Numerical Simulation of Three-Dimensional Fracture Propagation During Supercritical Carbon Dioxide Fracturing

Abstract By establishing a comprehensive thermal-hydro-mechanical coupled numerical model, this research elucidates the fracture propagation behaviors in unconventional reservoirs subjected to supercritical carbon dioxide (SC-CO2) fracturing. A comparative analysis is conducted to elucidate the discrepancies between conventional hydraulic fracturing and SC-CO2 fracturing, and the influence of engineering parameters on the fracture geometries during SC-CO2 fracturing is revealed, providing useful insights for optimizing SC-CO2 fracturing strategies and catalyzing the sustainable utilization of carbon dioxide. An integrated numerical model for three-dimensional (3D) fracture propagation during SC-CO2 fracturing in unconventional reservoirs is established based on the displacement discontinuity method (DDM) and the finite volume method (FVM). This model comprehensively captures the interactions among rock deformation, fluid flow and crack propagation, and incorporates the variations in rheological parameters of SC-CO2 and water under different pressure and temperature. A mixed fracture propagation criterion and fracture-matrix exchange model are utilized to describe the fracture trajectories and fluid flow between fractures and reservoir matrix, respectively. The proposed numerical model is solved by Newton-Raphson method and Picard method. Numerical simulations indicate that leak-off induced stress and thermal stress could change the geometries of hydraulic fracture (HF). Moreover, fracture trajectories and bottomhole pressure variations significantly differ between conventional hydraulic fracturing and SC-CO2 fracturing. Compared with water, fractures created by SC-CO2 exhibit smaller fracture width and fracture length under the same injection volume. The heightened diffusivity of SC-CO2 leads to substantial filtrate losses, triggering notable fluctuations in bottom hole pressure and stress interference that differs from those obtained by conventional hydraulic fracturing treatments. Therefore, the optimized cluster spacing for uniform propagation of multi-fracture differs between conventional hydraulic fracturing and SC-CO2 fracturing. Sensitive analyses show that large cluster spacing and high injection rate are favorable for increasing the fracture width and promoting uniform growth of multi-fracture under SC-CO2 fracturing.