Modeling of water-injected coal with fracture-pore structure and experimental study of gas-water two-phase transport characteristics

Abstract

Predicting the dynamic permeability of water-injected coal is essential for accurately guiding the development of water-injection technologies in coal seams. However, the impact of fracture morphology and complexity on the gas-water two-phase seepage process remains insufficiently explored. This study employed fractal theory to model a tree-like fractal bifurcation network that characterizes the fracture-pore network expansion caused by water injection. The distinct seepage behaviors of adsorption pores, seepage pores, and fractures were considered, and fractal dimensions were introduced to represent changes in pore size distribution and fracture aperture during water injection. Based on these considerations, a dynamic permeability model for water-injected coal with a complex network structure was developed, ensuring that all parameters retained explicit physical significance, free from empirical constants. Furthermore, sensitivity analysis and gray correlation analysis revealed that the number of network bifurcations, tortuosity, and roughness had the greatest influence on permeability. Critical pore diameters for effective and ineffective seepage regions were identified, with experimental results indicating that these diameters were indirectly affected by confining stress and water injection pressure. Changes in pore diameter distribution and water saturation demonstrated that increasing confining stress or reducing water injection pressure led to higher fractal dimensions for pore size distribution and tortuosity, while decreasing the fractal dimension for fracture aperture. This process also caused some bound pores and seepage pores to interchange.