Abstract
Molecular dynamics simulation method was used to systematically study the adsorption configuration, density distribution and adsorption energy of H2O, CO2, CH4, N2, C8H18 and fluorocarbon molecules C3F8 and C5F12 on kaolinite (0 0 1) surface. The enrichment characteristics of fluid molecules and the stability of adsorption configuration under reservoir temperature and pressure conditions are analyzed and discussed. The study shows that under the condition of reservoir temperature and pressure, the fluid molecules exhibit stratified adsorption characteristics on kaolinite (0 0 1) surface, in which the first adsorption layer has a higher regional density and the adsorption molecules are arrested by a higher free energy potential well, and the temperature and pressure have no significant influence on the potential barrier, meaning a adsorption phase state with ordered structure and stable configuration. In second and third adsorption layers, it was found that the free energy potentials well are lower, and the fluid is in a transition phase state with lower stability. The adsorption energy and free energy calculation results of the adsorption system show that the seven kinds of fluids are adsorbed on the kaolinite (0 0 1) surface in the order of H2O > C3F8 > C5F12 > C8H18 > CO2 > CH4 > N2. The water and fluorocarbon molecules, which have a much larger binding energy than the C8H18 and CH4 molecules, can displace oil and gas molecules in kaolinite reservoirs, while carbon dioxide can only displace methane molecules adsorbed on the kaolinite surface. It has been shown that as the increase of temperature and pressure, the binding energy of water molecules and C3F8 decreases significantly, but the binding energy of C5F12, C8H18, CO2 and methane molecules increases by 20%, 36%, 50% and 90% respectively, indicating that water flooding and C5F12 surfactant flooding can enhance oil recovery under high temperature and pressure conditions.
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