Analysis of an Unsaturated Seepage Mechanism in Coal Seam Water Injection

The wetting process of coal seam water injection is a typical unsaturated flow, but the steady-state method for measuring liquid permeability cannot reflect the unsaturated flow process of water in coal. The principle of wetting and expanding process of liquid in coal medium is very complex, and the multiscale pore characteristics of coal make the liquid permeability show multiscale characteristics. Therefore, liquid-phase triaxial seepage experiments under different working conditions are carried out, and a one-way multiscale dynamic apparent permeability coefficient model D(t) = D ₀ exp(-βt) is established to analyze the influence of pressure, liquid wettability, and pore structure on the unsaturated wetting process of coal seam water injection. The results show that the apparent permeability coefficient of liquid phase decreases gradually with time due to the multiscale pore characteristics of coal. The dynamic apparent permeability coefficient model can well describe the unsaturated seepage process of liquid in coal, which is helpful to characterize the liquid permeability of low permeability coal seam. The water injection pressure has a great influence on the attenuation coefficient of apparent permeability coefficient, and the higher the water pressure is, the smaller the attenuation coefficient is. The surfactant increases the permeability coefficient by enhancing the capillary wetting, while the capillary force, as an internal factor, has less influence on the model attenuation coefficient than the water pressure.