Investigation of the influence of gas fracturing on fracturing characteristics of coal mass and gas extraction efficiency based on a multi-physical field model

Abstract

Coal seam gas recovery is a critical way to achieve clean energy development and reduce greenhouse gas emissions. Gas fracturing, as a necessary and advantageous stimulation measure, can effectively achieve gas extraction from deep coal seams and ensure safe production in deep coal mines. In this study, a hydro-mechanical coupling model is established to analyze the damage characteristics of coal seams during gas fracturing. The model focuses on three aspects: (a). the fracture criterion of type-I fracture under the action of multiple stress states in the fracturing process; (b). the characteristics of post-peak strain softening of coal and its influence on coal permeability; (c). the influence of non-uniform stress state on directional compression deformation and directional permeability of coal. The difference between gas fracturing (nitrogen, CO2) and hydraulic fracturing is studied based on this coupling model. The development of fractures in coal seam gas reservoirs after gas fracturing is investigated, and the influences of the coal parameters of the gas extraction process after fracturing are determined. Results show that because of the different compression coefficients, viscosities of water and gas and the slippage effect, there are obvious differences in the fracturing pressure of gas and water. Under the same parameters, the hydraulic fracturing method needs larger fracture initiation pressure than the gas fracturing method (nitrogen, CO2). For two kinds of media (nitrogen, CO2) in gas fracturing, in addition to the parameter difference of gas itself, the adsorption of CO2 is also the main factor affecting the fracturing pressure. The increase of coal permeability and Biot's coefficient will reduce the fracture initiation pressure, and the increase of confining pressure will increase the fracture initiation pressure.