Wellbore Stability Management in Weak Bedding Planes and Angle of Attack in Well Planing

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Abstract Borehole instabilities during drilling are more common in shale formations than in most other rock formations. The assessment of in-situ stress and analysis of borehole failure due to instability and weak bedding planes represents one of the most critical factors when evaluating borehole stability that causes borehole failure. Significant amount of research have been done in this area which resulted in various mathematical models about the issue of borehole failure, stability and plane of weakness due to bedding. So far, unified decision about the plane of weakness and failure of borehole on shale is yet to be fully realized by the industry, the most common of these losses are: widening of the well, washouts, stuck pipes, cavings, and deformation of the casing, amongst others. This work highlights the effect of shale bedding plane failure on wellbore stability and the angle of attack for stable drilling conditions in weak bedding planes by developing a robust tool to account for the criterion/conditions for identifying and drilling weak bedding planes, the rock strength for weak bedding planes (Uniaxial Compressive Strength UCS), minimum mud pressure to prevent shear failure & wellbore slip or slip shear failure in weak bedding planes and suitable attack angle to the bedding planes was also developed. The proposed tool has increased drilling efficiency and has been validated with field data. The advantage of this process is reduction in drilling difficulties and non productive time (NPT) in field operations in the Niger Delta where well construction targeting deeper horizons are proving to be a challenge. The new rock strength for weak bedding planes (UCS) helps to predict the minimum mud pressure to prevent shear failure & wellbore slip or slip shear failure in weak bedding planes having determined the best angle to place the well (angle of attack).