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.

This paper reports on a comprehensive study of the CO 2 -EOR (Enhanced oil recovery) process, a detailed literature review and a numerical modelling study. According to past studies, CO 2 injection can recover additional oil from reservoirs by reservoir pressure increment, oil swelling, the reduction of oil viscosity and density and the vaporization of oil hydrocarbons. Therefore, CO 2 -EOR can be used to enhance the two major oil recovery mechanisms in the field: miscible and immiscible oil recovery, which can be further increased by increasing the amount of CO 2 injected, applying innovative flood design and well placement, improving the mobility ratio, extending miscibility, and controlling reservoir depth and temperature. A 3-D numerical model was developed using the CO 2 -Prophet simulator to examine the effective factors in the CO 2 -EOR process. According to that, in pure CO 2 injection, oil production generally exhibits increasing trends with increasing CO 2 injection rate and volume (in HCPV (Hydrocarbon pore volume)) and reservoir temperature. In the WAG (Water alternating gas) process, oil production generally increases with increasing CO 2 and water injection rates, the total amount of flood injected in HCPV and the distance between the injection wells, and reduces with WAG flood ratio and initial reservoir pressure. Compared to other factors, the water injection rate creates the minimum influence on oil production, and the CO 2 injection rate, flood volume and distance between the flood wells have almost equally important influence on oil production.

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