Abstract
During methane extraction, the permeability of a coal seam is the vital factor affecting recovery. Although the permeability of a coal seam and its relationship with porosity have been studied in a few works, the calculation process of coal seam permeability is usually too simplistic or neglects the influence of microscopic fracture structures. Statistical research shows that the permeability of coal seams with the same porosity and different fracture structures is quite different. For the purpose of quantitatively investigating the contribution of fractures and pore structure in coal seams, a fractal permeability model considering the microstructure of coal seam fracture is established in this paper. The correctness of the model is verified by comparing with the previous research results. Then, the influence of the microscopic fracture structure on the equivalent permeability is analyzed. The simulation results show that the permeability of fractured coal is directly proportional to the fractal dimension of the fracture, the maximum fracture length and the azimuth. It is inversely proportional to the tortuous fractal dimension and the dip angle of the fracture surface. This conclusion provides the foundation for revealing the microstructure mechanisms of macroscopic seepage characteristics of coal seams, and implementing effective strategies to enhance gas recovery rates under different geological structures.
Highlights
Published: 12 February 2022As the main channel for gas migration in a reservoir, fracture length is unevenly distributed, with varying degrees of tortuosity and a complex structural distribution
The coal fracture network is complex [4,5,6]; accurately exposing the connectivity of the reservoir structure and quantitatively investigating the interaction between the reservoir microstructure and the macroscopic behavior at the micro scale is the key to improving gas recovery [7,8,9,10]
An increasing number of studies have demonstrated that fractures in unconventional reservoirs have distinct fractal characteristics and that fractal theory is better able to quantitatively characterize the distribution of fracture structure networks
Summary
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou 221116, China. State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221116, China
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