Supercritical CO₂-water-shale interactions and their effects on element mobilization and shale pore structure during stimulation

There are many advantages to using supercritical carbon dioxide (ScCO₂) fracturing technology to exploit shale gas reservoirs in China, including minimal damage to the environment or formation, and displacing methane (CH₄) in the adsorbed state. When ScCO₂ enters fractures in the formation, ScCO₂-water-shale interactions may affect the physicochemical properties of shale. In this study, a high-pressure reaction system was adopted to simulate ScCO₂-water-shale interactions under ScCO₂ stimulation conditions. The element mobilization and pore structure before and after the reaction were measured using ICP-MS, XRF. The results show that the major elements, including Ca, Mg, Na, K, and Al, exhibit varying degrees of mobilization after the interactions because of dissolution of carbonate and silicate minerals in shale samples. Compared with the major elements, trace elements have a lower mobility, quantified as <13.97%. The specific surface areas and pore volumes of two shale samples increase at different degrees after the reaction. The interactions have a more significant influence on the micropores. In addition, fractal features of the shale pore structure were analyzed. The fractal dimensions of the shale samples increase after the reaction, indicating that pore surface roughness increases, and pore structure morphology gradually transforms from regular to complex.