Influences of CO2 Injection into Deep Coal Seams: A Review

For nearly 20 years, CO2 has been injected into coal seams to enhance the recovery of methane in a process known as enhanced coal bed methane (ECBM). However, there is a huge complexity associated with this process, mainly due to the generating complex coal chemicophysical structure rearrangement. This review paper aims to comprehensively discuss two main influencing factors upon CO2 injection in deep coal seams: (1) mobilization of hydrocarbon and (2) coal matrix swelling. CO2 injection into deep coal seams may remove available polycyclic aromatic hydrocarbons (PAHs) from the coal matrix and mobilize them in the coal seam. The amount of hydrocarbon that is mobilized from the coal matrix by the injected CO2 is dependent on coal rank, maceral content, type of available hydrocarbons in the coal mass (both dissolving and nondissolving types), and phase state of the injected CO2 in the seam. Supercritical CO2 has greater solvent ability and, therefore, has the ability to extract a greater percentage of hydrocarbon from the coal matrix. This mobilization of the organic constituents of the coal matrix by the injected CO2 causes many environmental issues. For examples, PAHs that exist in highly volatile bituminous coal are harmful to biota and the environment, even at relatively low concentrations. On the other hand, adsorption of the injected CO2 into the coal mass causes it to be swelled, leading to significant alternations in its internal coal mass structure, resulting in great modifications in its flow and strength properties. This CO2-adsorption-induced coal matrix swelling process is reduced with increasing temperature, exhibits an inverted-U shaped variation with coal rank, and is largely dependent on the pressure and the physical state of the injected CO2, where supercritical CO2 creates a much greater swelling effect, compared to gas/liquid CO2, because of its higher chemical potential. Potential coal seams for CO2 sequestration process are available at extremely deep locations and there is a high possibility of phase change from gas/liquid to supercritical state and, thus, they likely have high swelling rates.