Evolution and analysis of gas sorption-induced coal fracture strain data

Abstract

Although coal swelling/shrinking during coal seam gas extraction has been studied for decades, its impacts on the evolution of permeability are still not well understood. This has long been recognized, but no satisfactory solutions have been found. In previous studies, it is normally assumed that the matrix swelling/shrinking strain can be split between the fracture and the bulk coal and that the splitting coefficient remains unchanged during gas sorption. In this study, we defined the fracture strain as a function of permeability change ratio and back-calculated the fracture strains at different states. In the equilibrium state, the gas pressure is steady within the coal; in the non-equilibrium state, the gas pressure changes with time. For equilibrium states, the back-calculated fracture strains are extremely large and may be physically impossible in some case. For non-equilibrium states, two experiments were conducted: one for a natural coal sample and the other for a reconstructed one. For the fractured coal, the evolution of permeability is primarily controlled by the transition of coal fracture strain or permeability from local matrix swelling effect to global effect. For the reconstituted coal, the evolution of pore strain or permeability is primarily controlled by the global effect.

Differential Strain Index-Based Multiphysics Model for Coal Seam Gas Production

“…Zhang et al developed an effective strain-based coal permeability model, assuming that the sorption-induced matrix strain is the same as that of fracture. Under this assumption, the adsorption strain does not affect porosity changes and permeability evolution, which is not completely consistent with the experimental results . Due to the sharp contrast in permeability between fractures and matrix, the diffusion process from fracture to matrix lasts for an extended period following the gas injection, from a few months to years .…”

“…Under this assumption, the adsorption strain does not affect porosity changes and permeability evolution, which is not completely consistent with the experimental results. 43 Due to the sharp contrast in permeability between fractures and matrix, the diffusion process from fracture to matrix lasts for an extended period following the gas injection, from a few months to years. 28 Liu et al 7,44 introduced the concept of "internal expansion stress".…”

Novel Dynamic Multiscale Model of Apparent Diffusion Permeability of Methane through Low-Permeability Coal Seams

“…In China, CBM extraction has been showing low production due to the low permeability of coal seams in the past two decades. Coal is one of the porous gas reservoirs that contain pores and fractures. − Typically, the gas flow in coal fractures is described using Darcy’s law; thus, the fracture permeability is measured by the steady-state method . The gas flow in the coal matrix is usually described by Fick’s law.…”