Dual-Porosity Coupled Borehole Gas Flow Model: A New Method for Inversion of Coal Seam Permeability
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Original Paper
Dual-Porosity Coupled Borehole Gas Flow Model: A New Method for Inversion of Coal Seam Permeability Jia Liu,1 Yueping Qin ,1,3 Tianbai Zhou,1 and Yu Gao2 Received 2 November 2019; accepted 7 May 2020
The permeability of a coal seam is an important index for coal mine gas control and coalbed methane development, and its magnitude determines the degree of difficulty of gas drainage. To obtain the permeability value, a dimensionless mathematical model for dual-porosity borehole gas-coupled flow in a coal seam was established and adopted using a simulator developed by our group. A new method of inversion was developed to determine the fracture permeability coefficient kf and the matrix micro-channel diffusion coefficient Km by fitting the simulated results with onsite measured data. A range of simulations quantified the effects of different dimensionless parameters on gas migration. The results verified the feasibility of the inversion method based on the high matching degree of the fitted results, and the dimensionless mathematical model was accurate. The desorption and release of adsorbed gas from the center to the surface in coal matrices were heterogeneous, and unsteady states and gas migration times in coal matrices cannot be neglected. The new method can be introduced to analyze the problem of gas migration in different coal reservoirs, simplify the corresponding calculation and computational processes, and provide guidance in determining the permeability of coal seams. KEY WORDS: Inversion, Dimensionless, Fracture permeability coefficient, Matrix micro-channel diffusion coefficient.
INTRODUCTION Coal permeability is a crucial criterion for coal and gas outburst disasters, coalbed methane development and gas emissions in boreholes (Karacan et al. 2011; Xia et al. 2014; Zhou et al. 2016; Lin et al. 2019; Wang et al. 2019). Domestic and foreign scholars have performed several experimental and theoretical studies on the permeability of coal seams
1
College of Emergency Management and Safety Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China. 2 Madaotou Coal Company, Datong Coal Group, Datong 037000, China. 3 To whom correspondence should be addressed; e-mail: [email protected]
(e.g., Pan et al. 2010; Li et al. 2011; Liu et al. 2011; Mitra et al. 2012; Feng et al. 2017; Niu et al. 2017) and have established a variety of permeability evolution models (e.g., Gray 1987; McKee et al. 1988; Palmer and Mansoori 1998; Shi and Durucan 2004; Robertson and Christiansen 2008; Liu et al. 2010; Xue et al. 2017). These models can be divided roughly into two types: (1) stress-permeability models (Liu et al. 2010; Robertson and Christiansen 2008; Shi and Durucan 2004; Xue et al. 2017) and (2) porosity–permeability models (Gray 1987; McKee et al. 1988; Palmer and Mansoori 1998). Liu et al. (2010) stated that the adsorption deformation of a coal matrix only partially influences fractures, and they introduced a correction factor to establish a permeability evolution model based o
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