Microscopic Evaluation Method of Caprock Sealing Capacity for CO₂ Sequestration under the Coupling Thermal-Hydraulic-Mechanical Conditions

Caprock sealing integrity is critical to geological carbon sequestration, yet it can be altered by coupled thermo-hydro-mechanical (THM) conditions. To elucidate the microscale mechanisms governing CO₂ adsorption-transport within caprock, molecular simulations methods, including Monte Carlo and molecular dynamics simulations, are used to model CO₂ adsorption and transport in nanoscale caprock pore-throat systems under coupled THM conditions. Effects of formation temperature, pressure and pore size on the adsorption characteristics of CO₂ are systematically analyzed. Simulation results show that the adsorption and transport of CO₂ in the caprock exhibit strong dependence by multiple physical fields. Specifically, the adsorption of CO₂ in the nanopore throats decreases nonlinearly with the increased temperature and the decreased pressure. When the temperature rises from 300 K to 600 K, the reduction rates of CO₂ adsorption capacity and adsorption heat by nanoporous throats reach 71.4% and 54.1% respectively. Given increasing pressure from 1 MPa to 60 MPa, the corresponding adsorption capacity increases from 0.197 mmol/g to 5.818 mmol/g, and the adsorption heat increases by 85.8%. Meanwhile, the increased pore throat size leads to an increase in the adsorption capacity of CO₂, but reduces the reaction adsorption heat with the wall surface. This study can provide theoretical basis and methodological support for the microscale assessment of the sealing capacity of the caprock and the site screening of CO₂ geological storage.