Study of the influence of pore size on the occurrence characteristics and interphase microscopic mechanisms of fluid under the confined effect of nanopores

To elucidate the influence of pore size on the state of fluid occurrence in mixed wetting channels, and to reveal the influence mechanism of pore size on the interphase microscopic forces in the nanoconfined space, in this paper, the molecular dynamics simulation method is used to establish a shale mixed wetting nanopore model under different pore sizes to simulate the fluid occurrence and CO2 displacement process. The results show that the pore size is negatively correlated with the number proportion of adsorbed state oil molecules in the mixed wetting pore. The smaller the pore size, the stronger the confined effect of nanopores and the greater the attraction of the wall to the oil molecules. Moreover, if the pore diameter is too small, the water molecules in the pore will aggregate and form a water bridge under the synergistic action of strong wall attraction and hydrogen bonding, inhibiting the diffusion and migration of oil molecules. In addition, the larger the pore diameter, the stronger the competitive adsorption capacity of CO2 in the pore and the miscible effect with oil molecules, which can weaken the cohesion between oil molecules, reduce the resistance of oil molecules migration, increase the diffusion ability, and thus improve the oil displacement efficiency. Under different pore size conditions (3, 5, and 7 nm), the CO2 displacement efficiency is 53.51%, 87.83%, and 88.01%, respectively, and the CO2 storage efficiency can reach 34.28%, 47.35%, and 49.41%, respectively.