Investigation into shale softening induced by water/CO2-rock interaction

To investigate the shale softening behavior induced by water/Carbon dioxide (CO2)-rock interaction, a series of scanning electron microscope (SEM) observation and nanoindentation tests were performed on carbonate-rich shale specimens after high-temperature and high-pressure static soaking treatments. The characteristics of shale surface softening and the evolution of micromechanical properties were analyzed. It was indicated that water/CO2-rock interaction resulted in different extents of micromechanical degradation, which was closely related to both the type of soaking fluid and the soaking time. According to the shale surface observation by SEM images, the aperture of hydration-induced fractures after soaking with water was larger than that of adsorption -induced fractures after soaking with CO2. The statistical results of nanoindentation tests show that when soaking with water, the Young's modulus and the hardness ultimately decreased by 34.6% and 57.6% with the soaking time increased from 2 to 6 d, respectively. When soaking with CO2, the extent of shale softening was alleviated. After soaking for 6 d, the ultimate degradation extents were 14.8% and 30.0% for the Young's modulus and the hardness, respectively. Furthermore, due to the higher penetrability of CO2, the micromechanical degradation of the shale surface mainly occurred in the first 2 d. Indents located in the vicinity of shale laminas or induced fractures tended to possess the lower Young's modulus and the hardness than those located in shale matrix, meanwhile resulting in crushed zones or fracture compaction.