Stability analysis of wellbore for multiple weakness planes in shale formations

Traditional prediction methods for formation integrity collapse pressure are difficult to describe for shale microfractures, joints, and weakness planes. In this study, for multiple weakness planes, a reasonable prediction model for shale wellbore collapse instability is constructed using the Mohr–Coulomb criterion and the multistructural plane strength theory, which can analyze the wellbore collapse pressure considering different weak planes. The results show that the collapse pressure increases with the increase in the number of weak planes. With an increase in the number of weak planes, the safe drilling window becomes narrower, which leads to an increase in the drilling difficulty. With the change in the drilling azimuth, the weak plane dip region where the maximum collapse pressure is located for different weak plane groups is transformed. For a weak plane group, the maximum collapse pressure changes from a dip angle of 30°–60° to 0° and 90° with the drilling azimuth. When the dip angle of two weak plane groups is 0°, the collapse pressure of the wellbore is the lowest, and the maximum collapse pressure changes from 60°–90° to 30°–60° with the change in drilling azimuth. When drilling along the minimum horizontal ground stress azimuth, there is only a difference when the included angle of inclination is 60°–180° and 240°–360°, and the other azimuth coincide, which indicates that the influence of the weak plane inclination on the collapse pressure is weakened when drilling along the azimuth. The effect of multiple weak planes on the collapse pressure is more significant in the later stages of drilling. The example analysis shows that under the action of multiple weak planes, the method can predict the formation collapse pressure more comprehensively and accurately than previous methods, which is meaningful in guiding drilling engineering in shale formations.