Gas flow field evolution around hydraulic slotted borehole in anisotropic coal

Abstract

Currently, hydraulic slotted borehole technology for strengthening gas drainage is one of the main measures to control gas disasters in deep mines. In this paper, to analyze the evolution of gas flow field around the hydraulic slotted borehole, the borehole gas drainage was simulated based on the anisotropy of coal structure and the established multi-field (namely, coal deformation field, diffusion field and seepage field) coupling model. Then, the influences of characteristic parameters, including, initial diffusion coefficient, diffusion attenuation coefficient, initial gas pressure and initial permeability, on the gas production rate and reservoir parameters of hydraulic slotted borehole gas drainage were analyzed. Finally, the results were verified by comparing the model simulation with field tests in Yangliu Mine. The results show that the anisotropy of coal affects the effect of borehole gas drainage. The initial diffusion coefficient D0 and the diffusion attenuation coefficient λ have opposite effects on key parameters of coal. With a larger D0, it is easier for gas in matrix pores to diffuse into the fracture system, and accordingly the gas production rate and relative permeability grow. A larger λ makes it harder for gas in matrix pores to diffuse into fractures, thus decreasing the gas production rate and relative permeability. The initial gas pressure pm0 and the initial permeability kfx0 and kfy0 exert the same effect on the key parameters of coal. The larger their values are, the more easily the gas in fractures can be extracted under the same negative pressure of drainage, which contributes to the diffusion of gas from the matrix into the fracture system under the action of pressure difference. Through combining actual monitoring results of 3# and 4# drilling fields in Yangliu Mine and the results of numerical simulation, it is acquired that the gas production rate measured in situ coincides with the rate simulated by the model, verifying the feasibility and engineering applicability of the model.