Fractal Characterization of Three-Dimensional Fracture Networks in Acidified Coal by CT Scanning

To enhance the permeability of low-permeability coal reservoirs, this study conducted acidification experiments on lean coal samples with 3% hydrochloric acid under realistic confining pressure conditions and systematically explored the structural evolution of 3D fracture networks in coal before and after acidification with the aid of micro-CT scanning and 3D reconstruction techniques. The research focused on the comparison of the applicability of three fractal dimensions, i.e., the fracture volume-surface area fractal dimension ( D VS ), the fracture number-based fractal dimension ( D N ), and the fracture volume-scale fractal dimension ( D V ), in characterizing acidification effects. The following beneficial findings were yielded: Acidification remarkably expands fracture volume and surface area while enhancing fracture connectivity, and this effect is particularly pronounced in large fractures. A more uniform initial fracture structure correlates with a better acidification modification effect. Among the three fractal dimensions, D VS derived from the geometric scaling law has the highest theoretical rationality and sensitivity to acidification response, making it the most robust fractal indicator for evaluating acidification effects. This study is expected to provide a new method and theoretical support for the quantitative evaluation and prediction of chemical modification effects in low-permeability coal reservoirs.