Storing carbon dioxide in deep unmineable coal seams for centuries following underground coal gasification

Underground coal gasification (UCG) has a huge potential in extracting energy from deep coal seams with a low environmental footprint. A vast void space created in a UCG operation serves as an effective sink to anthropogenic CO2 storage and sequestration. To advance the knowledge base regarding CO2 storage in post-UCG cavities, a 3D commercial-scale 990m × 60m × 24 m multi-cavity scenario was constructed to model the transport and fate of the stored CO2 plume at around 1000 m depth for centuries using a coalbed methane (CBM) and CO2 storage dedicated simulator. The effects of the import multiphysics were analyzed. It was revealed that transport phenomena entered a stabilized steady state in 50 years in a multicomponent adsorption system. In addition, the matrix-cleat diffusion exhibited a substantially strong trapping mechanism with an at-equilibrium spread-out speed of 0.24 m/year. Dynamic changes in a cleat system and an injection process were shown to play a minor role. Furthermore, two effective means were proposed for a protracted thousands-of-year simulation of CO2 storage in post-UCG multiple cavities. The relevant findings in this paper would promote the utility and applicability of UCG in the world's deep coal seams for the transition of the coal industry in a carbon-conscious world. •Explored the fate of the stored CO2 in deep coal seams for centuries under the effects of very important multiphysics.•Constructed a 3D 990m × 60m × 24 m multi-cavity for a commercial-scale modelling.•The differing importance of effects was analyzed.•Proposed useful ways for predicting CO2 transport in deep unmineable coal seams for hundreds to thousands of years.