Anthropogenic energy-related carbon dioxide (CO2) emissions are higher than ever and continue to increase. Injection of CO2 into deep coal seams has great potential to sequester CO2, while simultaneously enhancing coalbed methane recovery (CO2-ECBM). However, one of the main technical barriers in coal seams needs to be resolved: Injecting CO2 reduces coal permeability and well injectivity. Here, in-situ synchrotron X-ray microtomography coupled with flow experiments was conducted under reservoir conditions, which bridges pore scale observations to core scale behaviours. The dependency of fracture porosity, connectivity and permeability on stress and fluid injection (water, CO2 and N2) was directly observed. We provide the first observational evidence that injecting nitrogen (N2) can reverse much of this lost permeability by reopening fractures that have closed due to coal swelling induced by CO2 adsorption. Our findings support the notion that injecting minimally treated flue gas—a mixture of mainly N2 and CO2—is an attractive alternative for ECBM recovery instead of pure CO2 injection in deep coal seams.

CO2 sequestration,three-dimensional fracture network,X-ray computed tomography