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.
Highlights
The permeability of coal is a key transport property in determining coalbed methane production and CO2 storage in coal seam reservoirs
Energies 2018, 11, 2800 have been formulated to quantify permeability evolution from such laboratory experiments [16,17,18,19,20,21,22,23]. Most of these permeability models fail to explain stress-controlled results since they improperly idealize the fractured coal as a matchstick or cubic geometry, or assume local equilibrium between the matrix and fracture pressures, or ignore matrix–fracture interactions [2]
The fracture may be represented as a softer material [19,29], with the equilibrium pressure applied on the cross section and no sorption-induced strain
Summary
The permeability of coal is a key transport property in determining coalbed methane production and CO2 storage in coal seam reservoirs. Energies 2018, 11, 2800 have been formulated to quantify permeability evolution from such laboratory experiments [16,17,18,19,20,21,22,23] Most of these permeability models fail to explain stress-controlled results since they improperly idealize the fractured coal as a matchstick or cubic geometry, or assume local equilibrium between the matrix and fracture pressures, or ignore matrix–fracture interactions [2]. Many studies focused on developing governing equations in dual-continuum systems (fractured rock) when modeling coupled liquid flow and mechanical processes [31,32], the lack of consideration of the fracture–matrix interaction may cause unacceptable errors if these equations are directly used for the fractured coal This is because many studies only noted the temporal effect on aperture change but ignored the influence on bulk deformation [28,33,34].
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