Evolution of Coal Permeability during Gas Injection—From Initial to Ultimate Equilibrium

Xingxing Liu, Jinchang Sheng, Jishan Liu and Yunjin Hu
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Xingxing Liu: College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing 210098, China
Jinchang Sheng: College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing 210098, China
Jishan Liu: Department of Chemical Engineering, School of Engineering, The University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia
Yunjin Hu: School of Civil Engineering, Shaoxing University, Shaoxing 312000, China

Energies, 2018, vol. 11, issue 10, 1-24

Abstract: The evolution of coal permeability is vitally important for the effective extraction of coal seam gas. A broad variety of permeability models have been developed under the assumption of local equilibrium, i.e., that the fracture pressure is in equilibrium with the matrix pressure. These models have so far failed to explain observations of coal permeability evolution that are available. This study explores the evolution of coal permeability as a non-equilibrium process. A displacement-based model is developed to define the evolution of permeability as a function of fracture aperture. Permeability evolution is tracked for the full spectrum of response from an initial apparent-equilibrium to an ultimate and final equilibrium. This approach is applied to explain why coal permeability changes even under a constant global effective stress, as reported in the literature. Model results clearly demonstrate that coal permeability changes even if conditions of constant effective stress are maintained for the fracture system during the non-equilibrium period, and that the duration of the transient period, from initial apparent-equilibrium to final equilibrium is primarily determined by both the fracture pressure and gas transport in the coal matrix. Based on these findings, it is concluded that the current assumption of local equilibrium in measurements of coal permeability may not be valid.

Keywords: equilibrium permeability; non-equilibrium permeability; matrix–fracture interaction; effective stress; coal deformation (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2018
References: View complete reference list from CitEc
Citations: View citations in EconPapers (2)

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