Determination of location of breakouts in transversely isotropic shales

Borehole stability during drilling operation is one of the most important tasks for oil and gas engineers to assure successful exploitation and production. Borehole instabilities frequently occur especially in organic rich shales because of the structural anisotropy and prevention requires careful study of stress field around wellbore, and the physical and geomechanical characteristics of shale. Borehole breakouts, which are one of the widespread borehole instability problems, may lead several problems such as poor cementing of the borehole annulus, difficulties with interpretation of logs, and even the loss of the well. Therefore investigation of breakouts-stress-induced extensions of borehole wall is crucial task for stable drilling and production. There is no so much research on borehole breakouts for unconvential shales, therefore, in this work study of the breakouts for two different type shale samples is discussed by investigation of the several laboratory experiments. Then, the effect of the presence of weakness planes on the breakout formation was analyzed and critical conditions for slip on the two different type of shales are determined using combination of Weakness Plane model and Kirsch solution. Finally parametric analysis is conducted on the shale samples to study how critical mud pressures change with the effect of each parameter.