Key role of CO2 thermophysical properties and phase behavior in coal: Optimizing CH4 recovery and CO2 geosequestration

The temperature- and pressure-dependent phase behavior of the injected CO2 in coal would impact the efficiency of CH4 recovery and carbon storage. To fully capture this effect, a sequenced combination of X-ray microcomputed tomography (micro-CT), digital rock physics technology, and finite element method is employed to reconstruct the coal cleat and replicate its flow response to the CO2 in a 3D. Three distinct complexities in response are specified in the simulations. We follow (i) the nonlinear change of CO2 thermophysical properties and phase behavior with temperature and pressure conditions as it transits from liquid to supercritical CO2 (LScCO2). Additionally, we catch up with a constant response with (ii) CO2 thermophysical properties for only liquid CO2 (LCO2) and (iii) for only supercritical CO2 (ScCO2), respectively. Results show that under LScCO2 conditions, fluid flow velocity in butt cleats and recovery coverage in face cleats are simultaneously enhanced, and the CH4 recovery efficiency has a 1.20 times improvement compared to ScCO2 conditions. The fluid temperature drops by 4.66 °C compared to ScCO2, and the decrease tends to enhance reservoir stability. Furthermore, the findings also provide recommendations for optimizing CO2 storage site selection. Reservoirs with lower geothermal gradients (<30 °C/km) have superior CO2 storage potential compared to higher ones. The coal reservoir characterized by abundant butt cleats is the optimal location for injection and production wells, whereas more face cleats are preferable as target seams for CO2 storage. This study contributes to aiding the design, planning, and economic evaluation of injection schemes before the implementation of CO2 storage.