Molecular Simulation Of Oil Detachment And Erosion Mechanism By Supercritical Carbon Dioxide

Supercritical carbon dioxide（sc CO₂） has the excellent solubility nonpolar substances, strong diffusion ability,strong adsorption ability, and can alter the wettability on the surface of rocks. sc CO₂ is applied to the extraction of crude oil, and useful for the retard of the oil shortage. The interaction between CO₂ and oil rock during the process of CO₂ flooding is complex, which greatly limits the usage of sc CO₂ in enhancing oil recovery and restricts the work process of oil displacement mechanism and theoretical research. This article adopted the method of molecular dynamics simulations to study the dissolving and detachment mechanism using paraffin oil/sc CO₂, sc CO₂/dodecane oil/silica models. During this work, we examined interactions of sc CO₂ and ankanes, sc CO₂ and rock, rock and alkanes for the well understanding of mechanism.The paraffin oil/sc CO₂ simulation showed that sc CO₂ has the property of dissolving nonpolar paraffin. But sc CO₂ has weaker capacity to dissolve the long chain alkanes due to the large molecular weight of alkanes. The calculation of solubility parameter showed that short paraffin alkanes has a smaller gap with sc CO₂, while C18, C22, C25 alkanes are large. It could be seen from interaction energy that dispersion effect is the most important reason for the difference of dissolving capacity. Meanwhile, long chain alkanes would bind with other molecules and lead to the poor dissolution in sc CO₂ due to the benging deformation.Oil film volume expansion and formation of CO₂ diffusion channels in oil film happen after the adsorption of oil on silica surface. Oil film will be stripped from the rock surface and gradually when CO₂ gather on the surface rock. The analysis of the interaction energy and density distribution shows that it could be divided into several stages for the detaching process of crude oil. We also studied the process of sc CO₂ diffusing to rock surface on the analysis. Diffusing ability of sc CO₂ and interaction between CO₂ and oil film is vital effects. Hydrogen bonds and electrostatic interactions lead to constantly adsorption of CO₂ onto rock surface. Dispersion effect between crude oil and sc CO₂ makes oil dissolves into sc CO₂.Based on the research about regulation of temperature, we acknowledged that sc CO₂ like liquid due to the low temperature of system, and the diffusion and adsorption of sc CO₂ will be affected largely. It will lead to alkanes gathering in sc CO₂ after detachment from surface. When CO₂ density is low, CO₂ could be regard as gas and only make the oil volume expansion with the film undetached. At the range of supercritical state, temperature’s effect on detachment is not in regular pattern. Increasing of CO₂ density makes CO₂ diffusion ability weaken, carrying ability decrease, which is bad for the oil detachment by sc CO₂.