The influence of hydrostatic stress on cross-scale gas emission behavior in coal

Gas drainage by boreholes is an important means to exploit coal seam gas. However, the high hydrostatic stress in deep in situ coal seams significantly limits the gas extraction efficiency. Gas drainage involves the problem of cross-scale gas emission under in situ stress. Nevertheless, the deformation response of the pore–fracture structure of coal under different hydrostatic stresses and its effect on the gas emission behavior have not been clarified. Therefore, gas seepage and emission experiments for four coal samples under different hydrostatic stresses were first carried out, and the evolutions of the fracture and matrix bulk modulus under different hydrostatic stresses were analyzed. Then, the control mechanism of the magnitude of hydrostatic stress on the gas emission behavior was analyzed through the changes in gas diffusion coefficients. In addition, a cross-scale gas emission model was developed to analyze the competitive effects of matrix pores and fractures on gas emission and the transformation behavior of gas diffusion modes. The main conclusions are as follows: The gas emission of coal with a small fracture bulk modulus is controlled by the fracture bulk modulus, while the gas emission of coal with a large fracture bulk modulus is controlled by the matrix bulk modulus. When the hydrostatic stress is small, the gas emission is mainly affected by the fracture closure effect. When the hydrostatic stress further increases, the change in the scale of the matrix pores becomes significant, causing the gas in the bulk diffusion state to transform into the surface diffusion state.