Fractal Model for Spontaneous Imbibition in Coal Considering Gas Adsorption-Induced Swelling Deformation

To investigate the effects of gas adsorption on coal expansion and deformation during the injection of water into gas-containing coal seams as well as their permeability and absorption characteristics, based on the mechanism of gas adsorption in coal, a coal porosity model for expansion and deformation was derived in this study. The existing spontaneous coal infiltration model was improved by combining Hagen Poiseuille's law and fractal dimension; the result is a fractal model of spontaneous infiltration considering gas adsorption conditions. Through the use of a self-developed experimental system for spontaneous infiltration of moisture into gas-containing coal, experiments were conducted at different pressures to verify the accuracy of the model. Research has shown that coal expansion deformation induced by gas adsorption has a significant impact on spontaneous imbibition. As the gas pressure gradient increases, the coal expansion strain exhibits nonlinear enhancement characteristics. This deformation reduces the effective porosity by compressing pores, and the evolution of the pore structure significantly increases the capillary force-dominated imbibition driving force, thereby significantly increasing the maximum imbibition height Le. This study provides important theoretical guidance for the study of spontaneous infiltration and absorption of gas-containing coal and for improving water injection technology for gas-containing.