Enhanced gas recovery by CO2 sequestration in marine shale: a molecular view based on realistic kerogen model

Injection of CO2 into shale reservoir is regarded as one potential scenario for CO2 sequestration and enhanced gas recovery (CS-EGR). In this work, a realistic molecular model of kerogen in Chinese Silurian marine black shale was generated using molecular dynamics (MD) simulations. The competitive adsorption of CH4 and CO2 was simulated by the grand canonical Monte Carlo (GCMC) method under different reservoir pressures, temperatures, geological depths, CO2 mole ratios, and moisture contents of kerogen model. Results show that CO2/CH4 adsorption selectivity decreases with increasing reservoir pressure, indicating that CS-EGR can be more efficient if CO2 injection is conducted at the late development stage. The temperature has a negative effect on the selectivity, which indicates that thermal stimulation has an adverse effect on the efficiency of CS-EGR. Also, the selectivity decreases with increasing geological depth, suggesting that shallow shale formations are more suitable for CS-EGR. At low pressures, the selectivity increases with increasing CO2 mole ratio, while at high pressures, the selectivity decreases with the increase of CO2 mole ratio. This result suggests that CO2 mole ratio should be dynamically adjusted with the production so as to adapt to the changing reservoir pressure. At higher pressure condition, both the amounts of CO2 sequestration and CH4 desorption increase with the increase of CO2 mole fraction. However, the adsorption stability of CO2 weakens with increasing injection amounts of CO2. Moreover, the adsorption selectivity decreases initially, and then increases with the moisture content of kerogen. Thus, the performance of CS-EGR may be improved by increasing the kerogen moisture content for Silurian shale gas reservoirs. This study gains enhanced insights on the effect of reservoir pressure, temperature, geological depth, CO2 mole ratio, and kerogen moisture content on CO2/CH4 competitive adsorption, and the results can provide applicable guidances for CS-EGR in shale gas reservoirs.