Maintaining wellbore stable is one of the key tasks in the oil and gas industry, in order to reduce nonproductive time during drilling, because wellbore instability problems would lead to higher than necessary drilling costs and have a severe impact on drilling schedule. Wellbore stability is controlled by two type of factors, one type is the factors which are completely out of our control, such as in-situ stresses, pore pressure, and rock strength, the other type is the factors which we can optimize and design to minimize geomechanical related stability problems, such as well trajectory, casing seats, mud system, mud weight, and proper drilling practice including minimizing swab and surge while running pipe and reducing the stationary time during connection.

In this paper, only the mud weight optimization and casing seats design will be focused on, in order to avoid geomechanical related instability issues for drilling a recently planned appraisal well in an offshore field.

Mechanical properties including Young's modulus, Poisson's ratio, USC, friction angle, tensile strength, bulk density and pore pressure gradient obtained from two offset wells A1 and A2 were projected to the planned appraisal well A3 with tops were used as guidance. The same parameters optimized from those two offset wells were used to estimate horizontal stresses for the planned well, and the same failure criterion was used to carry out wellbore stability for the planned well. Meanwhile the mud weight corresponding to kick, breakout, loses and breakdown were obtained. Based on those results, the safe mud weight window was established and the casing seats were placed to prevent and reduce the geomechanics related instability problems.

This methodology allows us to predict and prevent the geomechanics related instability issues in advance before drilling starts, thereby to reduce the non-productive time and drilling costs. Continuous updating of the geomechanical model is necessary during drilling if the geological environment is complicated.

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