A porochemothermoelastic coupling model for continental shale wellbore stability and a case analysis

Abstract

Maintaining wellbore stability is a significant task when drilling through shale sections. Continental shale expands quickly when hydrated because it usually has a high clay mineral content. When the water influx invades the shale formation during drilling, the mechanical strength of the shale will weaken to a certain extent, resulting in borehole instability or more serious problems. The shale formation is assumed to be a porochemothermoelastic medium, and the dynamic hydration of shale soaked in drilling mud are considered in terms of the shale matrix and bedding plane; then, a coupled porochemothermoelastic solution for the borehole stress field is presented. A model that includes shale hydration is built to evaluate borehole stability in continental shale. By analyzing the borehole collapse pressure in a field case of continental shale drilling, this study reveals the borehole instability mechanism and optimizes a drilling mud scheme.