Study on nonlinear seepage mechanism of water injection coal under the action of threshold pressure gradient

Abstract

During the initial stage of water injection into deep, low-permeability gas-bearing coal seams, a significant threshold pressure gradient causes the seepage behavior to deviate from Darcy's linear flow law. Consequently, the pressure gradient of pressure-driven water increases during transport through the porous medium, which restricts the effective infiltration range of water injection. Based on fractal theory and seepage equations, this study develops permeability and threshold pressure gradient models for gas-bearing coal by considering the effects of the boundary layer, gas pressure, surface tension, and contact angle. An influencing-factor analysis was further conducted to clarify the roles of different parameters in the nonlinear seepage process. Moreover, considering the physical and chemical properties of the fluid, a mechanism for permeability enhancement during the initial nonlinear seepage stage of water injection is proposed. The results indicate that the initial threshold pressure gradient during water injection is positively correlated with gas pressure and surface tension, but negatively correlated with pore radius and contact angle. The boundary layer effect reduces the effective flow radius, thereby decreasing coal permeability. After the addition of sodium dodecyl benzene sulfonate to the injected water, the surface tension and contact angle are significantly reduced, resulting in a lower threshold pressure gradient, enhanced coal permeability, and a substantially larger water infiltration range. These findings provide a theoretical basis and technical support for optimizing water injection parameters and improving disaster prevention and control in deep, high-gas coal seams.