Wellbore instability is one of the most critical challenges affecting drilling and production. Borehole instability in both shale and sand formations, hole collapse, lost circulation, stuck pipe, sanding, and other related well failures can be avoided by meticulous and careful design of drilling plan, specially by determining the critical mud weights and mud fluid components which will provide sufficient support for the wellbore during drilling operation. This paper presents a systematic program and guideline of comprehensive geomechanics investigation and evaluation that includes drilling and log data analysis for rock strength and earth stress model construction, stress regime influence, laboratory triaxial stress-strain tests on well core samples considering bedding plane inclination effects, failure criterion consideration, and optimum mud-weight (MW) window for secure and stable well drilling. The continuous well production can also cause problems for in-field drilling as the severe pressure depletion and reservoir compaction would inevitably induce significant reduction in stresses and formation fracture gradient both inside the reservoir and significantly upward into the caprock formations depending on the reservoir/formation stiffness contrast, reservoir size, thickness and depth, etc. This paper addresses those issues with the optimum MW reexamined and recalculated based on the largely altered in-situ stresses and wellbore strengthening application if necessary. This paper will describe theories, some issues in wellbore stability work, discuss some uncertainties from authors’ experience, lessons learned and guidelines for wellbore stability evaluation.


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