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非均匀页岩气在超致密约束下的非平衡输运(CS.CE)

在许多工程应用中都遇到过表面高度封闭的非均匀致密气体的非平衡输运问题。例如,在页岩气生产过程中,甲烷是在高压下从超致密孔隙中提取出来的,因此气体是不均匀且致密的。目前,非均匀致密气体的复杂非平衡输运是分子动力学或连续假设基础上的研究热点,其中气体表面相互作用起着关键作用。本文采用基于广义Enskog方程和平均场理论的可处理动力学模型,耦合了体积排斥和分子间长程引力的影响。利用10-4-3 Lennard-Jones势模拟气体分子与约束表面的相互作用,可以捕获气体表面吸附。不同围压条件下甲烷的横截面密度分布与文献报道的分子动力学结果吻合较好,并通过非平衡分子动力学验证了甲烷的输运行为。由于页岩基质中甲烷在纳米孔中的致密特性,其流动速度呈塞状。定量分析了压力、温度、孔隙大小和页岩组成对密度和速度分布的影响。结果表明,Klinkenberg修正量不适用于生产过程中页岩气流动模型;利用二阶滑移边界条件建立的Navier-Stokes模型不能得到合适的速度分布,因而不能准确预测纳米孔内的流动速度。该研究为理解页岩地层非平衡致密气流动的物理机制提供了新的思路。

原文题目: Non-equilibrium transport of inhomogeneous shale gas under ultra-tight confinement

原文:The non-equilibrium transport of inhomogeneous and dense gases highly confined by surface is encountered in many engineering applications. For example, in the shale gas production process, methane is extracted from ultra-tight pores under high pressure so the gas is inhomogeneous and dense. Currently, the complex non-equilibrium transport of inhomogeneous and dense gases where gas surface interactions play a key role is commonly investigated by molecular dynamics or on a continuum-assumption basis. Here, a tractable kinetic model based on the generalized Enskog equation and the mean-field theory is employed to couple the effects of the volume exclusion and the long-range intermolecular attraction forces. The interactions between gas molecules and confined surface are modelled by a 10-4-3 Lennard-Jones potential, which can capture gas surface adsorption. The cross-sectional density profiles of methane under different confinements are in good agreement with the molecular dynamics results reported in the literature, and the transport behaviors are validated by the non-equilibrium molecular dynamics. The velocity of methane flow in shale matrix is plug-like due to its dense characteristics in nanopores. The influence of pressure, temperature, pore size and shale composition on density and velocity profiles is analyzed quantitatively. Our results show that the Klinkenberg correction is not applicable to model shale gas flow in the production process; the Navier-Stokes model using the second-order slip boundary condition cannot produce the proper velocity profiles, and consequently fails to predict the accurate flow rate in nanopores. This study sheds new light on understanding the physics of non-equilibrium dense gas flows in shale strata.

原文作者:Baochao Shan, Runxi Wang, Peng Wang, Yonghao Zhang, Liehui Zhang, Zhaoli Guo

原文地址:https://arxiv.org/abs/2005.05622

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