Investigation of collapse pressure in layered formations based on a continuous anisotropic rock strength criterion

Abstract

Wellbore instability severely constrains the exploration and development of shale gas. In order to evaluate the impacts of anisotropy and water on wellbore instability, three different types of criteria are fitted to strength data of LMX shales with different moisture contents. A new model of transversely isotropic borehole stability considering compliance incremental tensor induced by natural fractures is proposed, then a more preferred drilling direction is performed by the new model. Results indicated that, Pariseau’s model is more attractive in predicting shale strength, including the difference of strength between vertical bedding and parallel bedding. Based on Pariseau’s model, the prediction accuracy of shale strength is improved by 33.04%. The Pariseau’ model gives a disparate collapse pressure from Jæger’s weak plane criterion, the most unstable drilling area shifts from northeast and southwest to the central area corresponding to relatively lower inclination. The collapse pressure only decreased by 0.55 MPa with considering the anisotropy of elastic parameters, but the strength criteria have a distinct influence. Compared with the results predicted by Jæger’s plane of weakness, the collapse pressure increased by 8.55 MPa using Pariseau’ model. Besides, invasion of water in bedding plane will aggravate borehole instability, especially in late drilling period, collapse pressure for the vertical wellbore increased by 6.35 MPa. Shale strength depends not only on the hydrostatic pressure and orientation angle, but also on the water content, which should be considered in mud weight design and well trajectory optimum.