Quantitative evaluation of coal fracability based on 3D CT reconstruction and fractal characteristics

Fracability is a critical indicator for evaluating the exploration and development potential of coalbed methane reservoirs and assessing the effectiveness of hydraulic fracturing stimulation operations. Its core function is to characterize the complexity of the induced fracture network and the resulting effective stimulated volume. In this study, we quantified fracture area and geometric complexity using true triaxial fracturing experiments and computed tomography three-dimensional (3D) reconstruction technology, combined with the box-counting method to calculate the 3D fractal dimension of the fracture surfaces. The results revealed that the total fracture surface area per unit volume of the stimulated reservoir effectively characterized reservoir fracability; specifically, both a larger total fracture surface area and a higher fractal dimension corresponded to better reservoir fracability. Fracture complexity was enhanced by a decrease in the horizontal principal stress difference or an increase in the injection rate. Under optimal conditions of a 3 MPa stress difference and an injection rate of 60 mL/min, fracability improved by 27.6 %. Furthermore, liquid carbon dioxide (CO 2 ) improved fracability by 50.7 % compared to using water as the fracturing fluid, a result attributed to its low viscosity and strong diffusion capacity, which activated a greater number of natural fractures. A fracability evaluation model integrating brittleness, fracture toughness, and dimensionless net pressure was developed using regression analysis, which demonstrated high reliability with a strong determination coefficient ( R 2 ) of 0.9019. This study clarifies the logical relationships among fracture area, complexity, and fractal dimension, providing a novel method for evaluating the fracability of coal reservoirs.