Molecular dynamics investigation on CO2-cosolvent flooding and storage synergy in shale nanopores

• Comparing oil displacement and storage effects of different CO 2 -cosolvent systems in inorganic nanopores. • Revealing the molecular-scale competition mechanism of displacement and storage in CO 2 -cosolvent system. • GDME showed a balanced performance in shale oil displacement and CO 2 storage. • The effects of multiple factors of pore environment (wall properties, pore size, water content, shale oil type) on the recovery and storage effect of CO 2 -GDME system were discussed. In order to explore the effect of cosolvent-assisted CO 2 flooding and storage in shale reservoirs, the synergistic mechanism of oil displacement and storage of CO 2 -cosolvent system in shale nanopores based on molecular dynamics simulation is analyzed in this study. It is found that dimethyl ether significantly improves the diffusion coefficient of shale oil (D is 1.0787Å 2 /ps) due to low polarity, showing the optimal displacement performance. The order of oil displacement efficiency is dimethyl ether > glycol dimethyl ether > ethyl acetate > acetone > ethanol > pure CO 2 . The dissolution of CO 2 and shale oil assisted by glycol dimethyl ether dominates and achieves the optimal storage effect. The order of storage efficiency: glycol dimethyl ether > ethyl acetate > dimethyl ether > ethanol > acetone > pure CO 2 . Based on the analysis of mean square displacement(MSD), radial distribution function(RDF) and interaction energy, glycol dimethyl ether(GDME) is selected as the best cosolvent due to the balance of shale oil displacement and CO 2 stable storage. Further analysis shows that the kerogen wall promotes oil phase stripping through organic action (oil displacement efficiency reaches 42.7 %), while the quartz wall enhances CO 2 adsorption by polar sites (storage efficiency reaches 26.84 %). Although the increase of pore size can improve oil recovery (the oil displacement efficiency increases by 26.12 % from 10 nm to 15 nm.), it leads to the decrease of CO 2 storage stability. Under the condition of water-containing, the oil displacement/storage efficiency is reduced by 17 % and 36.92 %, respectively, by water molecules through hydrogen bond competition compared with waterless conditions. The type of shale oil also has different effects on the recovery and storage of the system. Type II shale oil is most easily displaced due to high light hydrocarbon content, while type I shale oil promotes CO 2 storage efficiency due to strong polar components. This study reveals the molecular scale competition mechanism of CO 2 displacement and storage in nanopores in CO 2 -cosolvent system. The influence of different environmental factors on CO 2 extraction and storage is also discussed, which provides theoretical support for the cosolvent-assisted CO 2 synergistic development of unconventional oil and gas reservoirs.