Direct Evidence of Coal Swelling and Shrinkage with Injecting CO2 and N2 Using in-situ Synchrotron X-ray Microtomography

Abstract

Deep coal seams are one of the world’s most widespread deposits for carbon dioxide (CO 2 ) disposal and are generally located near large point sources of CO 2 emissions. The injection of CO 2 into coal seams has great potential to sequester CO 2 while simultaneously enhancing coalbed methane (CO 2 -ECBM) recovery. Pilot tests of CO 2 -ECBM have been conducted in coal seams worldwide with favorable early results. However, one of the main technical barriers in coal seams needs to be resolved: Injecting CO 2 reduces coal permeability and well injectivity. Here, using in situ synchrotron X-ray microtomography, we provide the first observational evidence that injecting nitrogen (N 2 ) can reverse much of this lost permeability by reopening fractures that have closed due to coal swelling induced by CO 2 adsorption. Our findings support the notion that injecting minimally treated flue gas—a mixture of mainly N 2 and CO 2 —is an attractive alternative for ECBM recovery instead of pure CO 2 injection in deep coal seams. Firstly, flue gas produced by power plants could be directly injected after particulate removal, thus avoiding high CO 2 -separation costs. Secondly, the presence of N 2 makes it possible to maintain a sufficiently high level of coal permeability. These results suggest that flue-gas ECBM for deep coal seams may provide a promising path toward net-zero emissions from coal mines.