Understanding roles of moisture in CO2 adsorption and desorption hysteresis on deep gas-bearing shales under high temperature and pressure

Abstract

To understand injectivity and storage stability of CO2 into gas-bearing shale reservoirs, CO2 adsorption and desorption on the dry and moist shales with diverse properties were conducted at 353.15 K and pressures up to 14 MPa. Multiple characterizations were combined to examine pore characteristics of the shales before and after CO2 adsorption/desorption. The resultant CO2 adsorption and desorption hysteresis on the shales was revealed ultimately. Results indicated that both the excess and absolute adsorption isotherms of CO2 witness slow increase stage, rapid increase stage, and decrease stage. The adsorption capability of the shales positively depends on the TOC content and pore abundance. Moreover, the clay minerals and oxygenic functional groups also benefit CO2 adsorption. The moisture significantly reduces the experimentally-determined maximum absolute adsorption capacity of the shales by 5.78–46.39 % due to pore space occupation. During the adsorption/desorption, the CO2 molecules could interact with the shales, thereby initiating extraction of partial organic matter, transformation of CO2 from supercritical state to gas state, and dissolution of inorganic minerals. Those effects consume CO2 and dominantly change the micropore structure of the dry shales and the mesopore structure of the moist shales, thus leading to CO2 adsorption and desorption hysteresis. Notably, the moisture dramatically raises the hysteresis degree on the shales by 8.42–59.45 % because of its solubility to CO2 and promotion to mineral dissolution. Overall, shale reservoir moisture could weaken injectivity but strengthen storage stability of CO2. Hence, the critical moisture content and occurrence pattern of organic matter and clay minerals are required for target shale reservoir selection.