Effect of the water injection pressure on coal permeability based on the pore‐fracture fractal characteristics: An experimental study

Coal seam water injection is a commonly used method by which to improve coal seam permeability. To reveal how this method can enhance coal permeability from the perspective of the pore‐fracture structure, the permeability and porosity of raw coal samples under different water injection pressures were tested. Low‐temperature nitrogen adsorption (N2GA) and high‐pressure mercury intrusion porosimetry (MIP) were employed to characterize the changes in the pore‐fracture structures of water‐injected coal samples. Moreover, the relationship between the fractal dimension and coal permeability was studied using fractal theory. The results revealed the following. The internal pore‐fracture structures of the coal samples were rich in variety, and the pore span was large. With the increase of the water injection pressure, the volume ratios of macropores and mesopores gradually increased. Moreover, in the test interval, the fractal dimension of the water‐injected samples did not fluctuate substantially. With the increase of the water injection pressure, the integral dimension of seepage pores became positively correlated with permeability, while the fractal dimension of diffusion pores became negatively correlated with permeability. The results indicate that the increase in the water injection pressure not only increases the proportion of seepage pores, but also densifies the diffusion pores. Thus, the uniformity of diffusion pores can well reflect the permeability level of coal samples. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.