Changes of wettability of shale exposed to supercritical CO2-water and its alteration mechanism: Implication for CO2 geo-sequestration

The wettability of shale are the key factors determining the shale gas production efficiency and CO2 storage safety. However, the effects of the supercritical CO2 (ScCO2)-water-shale interactions on the wettability in shale is unclear. To address this issue, water wettability alterations in shale at different ScCO2-water exposure pressure and temperature conditions were evaluated based on a nuclear magnetic resonance (NMR) test method, and the mineral composition, element mobilization and pore of shale were analyzed to interpret the alterations. After ScCO2-water exposure, the water wettability of shale was reduced, and the reduction enhanced with the increase of exposure pressure while weakened with the increase of exposure temperature. The weakening of water wettability in shale is closely related to the exposure pressure and temperature dependent mineral and pores alteration in shale. ScCO2-water exposure induced the increase in the content of hydrophobic quartz, and the decrease in the contents of hydrophilicity clay and carbonate minerals, leading to the weakening of water wettability in shale. In addition, the micro and mesopores gradually transfer to larger pores after ScCO2-water exposure, reducing capillary pressure and the hydrophilicity of shale, also responsible for the decrease of water wettability in shale. The weakening of water wettability may enhance the sweep efficiency of CO2 in shale reservoirs or reduce the capillary trapping capacity of CO2 when shale served as caprocks. Thus, this study contributes to the evaluation of CO2 storage capacity and security in shale reservoirs.