Micro/Nano-pore Network Analysis of Gas Flow in Shale Matrix.

Pengwei Zhang,Shengyan Gao,Jay N Meegoda,Liming Hu

doi:10.1038/srep13501

Pengwei Zhang, Shengyan Gao + Show 2 more

Open Access

https://doi.org/10.1038/srep13501

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Abstract

The gas flow in shale matrix is of great research interests for optimized shale gas extraction. The gas flow in the nano-scale pore may fall in flow regimes such as viscous flow, slip flow and Knudsen diffusion. A 3-dimensional nano-scale pore network model was developed to simulate dynamic gas flow, and to describe the transient properties of flow regimes. The proposed pore network model accounts for the various size distributions and low connectivity of shale pores. The pore size, pore throat size and coordination number obey normal distribution, and the average values can be obtained from shale reservoir data. The gas flow regimes were simulated using an extracted pore network backbone. The numerical results show that apparent permeability is strongly dependent on pore pressure in the reservoir and pore throat size, which is overestimated by low-pressure laboratory tests. With the decrease of reservoir pressure, viscous flow is weakening, then slip flow and Knudsen diffusion are gradually becoming dominant flow regimes. The fingering phenomenon can be predicted by micro/nano-pore network for gas flow, which provides an effective way to capture heterogeneity of shale gas reservoir.

Highlights

The gas flow in shale matrix is of great research interests for optimized shale gas extraction
The gas flow in the nano-scale pore may fall in flow regimes such as viscous flow, slip flow and Knudsen diffusion
Knudsen number (Kn) is defined to separate different dominant flow regimes, and expressed as K n = λ/r, which is the ratio between gas molecular mean free path (λ) and gas flow characteristic size (r) in porous media[14,15,16,17,18]

Summary

Introduction

The gas flow in shale matrix is of great research interests for optimized shale gas extraction. A 3-dimensional nano-scale pore network model was developed to simulate dynamic gas flow, and to describe the transient properties of flow regimes. Knudsen number (Kn) is defined to separate different dominant flow regimes, and expressed as K n = λ/r , which is the ratio between gas molecular mean free path (λ) and gas flow characteristic size (r) in porous media[14,15,16,17,18]. The Knudsen number accounts for the frequency of molecule-molecule and molecule-wall collisions based on gas molecular dynamics theory[19,20], which can be used to separate different flow regimes. Civan (2010) proposed a simple inverse power-law relationship of rarefaction factor and correlated with experiment data These unified models were obtained from straight tube and the validity for nano-scale shale matrix is uncertain. The method proposed by Chen et al (2015) is quite fundamental but computationally very intensive, a fairly accurate but simpler method using typical shale reservoir data is proposed in this research

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