Quantitative Characterization of Water Fracturing and Supercritical CO2 Fracturing in Continental Shale: Synergistic Effects of Stress and Fluid Type

To elucidate the effects of fluid viscosity and stress on the characteristics of hydraulic fractures, a series of triaxial laboratory experiments were performed on continental shale using water and supercritical CO2 (Sc-CO2). Utilizing advanced computed tomography scanning and three-dimensional reconstruction techniques, we quantitatively assessed the fractal dimension, connectivity, and volume of the induced hydraulic fractures. To provide a rigorous quantitative evaluation of the hydraulic fracturing effectiveness, we introduced a novel comprehensive fracturing index (CFI). Our experimental results reveal that the bedding planes of the Chang73 Formation continental shale significantly influence the morphology of hydraulic fractures. Under consistent differential stress conditions, Sc-CO2 preferentially initiates fractures along the bedding planes, exhibiting limited vertical propagation. Notably, a higher differential stress is required to promote vertical propagation of the fractures, with approximately 30 MPa necessary for Sc-CO2 in this study to induce vertical propagation. This study represents the first detailed analysis of the fracturing performance of continental shale under various confining pressures using Sc-CO2. At a confining pressure of 20 MPa, the height of hydraulic fractures was only 14.5% of that observed without a confining pressure under the same differential stress. Through the application of the proposed CFI, we observed that, under identical differential stress conditions, the CFI for Sc-CO2 fracturing was lower compared to that for water fracturing. With increasing differential stress, the CFI for water-based fracturing exhibited an initial increase, followed by a decrease. When the differential stress was sufficiently high to make fractures to propagate vertically, the complexity of the fracture morphology induced by Sc-CO2 increased significantly, leading to a notable rise in CFI. This research provides critical empirical insights for the selection of fracturing fluids and the optimization of fracturing techniques for continental shale formations.