CO2 utilization and sequestration in organic-rich shale from the nanoscale perspective

As the main components of shale gas, the adsorption behavior of CH4 and n-C4H10 in organic matter is of great significance to the recovery of shale gas. In this work, organic matter is selected as the object, and different types and scales of kerogen slit nanopore models are constructed based on the improved diamond-filling method. Moreover, the adsorption behaviors of CH4/CO2 and n-C4H10/CO2 mixture with organic matter are investigated at different pressure, temperature, and pore size conditions, and the subsequent implications for shale gas recovery and CO2 sequestration are summarized. The results show that the density of CO2 formed near the surface of organic nanopores is significantly higher than that of CH4, while the density of n-C4H10 formed near the surface of organic nanopores is significantly higher than that of CO2, and the order of the interaction energy of the three fluid molecules on the kerogen is: n-C4H10 > CO2 > CH4. Therefore, CO2 injection performs a positive effect on light hydrocarbon component CH4 recovery, whereas is detrimental to the recovery of heavy hydrocarbon component n-C4H10. Interestingly, the three kinds of fluid molecules exhibit stronger adsorption on type II kerogen than on type I kerogen, suggesting that when implemented in rich in type II kerogen shale reservoirs, the available reserves of shale gas will be higher and the CO2 sequestration strength will be greater. The proportion of dissolved phase and adsorbed phase CO2 in micropores is significantly higher than that in mesopores, indicating that the micropore is the ideal space for CO2 sequestration. Specifically, the sensitivity of CH4 and organic matter adsorption to reservoir pressure is stronger than that of n-C4H10, and the optimal pressure for CO2 recovery of shale gas is about 25 MPa. Particularly, the free phase density of n-C4H10 is the highest at 333.15 K, which implies that the recovery of n-C4H10 is the highest at temperature. In contrast, low temperature is beneficial to maintaining a more stable adsorption state of CO2 and exists a positive impact on CO2 sequestration. We observe that the maximum adsorption density of CO2 in 3 nm kerogen was significantly higher than that in 1 nm pores, thus CO2 in mesopores is more favorable to CH4 recovery than in micropores. Note that CO2 injection is more conducive to improving the CH4 recovery in the micropores of type I kerogen and the n-C4H10 recovery in the mesopores of type II kerogen. This study helps to deepen the understanding of the adsorption behaviors of shale organic matter with CH4, n-C4H10, and CO2, Furthermore, it may provide guidance for improving shale oil and gas recovery and CO2 sequestration in future sites.