuli.Due to non-linear relationships betweenflow rate, pipe size and pressure drop,this practice may not be appropriate toapply in drilling operations, according to the authors.To verify the applicability of existing practice, the study described by theauthors focuses on hydraulics of aerat-ed drilling fluid flow through an inclinedannulus, typical for drilling operationsin directional wells.Extensive experiments were performedin a unique field-scale low-pressure flow loop (8-in. x 4.5-in. annular geometry, 90ft long) in inclined positions (15°, 45°from vertical) with and without drill piperotation.The liquids used were water and aque-ous polymer solution (CMC+XCD+ water) at flow rates in the range of 100-325 gpm and air in the range of 8-85scfm.Gas/liquid ratios are chosen to simulatethe exact gas/liquid ratios under down-hole conditions, correcting for the factthat the actual pressure in the flow loopis much lower than the bottomhole con-ditions.Pressure drop and average liquidholdup over the entire annular section were measured. The authors report thatthey are not aware that such data hasbeen published before.The two-phase flow patterns were iden-tified by visual observations. Bubbly flow and slug flow are the two flow pat-terns observed over the ranges of thechosen test matrix. The presence of slug flow does not justify many of the exist-ing field and simulation practices thatassume homogeneous gas-liquid flow. Also, the flow pattern boundaries wereshifted compared to pipe flow. The tran-sition between bubbly flow and slug flow (for 15° inclination) was observed at a void fraction of 0.32 as compared to thatreported for pipe flow of 0.25.But for air-aqueous polymer fluid flow, it was the same as for air-water pipe flow (0.25).For flow with drill pipe rotation, churnflow was observed instead of slug flow due to the churning of slugs by the rotat-ing drill pipe. The authors report thatthere was no significant effect of drillpipe rotation on the pressure drop forair-water flow, while the pressure dropdecreased in case of air-aqueous poly-mer fluid flow with drill pipe rotation. A higher pressure drop was observed incase of air-aqueous polymer fluid flow compared to air-water flow. An existing unified pipe flow model wasmodified based on the experimentalresults and was evaluated againstexperimental and field data.The comparison shows that the pres-sure drop prediction by the modifiedmodel for inclined wells is better thansome other existing models, though itstill under-predicts the experimentalpressure drop measurements. REAL TIME SUPPORT  A successful underbalanced drilling operation requires control of downholepressures and management of the fluidsflowing from the well. Downhole pres-sures and the amount of fluid flowing out of the well are affected by injectionof fluids, reservoir inflow performanceand operational procedures. These parameters are inevitably subjectto fluctuations that cause transientresponses to the multiphase system.In Alternate SPE/IADC paper 67693,“Underbalanced Drilling: Real TimeData Interpretation and Decision Sup-port,” the authors describe a model usedto predict these changes. The paper wasprepared for the Drilling Conference by  R J Lorentzen and E H Vefring  , RF-Rogaland Research ; C V M Lage , Petrobras ; and K-K Fjelde , RF-Roga-land Research .During an underbalanced drilling opera-tion several parameters like injectionrates, downhole pressure, choke pres-sure and outlet rates can be measured.Methodology has been developed forincorporating the measured data in atransient model.The transient model is continuously updated with the measured data. Theidea is to estimate the different parame-ters in the multiphase model and thereservoir model to give a correct inter-pretation of the conditions in the well. Based on this interpretation, the futurestate of the underbalanced drilling sys-tem can be predicted.The model can be used to predict theconsequences of change in the under-balanced drilling system such as con-nections and shut down.The methodology has been applied tomeasured data from a 1,300 m deep well. Several transient cases have beenconsidered with drill string injectionand parasite string injection.   24  D R I L L I N G  C O N T R A C T O R  January/February 2001  May/June 2000  D R I L L I N G  C O N T R A C T O R  25
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