Analysis of coal permeability rebound and recovery during methane extraction: Implications for carbon dioxide storage capability assessment

Coal permeability rebound and recovery severely affect the efficiency of coalbed methane extraction and CO2 storage capability of coal seams, but theoretical research on it is insufficient now. Besides, ambiguity still remains in evolution laws of coal permeability rebound and recovery pressure with the change of various influencing factors. In this work, the focus is first placed on the influences of effective stress (considering engineering-strain and natural-strain) and adsorption-induced swelling (considering matrix bridge) on fracture aperture; then, a new evolution model of coal fracture aperture is established by adopting the competition mechanism of the two. At the same time, based on the correlation between the variation of fracture aperture and permeability using the classical cubic law, the evolution model of coal permeability is set up, of which is on the basis together with some reasonable assumptions to obtain the factors influencing permeability rebound and the recovery pressure. The evolution laws of coal permeability rebound and recovery under the influence of main factors are in detailed analysis. Specifically, permeability rebounds and recovers when the initial coal reservoir pressure is greater than its switching threshold. The greater the initial pressure, the larger the numerical range dropping from the initial value to rebound value, so does the effect of coal cleat compressibility. However, only on condition that the internal swelling coefficient is smaller than its switching threshold, the permeability will rebound and recover. Besides, the influencing mechanism of CO2 storage and CBM extraction on permeability evolution is the same, while the variation laws of permeability, of rebound and recovery especially, exert strong impact on CO2 storage capability. Therefore, the influence of various permeability evolution laws on CO2 storage capability is discussed macroscopically for valid assessment of it, providing guidance to select appropriate coal seams for CO2 storage.