Mechanism of Hexane Displaced by Supercritical Carbon Dioxide: Insights from Molecular Simulations

Abstract

Supercritical carbon dioxide (sCO2) has great potential for displacing shale oil as a result of its high solubility and low surface tension and viscosity, but the underlying mechanisms have remained unclear up to now. By conducting equilibrium molecular dynamics (EMD) simulations, we found that the displacing process could be divided into three steps: the CO2 molecules were firstly injected in the central region of shale slit, then tended to adsorb on the SiO2-OH wall surface and mix with hexane, resulting in loose hexane layer on the shale surface, and finally displaced hexane from the wall due to strong interactions between CO2 and wall. In that process, the displacing velocity and efficiency of hexane exhibit parabolic and increased trends with pressure, respectively. To gain deep insights into this phenomenon, we further performed non-equilibrium molecular dynamics (NEMD) simulations and found that both the Onsager coefficients of CO2 and hexane were correlated to increase with pressure, until the diffusion rate of hexane being suppressed by the highly dense distribution of CO2 molecules at 12 MPa. The rapid transportation of CO2 molecules in the binary components (CO2 and hexane) actually promoted the hexane diffusion, which facilitated hexane flowing out of the nanochannel and subsequently enhanced oil recovery efficiency. The displacing process could occur effectively at pressures higher than 7.5 MPa, after which the interaction energies of the CO2-wall were stronger than that of the hexane-wall. Taking displacing velocity and efficiency and hexane diffusion rate into consideration, the optimal injection pressure was found at 10.5 MPa in this work. This study provides detailed insights into CO2 displacing shale oil and is in favor of deepening the understanding of shale oil exploitation and utilization.

Molecular Dynamics Study on the Diffusion Mass Transfer Behaviour of CO2 and Crude Oil in Fluids Produced via CO2 Flooding

“…Li C et al [22] studied the mutual solubility behaviour of CO 2 and crude oil during EOR and compared the displacement efficiencies of different gases by calculating the solubility of gas, the volume swelling factor of crude oil, the diffusion coefficient of crude oil and the minimum miscibility pressure (MMP). However, the application of molecular simulations to study the interaction between carbon dioxide and crude oil mostly focuses on reservoir conditions where the temperature and pressure are significantly different from those at the surface where gathering and transportation occur [23,24]. Moreover, there is no relevant research considering the influence of CO 2 and water content on the diffusion mass transfer of CO 2 in crude oil.…”