Abstract
The three-dimensional (3D) structures of pores directly affect the CH4 flow. Therefore, it is very important to analyze the 3D spatial structure of pores and to simulate the CH4 flow with the connected pores as the carrier. The result shows that the equivalent radius of pores and throats are 1–16 μm and 1.03–8.9 μm, respectively, and the throat length is 3.28–231.25 μm. The coordination number of pores concentrates around three, and the intersection point between the connectivity function and the X-axis is 3–4 μm, which indicate the macro-pores have good connectivity. During the single-channel flow, the pressure decreases along the direction of CH4 flow, and the flow velocity of CH4 decreases from the pore center to the wall. Under the dual-channel and the multi-channel flows, the pressure also decreases along the CH4 flow direction, while the velocity increases. The mean flow pressure gradually decreases with the increase of the distance from the inlet slice. The change of mean flow pressure is relatively stable in the direction horizontal to the bedding plane, while it is relatively large in the direction perpendicular to the bedding plane. The mean flow velocity in the direction horizontal to the bedding plane (Y-axis) is the largest, followed by that in the direction horizontal to the bedding plane (X-axis), and the mean flow velocity in the direction perpendicular to the bedding plane is the smallest.
Highlights
The coal reservoir is a complex porous medium, which contains matrixes, pores, and minerals (Ni et al 2017; Li et al 2019; Liu et al 2019)
The pores extracted from the Bofang Mine (BF) sample in this study are macro-pores based on the pore structure classification system mentioned above
In order to better understand the geometrical and topological structures of the pore network model, the following parameters are defined (Lindquist et al 2000; Sok et al 2002; Herring et al 2013; Zhao et al 2018; Liu et al 2019): (1) pore radius is the radius of the equivalent sphere with the same volume; (2) throat radius and length are the radius and length of the channel connecting two interconnected pores; (3) coordination number is the connective number between one pore and other pores; (4) Euler’s characteristic is known as the connectivity function, which can be obtained by calculating the relative Euler characteristics of the minimum radius of the pores and throats
Summary
The coal reservoir is a complex porous medium, which contains matrixes, pores, and minerals (Ni et al 2017; Li et al 2019; Liu et al 2019). The pore structure directly affects the physical property of the coal reservoir (Fu et al 2017; Su et al 2019) and further affects the CH4 flow (Naveen et al 2018; Zhou et al 2018; Liu et al 2019). In the study of pore structure, traditional methods such as the mercury intrusion porosimetry and the low temperature liquid nitrogen adsorption analysis (Guo et al 2019; Wang et al 2018) often destroy the original pore structure during the sample preparation process, so the real 3D pore structures cannot to be characterized. The FIB-SEM and Nano-CT can only characterize the nanopore structure, and their application and promotion are further limited by the high resolution, long experimental period and strict requirements of sample preparation (Ni et al 2017). Due to the limited sample size observed, the analysis of the pore structure of coal reservoir with strong heterogeneity is not very representative (Ni et al 2017)
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