Mobilisation of elements from coal due to batch reactor experiments with CO2 and water at 40°C and 9.5MPa

Injection of CO2 into coal seams, with or without enhanced coal bed methane recovery (CO2-ECBM), is one method for sequestering CO2 greenhouse gas emissions in the subsurface. Conventional saline formations that could be used for CO2 sequestration also may have coal seams within the seal-rock units. However, relatively little is known concerning the potential interactions between CO2-acidified water and coal. This geochemical study investigates the short-term mobilisation of elements from selected samples of coal exposed to highly reactive conditions expected in the mixing front between the injected CO2 plume and groundwater. Coals were obtained from both the Permian Bowen Basin of Queensland, Australia, and the Carboniferous Ruhr Basin of Germany. Within a custom built batch reactor, cubes of coal were reacted with supercritical CO2-saturated water at 9.5MPa and 40 degrees Celsius for 72h. Carbonate minerals within the coal cleats and matrix were most affected by the CO2-water experiments, as indicated by both SEM images and reaction water chemistry. An experiment with N2 saturated-water to compare water-only mobilisation with CO2-water mobilisation was also done for a single sample in each of the seven sets of coals. The effluent concentrations of several elements including Al, Ba, Ca, Fe, Mg and Mn consistently were greater during the CO2-water-coal experiments in comparison to water-only mobilisation from the same samples, whereas elements such as Ag, B, Hg, K, Li, Na and Sb were much less likely to be preferentially mobilised by CO2-acidified water