Supercritical CO2 Breaking Through a Water Bridge and Enhancing Shale Oil Recovery: A Molecular Dynamics Simulation Study
B Liu and WY Liu and ZM Pan and LY Yu and ZY Xie and GZ Lv and PH Zhao and DM Chen and WJ Fang, ENERGY & FUELS, 36, 7558-7568 (2022).
CO2 injection has been proved to be the most promising enhanced oil recovery ( EOR) method for shale reservoirs. Water bridges impeding oil flow in shale nanopores have a significant impact on CO2-EOR, and the microscopic mechanism of this effect is still unclear, seriously hindering the design of CO2 injection technologies. In this work, molecular dynamics simulations were employed to study the microscopic process of fluid transport following CO2 injection into shale nanopores where oil and a water bridge coexist. Our results confirm that CO2 can break through the water bridge in shale nanopores to form fluid channels and improve the mobility and recovery of crude oil. The whole process can be divided into three stages: (i) CO2 diffuses into the nanopore, while oil diffuses into the fracture under concentration gradient; (ii) CO2 breaks through the water bridge; and (iii) CO2 drives oil out of the nanopore under a pressure gradient. Furthermore, four major microscopic mechanisms of CO2 breaking through a water bridge are summarized: (i) "porous" distribution of water molecules in the water bridge; (ii) less and weaker hydrogen bonds in the center of the water bridge; (iii) CO2 flushing the water bridge; and (iv) CO2 dragging water molecules through hydrogen bonding. Finally, the total oil recovery factor keeps increasing in the synergy of pressure flooding and interdiffusion of CO2 and oil. It is worth noting that CO2 injection and shut-in need to be properly regulated to avoid the reinjection of recovered oil into the nanopore owing to continuous pressure flooding. CO2 injection induces the swelling of oil and increases the mobility of oil, which can be further enhanced after the breakthrough of the water bridge. Our results advance the understanding of the microscopic mechanism of CO2-EOR of water-bearing shale reservoirs and the exploitation of unconventional resources.
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