Zhao Yinuo, Li Wenhui, Zhan Shiyuan, Jin Zhehui
School of Mining and Petroleum Engineering, Department of Civil and Environmental Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada.
College of Energy, Chengdu University of Technology, Chengdu 610059, China.
Phys Chem Chem Phys. 2022 Jul 21;24(28):17195-17209. doi: 10.1039/d2cp01643e.
As one common unconventional reservoir, shale plays a pivotal role to compensate the depletion of conventional oil resources. There are numerous nanoscale pores and ultra-narrow pore throats (sub 2 nm) in shale media. To displace oil through ultra-narrow pore throats by water, one needs to overcome excessively-high capillary pressure. Understanding the water-oil two-phase displacement process through pore throats is critical to numerical simulation on tight/shale oil exploitation and ultimate oil recovery estimation. In this work, we use molecular dynamics simulations to investigate oil (represented by -octane) displacement by water through a ~2 nm kerogen (represented by Type II-C kerogen) pore throat. Besides, the applicability of the Young-Laplace equation to the ultra-narrow kerogen pore throat has been assessed. We find that although the Type II-C kerogen is generally oil-wet, water has an excellent displacement efficiency without the oil film on the substrate, thanks to the hydrogen bonding formed between water and heteroatoms (such as O, N, and S) on the kerogen surface. Unlike previous studies, the capillary pressure obtained from the widely used Young-Laplace equation shows a good agreement with the breakthrough pressure obtained from MD simulations for the ∼2 nm kerogen pore throat. Our work indicates that explicitly considering intermolecular interactions as well as atomistic and molecular level characteristics is imperative to study the two-phase displacement process through ultra-narrow pore throats.
作为一种常见的非常规储层，页岩对于弥补常规石油资源的枯竭起着关键作用。页岩介质中存在大量纳米级孔隙和超窄孔喉（小于2纳米）。要通过水驱替超窄孔喉中的油，需要克服过高的毛细管压力。了解通过孔喉的水 - 油两相驱替过程对于致密油/页岩油开采的数值模拟和最终采收率估算至关重要。在这项工作中，我们使用分子动力学模拟来研究水通过约2纳米干酪根（以II - C型干酪根表示）孔喉驱替油（以正辛烷表示）的过程。此外，还评估了杨氏 - 拉普拉斯方程对超窄干酪根孔喉的适用性。我们发现，尽管II - C型干酪根通常是亲油的，但由于水与干酪根表面的杂原子（如O、N和S）之间形成了氢键，在没有油膜覆盖基底的情况下，水具有出色的驱替效率。与以往研究不同，从广泛使用的杨氏 - 拉普拉斯方程获得的毛细管压力与针对约2纳米干酪根孔喉的分子动力学模拟获得的突破压力显示出良好的一致性。我们的工作表明，明确考虑分子间相互作用以及原子和分子水平的特征对于研究通过超窄孔喉的两相驱替过程至关重要。
