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.