Study on the micro-fracture-structure and permeability behavior of coal under the action of CO2 based on micro-CT

Permeability and porosity are critical parameters in CO2-ECBM and depend on the fine-scale fracture structure of coal. A self-developed high-pressure gas adsorption/desorption experimental system was used to conduct adsorption tests on coal at pressures ranging from 1 to 6 MPa. The CT scanning system was employed to scan coal samples before and after adsorption. This process established the three-dimensional fracture structure of the coal and simulated seepage behavior under different CO2 pressures. After CO2 adsorption, the fractures in the coal expanded significantly. The CO2 pressure had a notable impact on internal fracture development. As CO2 pressure increased, fracture structure parameters also increased, the fracture network became more complex and better connected, leading to an increase in the coal's absolute permeability (K). The permeability was positively correlated with both the fractal dimension and porosity. The growth, expansion, intersection, and connectivity of microfractures caused by higher CO2 pressure were the main reasons for the increase in porosity and permeability. These findings provide theoretical guidance for CO2 geological sequestration and the development of coalbed methane resources.