Micro-mechanical properties of shale due to water/supercritical carbon dioxide-rock interaction

To investigate the impacts of water/supercritical CO2-rock interaction on the micro-mechanical properties of shale, a series of high-temperature and high-pressure immersion experiments were performed on the calcareous laminated shale samples mined from the lower submember of the third member of Paleogene Shahejie Formation in the Jiyang Depression, Bohai Bay Basin. After that, grid nanoindentation tests were conducted to analyze the influence of immersion time, pressure, and temperature on micro-mechanical parameters. Experimental results show that the damage of shale caused by the water/supercritical CO2-rock interaction was mainly featured by the generation of induced fractures in the clay-rich laminae. In the case of soaking with supercritical CO2, the aperture of induced fracture was smaller. Due to the existence of induced fractures, the statistical averages of elastic modulus and hardness both decreased. Meanwhile, compaction and stress-induced tensile fractures could be observed around the laminae. Generally, the longer the soaking time, the higher the soaking pressure and temperature, the more significant the degradation of micro-mechanical parameters is. Compared with water-rock interaction, the supercritical CO2-rock interaction caused a lower degree of mechanical damage on the shale surface. Thus, supercritical CO2 can be used as a fracturing fluid to prevent the surface softening of induced fractures in shale reservoirs.