Intrinsic mechanisms of shale hydration-induced structural changes

The interaction between a water-based working fluid and shale causes a change in shale structure, resulting in wellbore instability and greatly increasing drilling costs. On the basis of the black rock samples of the Longmaxi Formation, the structural change characteristics and internal mechanisms of shale hydration are investigated using scanning electron microscopy, fixed-point observation experiments, X-ray diffraction experiments, ion analysis, and energy spectroscopy analysis. In this study, the changes in the microscopic elements during the propagation of macroscopic cracks to hydration were studied and the ion exchange of brine and mineral hydration was analyzed. The results indicate that the interaction between water and shale leads to shale fracture along the bedding plane, the expansion of primary fractures, the generation of new fractures, and the occurrence of a high number of dissolution phenomena. The ion exchange cation precipitation of shale and the fluid is dominated by Ca2+, and the anion precipitated is mainly SO42−. Through the energy spectrum analysis of the whole-rock mineral composition changes and shale structural changes, it is believed that shale dissolution is mainly from carbonatite minerals, such as calcite. While the oxidation of pyrite in the aqueous environment provides a large amount of SO42− and H+ and fracture propagation mainly occurs in areas where non quartz inorganic minerals are distributed on the bedding plane. The intrinsic mechanism of shale hydration structural changes is revealed through the structural changes, mineral composition changes, and chemical reactions present during the water–shale interaction, which is significant for ensuring the wellbore stability of drilling.