The competitive adsorption of CO2 and CH4 binary mixture on kerogen and clay surfaces significantly affects CO2 sequestration for enhanced gas recovery (EGR) applications in shale-gas reservoirs. Therefore, the microscopic competitive adsorption behavior of CO2 and CH4 in the kerogen and clay structures needs quantitative investigation. In this paper, we aim to (a) quantify the effects of kerogen composition, pore structure, and thermal maturity on the CO2:CH4 competitive adsorption behavior, (b) quantify the effects of clay surface chemistry on the CO2 and CH4 adsorption capacity of organic shale formations, and (c) quantify the impacts of different moisture and oil content on the adsorption capacity of kerogen and clay structures.
We first transformed kerogen molecular models of different types (i.e., types IA, IIA, and IIIA) and different thermal maturity levels (i.e., IIA, IIB, IIC, and IID) into dense porous compounds to mimic the actual kerogen structures. Moreover, we modeled illite and kaolinite clay structures honoring their actual chemical composition, surface charge, and pore size. We then performed Grand Canonical Monte Carlo (GCMC) simulations to evaluate the CO2 and CH4 adsorption isotherms for different kerogen and clay structures. Adsorption isotherms were constructed for a pressure range of 1–20 MPa under reservoir temperatures of 330 K and 400 K.
The change in kerogen aromaticity and pore structure from type IA to type IIIA led to a corresponding increase in the available pore volume and adsorption sites for CO2. This led to an increase in the CO2 adsorption capacity from 1.42 mmol/g for kerogen IA to 5.56 mmol/g for kerogen IIIA in the presence of methane gas. Moreover, increasing the temperature from 330 K to 400 K led to a decrease in the CO2/CH4 selectivity from 3.06 to 2.45 at a pressure of 20 MPa. Changing kerogen thermal maturity from the immature type IIA to the postmature type IID led to double the CO2 adsorption capacity from 2.61 to 4.86 mmol/g. In addition, changing the moisture content of kerogen type IID from 0 wt% to 3 wt% led to a decrease in the CO2 adsorption capacity by 10%. Meanwhile, The CH4 and CO2 adsorption capacities of illite and kaolinite clay structures were highly affected by the charge of the clay surface and the moisture and oil contents. K-illite clay with a positive surface charge demonstrated a CO2/CH4 selectivity of 11.95 compared to 0.95 and 1.25 for the cases of negatively surface-charged illite and neutral kaolinite.
The outcomes of this paper reveal the micromechanisms of CO2 and CH4 adsorption/desorption on kerogen and clay surfaces within shale-gas reservoirs. Moreover, we provided enhanced insights on the screening and optimization of CO2 storage operations in different candidate shale-gas reservoirs with a wide range of composition, thermal maturity, pressure, and temperature.
