Zhou Wenning, Yang Xu, Liu Xunliang
School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China.
Beijing Key Laboratory of Energy Conservation and Emission Reduction for Metallurgical Industry, Beijing 100083, China.
Phys Rev E. 2022 May;105(5-2):055308. doi: 10.1103/PhysRevE.105.055308.
This study proposes a multiscale model combining molecular simulation and the lattice Boltzmann method (LBM) to explore gas flow behaviors with multiple transport mechanisms in nanoporous media of shale matrix. The gas adsorption characteristics in shale nanopores are first investigated by molecular simulations, which are then integrated and upscaled into the LBM model through a local adsorption density parameter. In order to adapt to high Knudsen number and nanoporous shale matrix, a multiple-relaxation-time pore-scale LBM model with a regularization procedure is developed. The combination of bounce-back and full diffusive boundary condition is adopted to take account of gas slippage and surface diffusion induced by gas adsorption. Molecular simulation results at the atomic scale show that gas adsorption behaviors are greatly affected by the pressure and pore size of the shale organic nanopore. At the pore scale, the gas transport behaviors with multiple transport mechanisms in nanoporous shale matrix are explored by the developed multiscale model. Simulation results indicate that pressure exhibits more significant influences on the transport behaviors of shale gas than temperature does. Compared with porosity, the average pore size of nanoporous shale matrix plays a more significant role in determining the apparent permeability of gas transport. The roles of the gas adsorption layer and surface diffusion in shale gas transport are discussed. It is observed that under low pressure, the gas adsorption layer has a positive influence on gas transport in shale matrix due to the strong surface diffusion effect. The nanoporous structure with the anisotropy characteristic parallel to the flow direction can enhance gas transport in shale matrix. The obtained results may provide underlying and comprehensive understanding of gas flow behaviors considering multiple transport mechanisms in shale matrix. Also, the proposed multiscale model can be considered as a powerful tool to invesigate the multiscale and multiphysical flow behaviors in porous media.
本研究提出了一种结合分子模拟和格子玻尔兹曼方法（LBM）的多尺度模型，以探索页岩基质纳米多孔介质中具有多种传输机制的气体流动行为。首先通过分子模拟研究页岩纳米孔隙中的气体吸附特性，然后通过局部吸附密度参数将其整合并放大到LBM模型中。为了适应高克努森数和纳米多孔页岩基质，开发了一种具有正则化过程的多松弛时间孔隙尺度LBM模型。采用反弹和全扩散边界条件的组合来考虑气体吸附引起的气体滑移和表面扩散。原子尺度的分子模拟结果表明，气体吸附行为受页岩有机纳米孔隙的压力和孔径影响很大。在孔隙尺度上，利用所开发的多尺度模型探索了纳米多孔页岩基质中具有多种传输机制的气体传输行为。模拟结果表明，压力对页岩气传输行为的影响比温度更为显著。与孔隙率相比，纳米多孔页岩基质的平均孔径在决定气体传输的表观渗透率方面起着更重要的作用。讨论了气体吸附层和表面扩散在页岩气传输中的作用。观察到在低压下，由于强烈的表面扩散效应，气体吸附层对页岩基质中的气体传输具有积极影响。具有与流动方向平行的各向异性特征的纳米多孔结构可以增强页岩基质中的气体传输。所得结果可为考虑页岩基质中多种传输机制的气体流动行为提供潜在的全面理解。此外，所提出的多尺度模型可被视为研究多孔介质中多尺度和多物理流动行为的有力工具。
