Scale-Dependent Transition from Single Fracture Interaction to Fracture Network Evolution during Supercritical CO2 Fracturing of Naturally Fractured Shale

Abstract

Supercritical CO2 (ScCO2) fracturing is a promising method for shale gas stimulation because it can reduce water consumption and provide a potential route for CO2 utilization. In naturally fractured shale, stimulation performance is controlled not only by the extension of induced fractures, but also by their interaction with pre-existing discontinuities under in situ stress contrast. In this study, small-scale true-triaxial fracturing tests, micro-CT imaging, and a two-dimensional thermo-hydro-mechanical-damage (THMD) model with explicit natural fractures were used to investigate ScCO2-induced fracture interaction from a single-fracture scale to a fracture-network scale. True-triaxial tests with water and ScCO2, combined with micro-CT observations, were used to identify representative fracture-weak plane interactions and the principal propagation, branching, and intersection patterns. The observed fracture morphologies provided the benchmark for a qualitative evaluation of the numerical model. The evaluated model was then used for systematic parametric analyses of discrete fracture networks with different natural fracture orientations, densities, and horizontal stress differences. The results show that ScCO2 fracturing produces larger fractured areas and higher fracture complexity than water fracturing under the studied conditions. For a single natural fracture, the most favorable interaction was obtained at an intersection angle of 45°. At the fracture-network scale, the response became nonmonotonic. More developed fracture networks were obtained only under moderate fracture density and stress contrast. In the influencing factor numerical analyses, a dominant NF orientation of 30°, P20 = 4000 m–2, and Δσ = 6 MPa each produced the largest fracture-region area within its respective investigated range. These results suggest that natural fractures can promote ScCO2-driven fracture-network development when their orientation, density, and stress conditions are favorable. The findings provide useful evidence for designing water-saving shale gas stimulation strategies with potential CO2 utilization.