Mechanism and experimental study of deformation and permeability of low-permeability coal structures under stress-desorption effects

Abstract

To enhance gas extraction efficiency, it is crucial to elucidate the factors influencing desorption deformation and permeability in low-permeability coal structures, as well as their interrelationships. In this study, using a self-developed triaxial diffusion-permeability device, desorption-apparent permeability experiments were conducted for methane (CH₄) and helium (He) under different gas pressures and confining stress conditions. A method for the determination of apparent desorption permeability was proposed. The relationship between coal structure desorption deformation and gas pressure, confining stress, and gas types was explored. Additionally, the relationship between apparent desorption permeability and coal structure deformation was analyzed, identifying the dominant influencing factor. A strain-apparent desorption permeability model was developed and experimentally validated. The results indicated that under constant stress, decreasing CH₄ desorption pressure initially resulted in radial shrinkage followed by expansion, while axial expansion initially decreased and then increased. The apparent desorption permeability decreased and then increased with decreasing gas pressure, exhibiting a 45.26-fold increase when the gas pressure decreased sixfold. Furthermore, apparent desorption permeability increased exponentially with coal column volumetric strain. Subsequently, under constant gas pressure, radial coal cylinder shrinkage increased linearly with decreasing confining pressure, while axial expansion and volumetric strains increased with decreasing He pressure. At high–pressure stages (1 MPa–3 MPa), CH₄ volumetric strain exhibited an 11.72–fold increase compared with He. However, in the low-pressure phase (1 MPa–0 MPa), the value of the He volumetric strain exhibited a 2.43-fold increase compared with CH₄. This study highlighted adsorption and slip effects as primary controls on desorption deformation in low-permeability coal structures.