@+vKthl 1998. SOC~[Y of Petroleum Engineers, IncThis paper was prepared for presenlalicm 81 me SP13SRM Eurock .98 held in Trondheim, Norway, &10 July 1998 'Ms paper was selected for presenmlicm by en SPE Prcgram CommiNea following retiew of Infonnawm contained in an abstract sulvnltlad by the author(s), Contents 01 the paper, as pmsanted, hava not been !Wiewed by the Society of Pelroleum Engineers and are subject 10 mmcfi.M by the author(s). The matetial, as presented, does not necessarily reffect any positim of the !3ciely of Pelmleum Engineam, its officers, or members. Papers pm.$ented at SPE meetings am subject 10 publication mvlew by Editodal Ccxnmitte.as of the Society of Petroleum Engineers. Eleclrcnic raproduclion, dlstnbutmn, of storage of any part of this paper la commercial purposes withcut Ihe written consent of the Society of Petroleum Engineers is pmhlblted. PermlsslmI to mpmduce m pflnt is restricted to an abslmct of not mom than 300 v?m% ilfustraths may not be coplacf The abstract must ccmtain conspicuous acknowledgment of where art-dby whom tlw paper was presented, Write Ubm,rian, SPE, P.0, SCM033826, Riiardson, TX 751YJ3.3K?4, U. S.A.. fax 01-972 -952 -943S, AbstractEarly efforts to driII wells in a gas field north of Bali Island, a tectonic region in Indonesia, were hampered by high costs associated with wellbore instabilities.In the recently completed late-phase development program, drilling eftlciencies and costs were improved with a strategy that included a rmk mechanics program, mud optimization, and batch &lIling. Wellbore instabilities occurred in the Ngimbang Shale, a hard, brittle, anisotropic formation, that is highly stressed. Field measurements of stresses, coupled with laboratory measurements of strength, demonstrated the presence of elevated horizontal stresses. The computed safe mud densities for weI1bore elastic stability ranged from 10 to 15 ppg, depending on location, well azimuth, hnd deviation. High mud densities did not necessarily alleviate instability, as indicated by stability-days, a method of tracking the stabletime periods of each well. Drilling-time and costs were also improved with the adoption of a synthetic mud system, a batch drilling schedule, and sound field engineering.Cost savings were substantial, and the wells are now in production.

IADCMembers %tiht IW, lADC/SPE Drilfii Confermce This pepar was prepared for Prasentaficfr at the 199S lADC/SPE Drilling Conference bald in Dal!as, Tesss 2-S March IM. This pspar was seleotad for presantafion by an fA~PE Program hmiffaa following ratiew of information contained in an abstract subm iffad by the auftmr(a). Mtanta of the papar, aa presented, have not bean re~ad by the International Association of Drilling Contractors or the ti!ety of Patmleum Enginaem end are aubjact to corraofiw by the author(s). The material, as presmtad, dues not necessarily reflect any poaifion of the IADC of SPE, fhah officers, or membars. Papers preaantad at the IADWPE m+lnga am subjaot to Wblicafim retiw by Ediorial Commiffeaa of the WDC and SPE. Electronic reproduction, distribution, or atomge of any part of this papar for wrrmemial purposes without the wriffan ccmaent of the Society of Petroleum Engineers la pmhibiad. Parmisslon to reproduce in print ia restricted to an abstract of nd more than~wordv Illuatratiwa may n~b oopied. The abstract must c~tain conspimroua acknowladgmant of where and by whm ffm paper was preaanted. Write Librarian, SPE, P,O, Sox 833030, Nchardsorr, TX 75~-333S, U,S.A,, fax 01-972-W2-9425. AbstractInitial development drilling programs in the Pagerungan Gas Field were plagued by severe hole problems caused by tectonic stresses and dispersive formations, shallow gas events and logging problems. An optimization study was conducted using a database generated &om 14 wells drilled over the past ten years. The rmults of this study showed that significant improvements in drilling performance could be achieved.A technique using wellbore stability days as an index was used to account for time-dependent effects of water-base mud systems on dispersive formations. This technique enabled an empirical refinement of a static-elastic wellbore stress model. Additionally, the use of a synthetic-base mud system eliminated the time-dependent effects allowing lower mud densities, casing string reductions, and dramatic increases in rates of penetration. A "batch" drilling/completion schedule accelerated the learning curve effect and optimized the program's logistical efficiency.By implementing the recommendations of the optimization study, improvements as high as 30% in drilling performance were achieved without compromising safety. As a result, significant financial savings were realized for the program.