Triaxial-mechanical and permeability properties of coal body under varying CO₂ pressures

The sequestration of CO₂ in coal seams has become an effective way to curb greenhouse-gas emissions. Coal mechanics and permeability properties are key factors affecting the safe sequestration of CO₂ in coal seams, and both are significantly affected by the CO₂ injection pressure. In this study, triaxial compression-permeability tests were conducted on coal under varying CO₂ and limited confining pressures using a measurement system to determine the coupled mechanical properties and adsorption permeability of coal. The effects of CO₂ pressure on the mechanical properties and evolution of coal permeability were investigated. A three-dimensional statistical damage constitutive model of coal that considers CO₂ adsorption damage was established. The results showed that the stress-strain curve of the coal was divided into four stages. During the first two stages, the amplitudes of the permeability changes were small, whereas at the peak stress point, a permeability "jump" phenomenon occurred, after which an increase in stress resulted in a slower amplitude increase in permeability. The CO₂ pressure had an evident damage-deterioration effect on the mechanical properties of the coal samples, and the primary failure characteristic was shearing damage. The higher the CO₂ pressure, the higher the degree of internal fragmentation and fracture network complexity of the coal body. During the triaxial compression-permeability experiment, the normalized permeability of the coal samples tended to increase slowly, followed by a rapid increase and back to a slow increase. However, with increasing CO₂ pressure, the initial permeability of the coal samples decreased, whereas the normalized permeability increased sharply. The rationality and accuracy of the constitutive model were verified by comparing the established constitutive model and test stress-strain curves of the coal samples. The results of this study can provide theoretical references for CO₂ geological storage and facilitate the selection of appropriate CO₂ injection pressures.