The role of brine in gas adsorption and dissolution in kerogen nanopores for enhanced gas recovery and CO2 sequestration

Understanding storage characteristics of CH4 and CO2 in shale media is important for enhanced gas recovery and geological CO2 sequestration. This work reports a molecular simulation study of CH4 and CO2 storage behaviors in kerogen nanopores partially saturated with brine. Molecular distributions of pure CH4 , pure CO2 and their mixtures in the kerogen nanopores are quantified and divided into three distinct zones: adsorption between kerogen surfaces in Zone 1, aggregation at the kerogen-brine (water) interface in Zone 2, and gas dissolved in the confined brine in Zone 3. The gas uptake is found to be affected by two different storage mechanisms: adsorption (i.e. in Zone 1) and dissolution (i.e. in Zones 2 and 3). Uptake of CH4 and CO2 decreases linearly with increasing salinity, but with different mechanisms. CH4 uptake is dominated by its adsorption in Zone 1, where its density distribution is not affected, but the available volume decreases as salinity increases. On the other hand, CO2 solubility in brine contributed by Zone 2 and Zone 3 can be comparable to its adsorption in Zone 1. As salinity increases, a significant decrease in CO2 solubility in brine is observed, besides the available volume reduction in Zone 1. For mixture sorption, the CO2 density is enhanced in the region of CH4/CO2/brine (water) interface, while that of CH4 is slightly decreased. Furthermore, increasing salinity leads to decreased amounts of recovered CH4 and sequestrated CO2. Our work provides new and important insights into enhanced gas recovery by CO2 injection and geological CO2 sequestration in shale reservoirs.