Seepage Characteristics of 3D Micron Pore-Fracture in Coal and a Permeability Evolution Model Based on Structural Characteristics under CO2 Injection

Sequestration of CO2 in unworkable coal seams is one of the most promising carbon reduction strategies. The mobility of CO2 in coal reservoirs is closely related to permeability, especially in low-permeability reservoirs. To this end, a method of reconstructing 3D pore-fracture networks by combining nuclear magnetic resonance and CT was proposed; and the relationship between the structural characteristics of 3D micron pore-fracture networks and permeability was studied. It was found that the average pore–throat diameter was positively correlated with permeability (R2 = 0.84) among the parameters reflecting the complexity of the pore-fracture networks, which plays a vital role in the permeability of micron pore-fractures. Subsequently, skeleton deformation and permeability evolution of 3D micron pore-fracture networks under high-pressure CO2 was studied using COMSOL software, and a permeability evolution model was established based on the structural characteristics. To evaluate the reliability of the model, it was verified to experimental results, resulting in an excellent linear positive correlation between theoretical and experimental permeability (R2 = 0.999). This showed that the permeability evolution model is instructive for engineering. In addition, the adsorption-induced strain swelling, slippage effects and fractures have a crucial effect on the permeability of coal, which is necessary for predicting reservoir permeability.