Characterization of Shale-Drilling Fluid Interaction Mechanisms Related to Wellbore Instability

Wellbore stability problems tend to occur during the drilling phase in shale sections, hence the understanding of shale-drilling fluid interaction mechanisms is essential if wellbore instability is to be avoided when drilling through shale sections. A review on the origin of shale electric potential is presented in this paper. Simple tests to be used at rig-site to characterize shale reactivity in the presence of drilling fluids were developed. These developed tests were verified by means of developing easy to use equipment made especially for this study. Borehole collapse problems that occur when drilling in shale sections were simulated using thick-walled shale cylindrical specimens. It was found that any increase in sample moisture content has led to a reduction in shale strength and decreased borehole stability, while the increase in wellbore pressure was found to improve borehole stability.