This papr was prepared for presentation at the fAOC/SPE Asia Pacific Drilling Tetinology '98Cmfefence and Exhibition held in Jakarta, Indonesia, 7-9 September 1998, This pa~r was selected for prasantation by an SPE program Commitfae foll~~g ravi~~f information contalnad in an abstract submitted by the author(s). Contents of the paper, as presented, hava not ben reviewad by the Swiety of Pa!roleum Engineers and are subjact to mrraction by the author(s). The material, as presentad, does not necessarily reflect anỹ ifion of the S%iety of Pakolaum Engineers, ita ofimrs, or members. Pa~rs presented at SPE meetings are subject to publication review by Editorial Committaaa of the Society of Pebu[eum Engineers, Electronic reprcdutilon, distribution, or storage of any part of this paper for coinmercial pu~aes withoul the written consent of the Society of Petroleum Engineers is titsd.Permission to reproduce in print is restricted to an abstract of not more than 3c0 words; il[uatrations may not ba copied. The abstract must contain conspicuous awledgment of where and by whom the paper was presentad. Wte Librarian, SPE, P.O. Box 833S38, Ri~ardaon, TX 750S3-3S3S, U,S.A, fax 01-972-952-9435. AbstractAn efficient approach to wellbore stability analysis and management of shale instability by taking into consideration the dominant instability mechanism(s) has been developed. The mechanism(s) is dependent on the type of shale, in-situ stress environment and drilling fluid system used. These factors determine whether a drilling fluid program can be developed using a mechanical (stress-induced) wellbore stability analysis or complex time-dependent drilling fluidshale interaction and thermal mechanisms need to be taken into account.A range of wellbore stability analytical tools for efficient management of shale instability is presented. A pragmatic approach to use drilling fluid design charts together with a shale property database and property correlations for designing optimal drilling fluids to manage shale instability efficiently is described. me utilisation of the tools is demonstrated through a field case study in which strategies are developed to control wellbore instability in horizontal wells. Parametric analyses conducted demonstrate the effects of shale and drilling fluid properties on time-dependent wellbore (instability. They highlight the conditions in which the drilling fluid-shale interaction mechanisms are critical and need to be incorporated in the analysis.