The technology used to enhance coalbed methane (CBM) recovery by injecting CO₂ (CO₂-ECBM) with heat, combining heat injection with CO₂ injection, is still in its infancy; therefore, theoretical studies of this CO₂-ECBM technology should be perused. First, the coupling equations of the diffusion–adsorption–seepage–heat transfer fields of gas are established. The displacement processes under different pressures and temperatures are simulated by COMSOL. Finally, the displacement effects, a comparison of the CO₂ storage capacity with the CH₄ output and the effective influencing radius of CO₂ injection are analyzed and discussed. The results show that (1) the displacement pressure and temperature are two key factors influencing the CH₄ output and the CO₂ storage capacity, and the increase in the CO₂ storage capacity is more sensitive to temperature and pressure than the CH₄ output. (2) The gas flow direction is from the injection hole to the discharge hole during the displacement process, and the regions with high velocity are concentrated at the injection hole and the discharge hole. (3) A reduction in the CH₄ concentration and an increase in the CO₂ concentration are obvious during the displacement process. (4) The effective influencing radius of injecting CO₂ with heat increases with the increase in time and pressure. The relationship between the effective influencing radius and the injection time of CO₂ has a power exponential function, and there is a linear relationship between the functional coefficient and the injection pressure of CO₂. This numerical simulation study on enhancing CBM recovery by injecting CO₂ with heat can further promote the implementation of CO₂-ECBM project in deep coal seams.

The interactions between supercritical CO₂ and coal and their effects on changes in the coal pore structure and organic groups play a critical role in the CO₂ geological storage-enhanced coalbed methane recovery. To investigate the effects of supercritical CO₂ on organic groups in coals of different ranks and its mechanisms under different temperature and pressure conditions, CO₂ sequestration processes in bituminous coals and high-rank coals were replicated using a high-pressure reactor. Four coal samples of different ranks were exposed to supercritical CO₂ and water under three temperatures and pressures for 240 h. Fourier transform infrared spectroscopy was used to provide semiquantitative ratios and Fourier transform infrared spectra of coal samples before and after the supercritical CO₂–H₂O treatment. The results show that interactions between supercritical CO₂ and coal were controlled by the coal macromolecular structure, and semianthracite is the inflection point of interaction characteristics for coal samples of different ranks. Bituminous coal, including high- and low-volatility bituminous coal, has a low degree of condensation of its aromatic structure, and its aromatic nuclei can facilitate addition reactions. Swellings primarily break cross-links between aromatic nuclei in the same aromatic layer. These characteristics favor the polymerization addition of aliphatic side chains of aromatic nuclei, causing an increase in the degree of condensation of the aromatic structures in bituminous coal. High-rank coals including semianthracite and anthracite have a high degree of condensation of their aromatic structures, and the aromatic nuclei favor substitution reactions. Swellings primarily break cross-links connecting different aromatic layers, and bond dissociation reactions and sulfuration reactions are more significant for high-rank coal. These characteristics cause a decrease in the degree of condensation of the aromatic structure in high-rank coal. Temperature and pressure have a great impact on interactions between supercritical CO₂ and coal and are controlled by the reaction types of the organic groups. With the increase in experimental temperature and pressure, the changes in the organic group content can be classified as the descending type, the rising type, the lower opening parabola type, and the upper opening parabola type. 45.0°C and 10 MPa is the inflection point of the changes in the organic group content. Descending- and rising-type changes favor addition, bond dissociation, and sulfuration reactions, which are endothermic. The reaction rate of supercritical CO₂ and the organic groups increases, and the effects caused by temperature and pressure decrease as the temperature and pressure increase. Lower opening parabola- and upper opening parabola-type changes favor substitution, oxidation, and addition polymerization reactions, which are exothermic. These changes were significantly affected by a variety of reactions and were suppressed by high temperature and pressure. When the temperature is ≤45.0°C and the pressure is ≤10 MPa, supercritical CO₂ has remarkable effects on alkyl and hydroxy groups and has a stronger effect on bituminous coal. When the temperature is >45.0°C and the pressure is >10 MPa, supercritical CO₂ has remarkable effects on oxygen- and sulfur-containing groups and has a greater effect on high-rank coals.

CO₂ sequestration in deep, unmineable coal seams has been proposed as a method for offsetting increasing anthropogenic CO₂ emissions, while increasing natural gas production. Through laboratory experiments on gaseous/supercritical CO₂ adsorption using a manometric procedure, we investigated anomalies in supercritical CO₂ adsorption and determined CO₂ density‐temperature effects on adsorption capacity, which are closely related to the CO₂ storage potential in deep coal reservoirs. Results indicate that the revised Dubinin‐Radushkevich (D‐R) model can fit supercritical CO₂ adsorption data well, and there is a significant difference between excess and absolute adsorption capacity as CO₂ density increases. In the supercritical isochore, a rapid change in CO₂ density results in a transition from Langmuir‐like monolayer adsorption to multilayer adsorption, and there is an unsaturated multilayer system during this transition. Above the temperature at the cross point, the increasing of density decreases, and a negative dependence on temperature causes a gradual decrease in adsorption capacity. Coal reservoirs near the boundary between the supercritical isochore stage and fully supercritical CO₂ stage should be the prime target for geological storage of CO₂. In order to better understand supercritical CO₂ adsorption behavior and identify target coal seams, further supercritical CO₂ adsorption experiments are needed in the southern Qinshui Basin.