Pore-scale simulations of rarefied gas flows in ultra-tight porous media
Pore-scale simulations of rarefied gas flows in ultra-tight porous media
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DOI：
10.1016/j.fuel.2019.03.106
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发表时间：
2019-08
期刊：
影响因子：
7.4
通讯作者：
M. Ho;Lianhua Zhu;Lei Wu;Peng Wang;Zhaoli Guo;Jingsheng Ma;Yonghao Zhang
中科院分区：
文献类型：
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作者：
M. Ho;Lianhua Zhu;Lei Wu;Peng Wang;Zhaoli Guo;Jingsheng Ma;Yonghao Zhang
深入了解天然气在超致密多孔介质中的运移是定量研究孔隙空间小到几纳米的页岩渗流特性的关键，而稀薄效应在页岩中起着主要作用。由于常规流体力学理论不能描述非平衡稀薄渗流，本文借助于气体运动理论，利用孔隙空间解析后的多孔介质数字图像，直接模拟了多孔介质中的气体流动。用离散速度法(DVM)求解Boltzmann模型方程，可以较准确地预测稀薄效应引起的渗透率提高。我们对不同多孔介质的模拟表明，常用的标准格子Boltzmann方法(LBM)不能描述稀疏效应，尽管有助于捕捉速度滑移的动力学边界条件可以将LBM的有效性扩展到滑移流动区域。针对所有流型提出的启发式Klinkenberg型模型往往包含许多未知的经验参数，这些参数可以通过我们的模拟进行校准。然而，对于不同的多孔介质，这些参数是不同的，而且还取决于流动条件，因此这些模型没有任何实际应用价值。相比之下，我们的动力学求解器可以在不引入任何经验参数的情况下准确地预测表观渗透率，这为扩大规模奠定了坚实的基础。由于流动阻力最小的大流道在整体渗透率中占主导地位，因此我们的DVM模拟对速度空间分辨率的要求大大降低，以便以可承受的计算成本预测准确的渗透率，这为数字岩石分析提供了一种有前途的新途径。
An in-depth understanding of gas transport in ultra-tight porous media is the key to quantifying flow properties of shale rocks with pore space as small as a few nanometers, where the gas rarefaction effects play a major role. As the conventional fluid mechanics theory fails to describe non-equilibrium rarefied flow, we resort to the gas kinetic theory and directly simulate gas flow inside the porous media utilising the digital images of porous media where the pore space is resolved. The Boltzmann model equation is solved by the discrete velocity method (DVM), which can accurately predict the permeability enhancement caused by rarefaction effects. Our simulations for different porous media show that the commonly-used standard lattice Boltzmann method (LBM) cannot describe rarefaction effects, although the kinetic boundary condition, which helps to capture velocity-slip, can extend the validity of the LBM to the slip flow regime. The heuristic Klinkenberg-type models proposed for all the flow regimes often involve many unknown empirical parameters, which may be calibrated by our simulations. However, these parameters are different for each porous medium and also depend on flow conditions, so these models are not of any practical use. By contrast, our kinetic solver can accurately predict apparent permeability without introducing any empirical parameters, which lays firm foundation for upscaling. As the large flow paths with least flow resistance dominate the overall permeability, the requirement on the velocity-space resolution is significantly reduced for our DVM simulations to predict accurate permeability with affordable computational costs, which offers a promising new way for digital rock analysis.