Experimental Investigations on Enhanced Coal Seam Methane Extraction by Injecting Gas: A Review

Enhanced coal seam methane extraction by injecting gas is an emerging technology to both reduce greenhouse gas emissions and improve coal seam methane recovery. In this paper, the experimental and field research progress on gas injection for driving and displacing coal seam methane are reviewed from the perspectives of the micro, meso, and macro scales. The microscopic scale perspective involves intuitively analyzing the interaction process and relationship between strong adsorption gases (CO2, LCO2, and SCCO2) and relatively weak adsorption gases (CH4, N2) injected into coal samples, the changes in the number of micropores, mesopores, and macropores in coal samples by using NMR and low-temperature nitrogen adsorption equipment, and visualizing the multiphase flow process in pore structures using microfluidic methods. For the mesoscopic scale, multiple equipment means (SEM, NMR, liquid nitrogen adsorption, etc.) were used to quantitatively analyze the effects of different fracture widths, different degrees of metamorphism, and composition on gas seepage and adsorption–desorption rate. For the macroscopic scale, the physical and mechanical properties of coal and rock were analyzed using different pseudotriaxial or true triaxial seepage experimental equipment during the changes of gas injection process parameters and the effects of axial pressure and confining pressure, especially in adsorption–desorption and seepage related experiments in single component and multicomponent gases. Moreover, from future perspectives, some key, emerging, and intelligent technologies and equipment of gas injection enhanced coal seam methane extraction will be put forward.