Reservoir simulation study of CO2 storage in formations containing both aquifers and coal seams

Abstract

Abstract Geological storage of CO 2 is a viable option for the mitigation of greenhouse gas emissions. Formations such as saline aquifers and coal seams have distinct storage mechanisms; in saline aquifers, the CO 2 is mainly stored by compression and/or dissolution in the formation fluid, whereas in coal seams, the CO 2 is primarily stored by adsorption. To investigate the impact of CO 2 dissolution in formation fluid on CO 2 storage in coal and enhanced coalbed methane production, two scenarios are considered (1) CO 2 injection and coalbed methane production for a single coal seam, (2) CO 2 injection in two coal seams with an aquifer in-between while coalbed methane is produced in the upper coal seam. It is found that although CO 2 dissolution in formation water is not the main storage mechanism in coal reservoirs, including CO 2 dissolution can lead to significant differences in the simulation results. In addition, including CO 2 dissolution leads to more accurate description of the ECBM process through more accurate prediction of water saturation and thus the gas effective permeability, the overall reservoir pressure and gas flow response to CO 2 injection. The results also suggest that for CO 2 storage in lower rank coals, which usually have higher porosity and permeability, CO 2 dissolution in the formation water should be considered in order to more accurately describe the CO 2 storage and ECBM behaviour. The results also show that water containing formations in between the coal seams, although often low in porosity and thus insignificant in overall CO 2 storage capacity, also have a significant impact on the overall CO 2 storage and enhanced coalbed methane recovery behaviours when considering CO 2 dissolution modelling in formation waters.