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Carbon dioxide (CO2)-enhanced coalbed methane recovery (CO2-ECBM) is a critical way to increase methane production and reduce greenhouse gas (CO2 and CH4) emissions. As captured CO2 is continuously injected in the coal seams, a low cost of CO2 sequestration and high efficiency of CH4 recovery can be achieved via the flooding and replacing effects driven by the injected CO2 flow. Scientific insights into the complex process of CO2-ECBM in experiments, modelings, and technological developments need to be made to propose appropriate countermeasures. This review first highlights the progress of CO2-ECBM under laboratory conditions, e.g., the binary gas competitive adsorption and gas displacement experiments in the macroscale and porous structure tests using technologies of nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and computed tomography (CT) in the microscale. Then, the advances of mathematical models for changing in coal permeability and porosity during CO2-ECBM are reviewed, accompanying with the multi-field and multi-phase coupling responses of competitive sorption, diffusion, gas–water seepage, heat transfer, and solid deformation. Furthermore, the field pilot tests of CO2-ECBM in various countries and regions are also covered to reveal the key technical challenges confronted with the development of CO2-ECBM technology. The perspectives in experiments, models, and field pilots of CO2-ECBM are made, which include but are not limited to the following: conducting a core CH4/CO2 flooding test under in situ conditions, modeling CO2-ECBM with real fractures/faults and coal failure, developing a new method for gas migration and leakage monitoring in the field, and enacting relevant standards, laws, and regulations to promote CO2-ECBM.

Owing to the exhaustion of shallow coal resources, deep mining has been occupied in coal mines. Deep buried coal seams are featured by the great ground stress, high gas pressure, and low permeability, which boost the risk of gas disasters and thus dramatically threaten the security about coal mines. Coal seam gas pressure and gas content can be decreased by gas extraction, which is the primary measure to prevent and control mine gas disasters. The coal mass is simplified into a continuous medium with dual structure of pores and fractures and single permeability. In consideration of the combined effects of gas slippage and two-phase flow, a hydraulic-mechanical coupling model for gas migration in coals is proposed. This model involves the equations of gas sorption and diffusion, gas and water seepage, coal deformation, and evolution of porosity and permeability. Based on these, the procedure of gas extraction through the floor roadway combined with hydraulic punching and ordinary drainage holes was simulated, and the gas extraction results were used to evaluate the outburst danger of roadway excavation and to verify the engineering practice. Results show that gas extraction can reduce coal seam gas pressure and slow down the rate of gas release, and the established hydraulic-mechanical coupling model can accurately reveal the law of gas extraction by drilling and punching boreholes. After adopting the gas extraction technology of drilling and hydraulic punching from the floor roadway, the remaining gas pressure and gas content are reduced to lower than 0.5 MPa and 5.68 m3/t, respectively. The achievements set a theoretical foundation to the application of drilling and punching integrated technology to enhance gas extraction.

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Effects of N and S functionalities on co-adsorption of CH 4 + N 2 and CH 4 + CO 2 on the coal vitrinite macromolecule model (CVMM) were explored using molecule simulation. Inclusion of the sulfoxide, amine, and sulfide functionalities can enhance S CO 2 /CH 4 (selectivity of CO 2 over CH 4 ), while the pyrrole and pyridine functionalities could not. The higher mole fraction of CO 2 could reduce the pressure required to reach the stable equilibrium state as well as S CO 2 /CH 4 . Microporous calculation results suggest that the microporous size distribution (MSD) of amine−CVMM (median pore size, D M , 8.24 Å; free volume probed by He, V d F , 7105.55 Å 3 /cell) and sulfoxide−CVMM (D M , 7.92 Å; V d F , 7812.96 Å 3 /cell) are most suitable for CH 4 adsorption, followed by pyrrole−CVMM (D M , 7.18 Å; V d F , 6812.96 Å 3 /cell). The inclusion of N and S functionalities can distinctly increase the adsorption affinity of the linear molecular configurations of adsorbates by changing the adsorption orientations. The target N and S functionalities in functionalized CVMMs contribute primarily to the adsorption process of these three gaseous adsorbates. The adjacent functionalities have critical influences on adsorption affinity for these three adsorbate species depending upon the relative adsorption affinity strength of the adjacent functionalities and the target functionalities.