Hydrogeological and Geomechanical Aspects of Underground Coal Gasification and its Direct Coupling to Carbon Capture and Storage

Underground coal gasification (UCG) using boreholes drilled from the surface has been feasible for almost a century, but is only now being implemented in market economies. Good analogues for the hydrogeological effects of UCG are provided by longwall mining. Careful planning of UCG void locations and dimensions can result in minimal disturbance of overlying aquifers as close as 40 m above the burn zone. Moreover, development of a pressure arch above the zone of net strata extension can provide a hydraulic seal to prevent vertical fluid migration. Injection of CO2 into former UCG voids ought to be possible where these are present at depths in excess of a parts per thousand 800 m. Analogies to longwall mining suggest that extensional deformation immediately above the burn zone will render sufficient pore space accessible to accommodate all of the CO2 arising from gasification. In the absence of adequate engineering, the boreholes used in the UCG-CCS processes represent the most likely leakage pathways for injected CO2. Hence high-quality borehole engineering will be required, taking full account of thermal performance of casing and grout, and geophysical testing of borehole integrity. An agenda for management of groundwater issues at all stages in the life-cycle of UCG and UCG-CCS has been developed, as an aide memoire for future developments.