Gas Transportation and Enhanced Coalbed Methane Recovery Processes in Deep Coal Seams: A Review

Coalbed methane (CBM) is a potential green energy supply for addressing the worldwide energy crisis. However, the recovery of economically viable amounts of methane requires the application of production-enhancement techniques. The greater effectiveness of enhanced coalbed methane (ECBM) recovery compared to traditional pressure depletion and hydraulic stimulation techniques has been identified in terms of higher CBM recovery with minimal pollution risk and the ability to contribute to CO2 sequestration. Gas transport behavior in a coal seam is the governing factor for ECBM recovery, which includes sorption/desorption and diffusion in the matrix and advective flux in cleats. The interactions among sorption, diffusion, and flow indicate the complexity and abstruseness of gas transport in coal. Therefore, the purpose of this paper is to provide comprehensive knowledge of the gas transport process in deep coal seams, particularly in relation to the ECBM process. According to the review, the dual-porosity system in coal provides sorption sites, and CO2 has much higher adsorption affinity to coal compared to that of CH4. Gas adsorption capacities for CH4 and CO2 are greatly reduced with temperature and the presence of moisture and increased with pressure. However, the adsorption capacity for supercritical CO2 decreases with increasing pressure due to changes in the associated CO2 properties. Regarding the diffusion process, CO2 has the highest diffusivity for its smallest kinetic diameter and the diffusion capability may be reduced with the existence of moisture for moisture adsorption-induced coal swelling. Seam temperature has a positive influence on gas diffusion due to the enhanced kinetic energy and, the effect of pressure on diffusion is still open to debate. Upon sorption/diffusion, gas moves toward the cleat system through gas flow, which is controlled by permeability and is in turn greatly altered by gas adsorption/desorption-induced swelling/shrinkage effects during ECBM recovery. With high chemical reactive potential, CO2 creates the greatest coal matrix swelling for its higher adsorption capacity. Seam permeability increases with increasing injection pressure due to the associated pore expansion and reduces with enhanced swelling. Coal mass swelling reduces with increasing temperature due to the exothermic nature of gas adsorption. Dewatering coal seams increases coal permeability through the reduced moisture content that provides more sorption places for CO2 adsorption. However, this in turn may cause reduced permeability through the enhanced swelling effect.