Dynamics characterization of N2 infiltration-displacement in non-homogeneous coal seams

Abstract For the limitations of laboratory testing for permeability of heterogeneous coal seams, an in-situ direct testing method for coal seam permeability was proposed. Based on the steady-state gas permeation theory, the permeability of coal seams was calculated in different orientations. And Weibull distribution was used to establish a random permeability field for coal seams. Meanwhile, based on gas seepage and diffusion theory, gas injection replacement and displacement model is established to numerically analyze the time-dependent characteristics of injecting N2 to replace CH4 in homogeneous and non-homogeneous coalbeds. The results show the permeability of coal seams has significant differences in spatial orientation, and the ratio can reach 2.96 for the maximum and minimum in-situ permeability in different orientations. Based on the stochastic permeability field of non-homogeneous coal seams, the mass fraction contour of N2 injected into coal seams is an irregular curve, and in the later stage of injecting N2 to replace CH4, the region area is increased by about 20 % when the N2 mass fraction reaches 0.9 compared with that in homogeneous coal seams. Compared with the effect in homogeneous coalbeds at 800d, the cumulative CH4 production increased by 10.22 % in the case of a 7.09 % decrease in the cumulative N2 storage capacity in non-homogeneous coalbeds. Research on enhanced coalbed CH4 extraction by injecting N2 into non-homogeneous coalbeds can determine suitable development schemes and optimize the location of production wells and improve coalbed methane production capacity and development efficiency.