CO₂ sequestration in unmineable coal seams

Carbon dioxide (CO₂) is a major greenhouse gas that is emitted in all activities. CO₂ sequestration in coal enhances methane production from the coalbed (ECBM) in addition to storing CO₂. CO₂ can be stored in coalbeds in three ways: free gas within the pore space or fractures in the coal, adsorbed on the organic surface of the coal, or dissolved in the groundwater within the coal. This paper addresses the effect of the salinity of the coal seam water, injected gas composition, and formation pressure on the CO₂ sequestration in volatile bitumen coal. In order to achieve these objectives, coreflood tests were conducted using coal cores. The displacement efficiency of water by CO₂ and the effective water permeability change due to CO₂ adsorption were measured. The coreflood experiments showed that the permeability reduction due to coal swelling was significant in the case of high NaCl concentrations. The permeability reduction changed from 22 to 32% with the salinity increasing from DI to 20 g/L NaCl. The displacement efficiency increased as the salinity increased. The water saturation after CO₂ injection decreased from 37 to 25% with salinity increasing from DI to 20 g/L NaCl. Also, as N₂ concentration increased in the injection gas, the permeability reduction and the sweep efficiency decreased. In the case of DI water and 50% N₂, the permeability reduction was 12%, and the residual water saturation was 47%. Based on these observations, CO₂ sequestration and ECBM into highly volatile bitumen coal seams with high salt concentration is more efficient compared to low salt concentration.