Shales are widely blamed for drilling problems. Such problems are frequently attributed to mud/shale problems are frequently attributed to mud/shale chemical interaction, abnormal pore fluid pressures and excessive overburden or tectonic earth stresses. Problems experienced include 'lost time' due to tight Problems experienced include 'lost time' due to tight hole, sloughing, and even loss of the hole. This paper attempts to evaluate the practical application of mechanical stability studies to directional well planning and emphasizes the importance of a planning and emphasizes the importance of a 'combined approach' when tackling specific shales, where problem severity is frequently best understood in terms of both borehole stress and chemistry. An analytical technique has been developed, which utilizes wireline sonic, bulk density and gamma ray data from offset wells, to estimate the mechanical integrity of the wellbore with depth and hence the mud weights required to support the borehole wall without fracturing weaker beds. Increasing hole inclination usually narrows the choice of safe operating mud weights. The log was evaluated in CONOCO's Ikan Pari development field in Block 'B', South China Sea Indonesia, where shale 'bridging' and 'sloughing' had plagued earlier drilling campaigns. These recent plagued earlier drilling campaigns. These recent efforts to identify the causes of hole problems in Block 'B' demonstrate the cost saving potential of stability studies to development well planning and should be applicable to many other areas. A stability log from the vertical well in an offshore development field can be re-run at the planned well trajectories; this can provide valuable input to casing seat and mud weight provide valuable input to casing seat and mud weight selection, cement program design, fracture gradient calculation, drilling procedures through problem sections and directional well planning, - (particularly high angle profiles). Introduction Predicting the behavior of rock under applied stress is still an inexact science. Despite expanding research efforts in recent years, mechanical borehole instability is still responsible for a large percentage of todays drilling problems are probably often identified as either pore pressure or filtrate invasion problems, and the pressure or filtrate invasion problems, and the symptoms are treated rather than the disease in the case of overpressure the cure is, luckily, increasing [he mud weight, but in the case of filtrate invasion extensive water-loss treatments may be implemented needlessly. As the search for oil ventures into more structurally complex traps, at higher wellbore angles and with greater lateral displacements than ever before he importance of preventing borehole instability has become increasingly urgent. The application of laboratory derived rock constitutive models to the imprecise and hostile environment of real borehole deformation is advancing to meet this challenge. With the increasing ingenuity of wireline logging methods and the rapid growth and improvement of measurements while drilling (MWD) and logging while drilling (LWD) technology wellbore conditions and processes are daily becoming less mysterious. processes are daily becoming less mysterious. Logbased estimates of in-situ rock properties are essential to the improvement of current models of rock behavior in that they go beyond the 'macro' approach of trend lines, derived from laboratory tests on cored samples, to explore the 'micro' world of foot-by-foot variations in rock strength. Serious drilling set-backs can occur because of narrow units which may deviate from the general trends of pore pressure or rock strength. P. 283
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