Key Laboratory of Low-Grade Energy Utilization Technologies and Systems, School of Energy and Power Engineering, Chongqing University, Ministry of Education, Chongqing 400044, China.
Molecules. 2022 Nov 29;27(23):8340. doi: 10.3390/molecules27238340.
Supercritical carbon dioxide (sCO) 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 CO molecules were firstly injected in the central region of shale slit, then tended to adsorb on the SiO-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 CO 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 CO and hexane were correlated to increase with pressure, until the diffusion rate of hexane being suppressed by the highly dense distribution of CO molecules at 12 MPa. The rapid transportation of CO molecules in the binary components (CO 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 CO-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 CO displacing shale oil and is in favor of deepening the understanding of shale oil exploitation and utilization.
超临界二氧化碳（sCO₂）由于其高溶解度、低表面张力和低粘度，在替代页岩油方面具有巨大潜力，但迄今为止，其潜在机制仍不清楚。通过进行平衡分子动力学（EMD）模拟，我们发现置换过程可以分为三个步骤：首先，CO 分子被注入页岩狭缝的中心区域，然后倾向于吸附在 SiO-OH 壁表面并与己烷混合，导致页岩表面的己烷层松动，最后由于 CO 和壁之间的强相互作用，从壁上置换出己烷。在这个过程中，己烷的置换速度和效率分别呈现抛物线和增加的趋势与压力有关。为了深入了解这一现象，我们进一步进行了非平衡分子动力学（NEMD）模拟，发现 CO 和己烷的 Onsager 系数都与压力呈正相关，直到 12 MPa 时 CO 分子高度密集分布抑制了己烷的扩散速率。二元组分（CO 和己烷）中 CO 分子的快速传输实际上促进了己烷的扩散，从而促进了己烷从纳米通道流出，随后提高了采油效率。置换过程可以在高于 7.5 MPa 的压力下有效发生，之后 CO-壁的相互作用能强于己烷-壁的相互作用能。考虑到置换速度和效率以及己烷的扩散速率，本工作发现最佳注入压力为 10.5 MPa。这项研究提供了对 CO 置换页岩油的详细了解，有利于加深对页岩油开采和利用的认识。
