Carbon Dioxide Sequestration in Coal Formations

Abstract Carbon dioxide (CO2) is the major greenhouse gas emitted in all activities. CO2 sequestration in coal seams has become an attractive carbon sequestration technology where the injection of CO2 enhances methane production from coalbeds (ECBM), in addition to storing CO2. CO2 can be retained in coal via three ways: free gas within the pore space or fractures in the coal, adsorbed molecules on the organic surface of coal, and dissolved in groundwater within the coal. This study investigates the effects of the salinity (NaCl) of coal seam water, injection flow rate, and CO2 state (formation pressure) on the CO2 sequestration in volatile bitumen coal. To achieve this objective; coreflood tests were conducted on different coal cores. The change in the effective water-coal permeability after CO2 injection was examined. Also, the displacement efficiency of water by CO2 was estimated. The experimental data was cross-matched with numerical simulation to estimate the relative permeability curves. The coreflood experiments showed that the effective water permeability in coal decreased during CO2 injection due to its adsorption onto the coal surface, and reduction of the relative water permeability. Higher formation pressure led to permeability reduction as a result of higher CO2 adsorption onto the coal surface. The effective water permeability decreased by 8% at 50 psi back pressure, compared to 48% at 1100 psi back pressure. Furthermore, as the injection flow rate increased, the contact time of CO2 to the coal surface decreased. Hence, the CO2 adsorption to the coal matrix decreased and the change on effective water permeability slightly decreased. Specifically, the effective water permeability decreased by 48% at 1 cm3/min comparing to 41% at 4 cm3/min. As salt concentration increased, the change in pressure drop across the core slightly increased, but this effect decreased as formation pressure increased. As the water salinity increased, the coal become more CO2 wet, and the residual gas saturation increased. Based on these observations, the injection of CO2 into highly volatile bitumen coal seams for CO2 sequestration and ECBM purposes is more efficient as the salt concentration increases and high injection pressure.