Effects of drilling fluids on shale mechanical properties and wellbore stability

Long-term soaking in drilling fluids has vital effects on shale with well-developed bedding and subsequently wellbore stability in shale. The uni- and tri- axial compression tests were performed using the Longmaxi Formation shale samples from South Sichuan that have been immersed in different drilling fluids, to investigate the variation of mechanical characteristics and failure modes and also the mechanisms of wellbore instability. The results showed that due to the bedding development, the shale presents considerable mechanical anisotropy and dependency on confining pressure. The lubrication effect of oil-based drilling fluids on bedding planes strengthens the anisotropy, shale is found with the shear failure along bedding planes, and the failure mode is controlled by bedding planes. In the case of water-based drilling fluids, hydration damage is caused to shale, and the anisotropy is reduced due to the propagation and bedding connection of micro fractures. The failure is often composite, and the failure mode is controlled jointly by the matrix and bedding plane, which leads to high proneness to wellbore instability. The root causes of wellbore instability are hydration of clay minerals and capillary effects of pores and fractures. The findings of this research provide references for wellbore stability analysis of shale with well-developed bedding.