Abstract
The parameters of pore-fracture structure and permeability have a controlling effect on the behaviors of gas adsorption/desorption and transportation in coal reservoir. A mathematic model for coal seams is of great significance to evaluate the mass migration within the coal fracture-matrix system. In this paper, the hydraulic-mechanical coupling model considering both dual-porosity dual-permeability and anisotropy characteristics is first established by using the methods of theoretical analysis, nuclear magnetic resonance (NMR) test, and numerical simulation. Then, this model is applied to simulate the gas migration characteristics of No. 3 coal seam in Changping Mine, China. Results show that the pore structure of No. 3 coal seam is characterized by small radius of pores and pore throats, which is determined by the NMR test, verifying the dual-permeability dual-porosity of coal seams. Both matrix permeability and fracture permeability increase approximately linearly with the process of mine gas extraction. The increased magnitude of the matrix permeability is greater than that of the fracture permeability. The permeability is inversely proportional to the anisotropy coefficient. The pressure gradient within the coal matrix and fracture increases first and then decreases with the extraction time. This pressure gradient is proportional to the anisotropy coefficient at the early stage of extraction and is inversely proportional to the anisotropy coefficient at the later stage. The seepage and diffusion flux are proportional to the anisotropy coefficient. The proportion of matrix-to-fracture seepage flux to the total flux increases first and then decreases to a certain value. The proposed model provides a guide for accurate designation of gas extraction from coal seams.
Highlights
Coalbed methane (CBM), a kind of unconventional natural gas, is a clean energy source for solving the problem of global warming [1, 2] and a valuable by-product of CO2 geological sequestration in coal seams [3,4,5]
Gas migration in coal seams is the core of gas extraction, which involves the interaction of coal and gas, namely, the hydraulic-mechanical coupling responses [11, 12]
E pore structure of the coal seam determines the law of gas adsorption/desorption, diffusion, seepage, and mass transfer between the coal fracture and matrix [13, 14]. e hydraulic-mechanical coupling model in the coal seam can fully reflect the relationship between gas seepage and coal deformation and capture the evolution of coal porosity and permeability [15]
Summary
Coalbed methane (CBM), a kind of unconventional natural gas, is a clean energy source for solving the problem of global warming [1, 2] and a valuable by-product of CO2 geological sequestration in coal seams [3,4,5]. Zhang et al pointed out the geostress and gas pressure control the effective stress on coal, which will change the porosity and Advances in Civil Engineering permeability within the coal seam, and developed a nonlinear coupled mathematical model [16]. Considering gas desorption-induced swelling, Connell developed a coupled model for coalbed methane extraction. In this model, the permeability in the coal seam is sensitive to stress and pore pressure changes [17]. We will creatively propose a hydraulic-mechanical coupling model by considering dualporosity and anisotropy characteristics of the coal seam on the basis of investigation of the pore structure of Changping coal samples. We will creatively propose a hydraulic-mechanical coupling model by considering dualporosity and anisotropy characteristics of the coal seam on the basis of investigation of the pore structure of Changping coal samples. en, the proposed model will be applied to simulate underground gas drainage and reveal the law of gas migration, as well as the influence of permeability anisotropy on gas drainage. is provides a theoretical basis for gas disaster prevention and control in gas-rich coal seam
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