Wellbore instability in certain formations offshore India is quite frequent and can cause serious problems to drilling, logging and production operations. The removal of rock during the drilling operation disturbs the natural rock formation that is being subjected to a state of compressive in-situ stresses at depth. The redistribution of the stresses around the borehole thus produces stress concentrations at or near the borehole wall. A lack of effective control to counter the high stress buildup around the wellbore may lead to a number of hole problems including stuck pipe, borehole collapse, sloughing shale and excessive fill. Wellbore instability can be managed by determining the critical mud weights that provide sufficient wellbore wall support to counteract the redistribution of stresses resulting from the creation of the wellbore. These mud weights are mainly dependent on the in-situ stress regime, in-situ pore pressures, borehole azimuth and inclination, and formation properties and drainage conditions. In this study, a geomechanical model was generated using all the available log and drilling data from a recently drilled well offshore India, which experienced significant drilling problems. The geomechanical model was then used to provide wellbore stability and pore pressure planning and drilling parameter optimisation for the nearby proposed well. The study evaluated the rock-mechanical properties and in-situ stress state of all the prognosed formations. It then interpreted the conditions required for wellbore stability during drilling the well. The geomechanical evaluation workflow included real-time calibration of the prognosed wellbore stability model by incorporating MWD and LWD logs. Moreover, at the end of each drilling section, the prognosed seismic velocities were calibrated to the acquired VSP and sonic logs and then used as inputs for wellbore stability evaluation over the next section, ahead of the bit. Drilling performance and wellbore conditions were improved significantly, which led to significantly lower drilling costs and better quality of logs. The caliper log was gauged all the way to TD indicating perfect hole conditions without any washouts over intersected shales, sands and silts. In fact, the hole was so gauged that several people looking at the caliper thought that it was logged in casing.
Real-time Drilling Performance Optimisation by Wellbore Stability Evaluation and Integration of Wireline, LWD and Seismic data in an exploration well, offshore India
Klimentos, Theodore, Saxena, Kamlesh, Gosh, Debajit, and Somesh Bahuguna. "Real-time Drilling Performance Optimisation by Wellbore Stability Evaluation and Integration of Wireline, LWD and Seismic data in an exploration well, offshore India." Paper presented at the SPWLA 46th Annual Logging Symposium, New Orleans, Louisiana, June 2005.
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