Self-diffusivity of dense confined fluids
Self-diffusivity of dense confined fluids
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DOI：
10.1063/5.0059712
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发表时间：
2021-08-01
影响因子：
4.6
通讯作者：
Zhang, Yonghao
中科院分区：
文献类型：
--
作者：
Corral-Casas, Carlos;Gibelli, Livio;Zhang, Yonghao
分子在致密多孔介质中的传输对页岩气勘探至关重要，但仍需要对元素物理有更深入的了解，特别是在高压和纳米尺度的限制下，在这些情况下，纳维尔-斯托克斯和玻尔兹曼解决方案不再有效。在这项工作中，我们利用事件驱动的分子动力学模拟，改变流体的稀疏性、限制和表面摩擦，对自扩散进行了基本和系统的研究。我们区分了流体-流体和流体-壁面碰撞，以确定支撑扩散机制的相互作用，即分子扩散和努森扩散。我们发现，只要孔道高度不小于五个分子直径，用于描述稀薄气体的Bosanquite公式也能够很好地半解析地描述受限稠密流体中的自扩散系数。重要的是，这使我们能够预测自扩散系数，而不考虑流体的稀疏性、禁闭状态和表面粗糙度，这在很大范围内是以前不可能实现的。作为争论的一个来源，我们在这里证明，尽管在这些条件下出现了强烈的流体不均匀性，但爱因斯坦自扩散系数仍然可以在菲克定律内使用，只要在使用菲克定律时考虑边界效应。最后，我们注意到，以前发现的自扩散系数随约束的线性标度仅在低密度和无摩擦壁面的极限下有效，这并不代表页岩储层。这项工作将作为研究在最近的稠密受限流体工作中观察到的异常气体传输行为的基础。
Molecular transport through tight porous media is crucial to shale gas exploration, but deeper insights of the elemental physics are still required, particularly under high pressures and nanoscale confinements, where Navier-Stokes and Boltzmann solutions are no longer valid. In this work, we carry out a fundamental and systematic study of self-diffusion using event-driven molecular dynamics simulations, varying fluid rarefaction, confinement, and surface friction. We differentiate between fluid-fluid and fluid-wall collisions to identify the interplay of the underpinning diffusive mechanisms, namely, molecular and Knudsen diffusion. We find that the Bosanquet formula, which has been used for describing rarefied gases, is also able to provide a good semi-analytical description of self-diffusivities in confined dense fluids, as long as the pore height is not smaller than five molecular diameters. Importantly, this allows us to predict the self-diffusion coefficient, regardless of the fluid rarefaction, confinement state, and surface roughness, in a wide range of Knudsen numbers that were not possible before. Often as a source of debate, we prove here that despite strong fluid inhomogeneities arising in these conditions, the Einstein self-diffusivity can still be used within Fick's law, provided boundary effects are considered when using Fick's setup. Finally, we notice that a previously identified linear scaling of self-diffusivities with confinement is only valid in the limit of low densities and frictionless walls, which is not representative of shale reservoirs. This work will serve as a foundation for investigating the anomalous gas transport behavior observed in the recent work of dense, confined fluids.