ANALYSIS OF PERMEABILITY EVOLUTION CHARACTERISTICS BASED ON DUAL FRACTAL COUPLING MODEL FOR COAL SEAM

Abstract

In study of gas migration process in coal seam, the permeability evolution rule with time has been one of hot topics in the area of coal seam for decades. At present, in view of the time-varying rule of coal seam permeability, the influence of microstructure parameters and adsorption effect are seldom considered simultaneously. In this paper, the fractal seepage model coupled with coal deformation, and the adsorption expansion effect of coal is proposed. A multi-field coupling model is established by considering the influence of matrix and fracture structure. The influence of structural parameters and adsorption constant of main fracture on the time-varying curve of macro permeability are analyzed, including: (1) fractal dimension for fracture lengths; (2) maximum fracture length; (3) Langmuir adsorption strain constant; (4) Langmuir adsorption pressure constant. The results show that, under the influence of in situ stress and adsorption effect, the permeability of coal seam decreases with time. The present results show that the permeability is proportional to the fractal dimension for fracture lengths and the maximum fracture length. The uniformity of permeability distribution is found to be directly proportional to the pressure constant of Langmuir and inversely proportional to the volume constant of Langmuir.