Abstract
Fractures in coal seams are channels for gas migration, and their geometric characteristics can affect permeability, thereby affecting the effectiveness of coalbed methane (CBM) extraction and carbon dioxide sequestration. The surface of natural fractures is rough and composed of minerals with complex shapes. At present, research on rough fracture is mainly focused on surface characteristics, with less attention given to the dynamic effects of rough structure in fractures on coal permeability considering effective stress and matrix adsorption expansion during different stages of methane extraction. In this work, we construct the complex geometric structure in the fracture surface into simplified rough elements. First, a physical model of fracture rough elements was constructed by using the analytical geometry method; meanwhile, the roughness applicable to this model was proposed, and the characterization of roughness degree of different-shaped rough elements was unified. Subsequently, a definition and an approach for selecting and computing effective fracture aperture were proposed. By taking into account the competition between effective stress and matrix adsorption expansion, the change of effective fracture aperture related to pore pressure was determined. Additionally, a novel permeability evolution model was formulated according to the cubic law and compared with the numerical simulation of other researchers for verification. The variation patterns of permeability, gas pressure and fracture aperture under diverse characteristic fracture roughness elements during CBM extraction were obtained by utilizing the proposed model. The findings indicate that the fractures with smaller roughness, higher stagger degree and gentler inclination angle of rough elements are more favorable for CBM extraction. This has inspired a new understanding of the mechanism of CO2-enhanced coal bed methane recovery (CO2-ECBM) technology in deep coal seams with regard to fracture rough element characteristics, which offers valuable theoretical support for the practical implementation of this technology.
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