Fracture propagation law of shale rocks using pre-fracturing technology with supercritical CO2 fluids of different volumes

Shale reservoirs are intrinsically tight, and conventional fracturing generally induces substantial reservoir damage while only simple fracture geometries are generated. This study investigated the effects of supercritical CO2 (Sc-CO2) volumes on artificial fracture morphology and propagation law during Sc-CO2 pre-injection gel fracturing. A true triaxial fracturing apparatus, combined with micro-CT and coupled thermal-hydraulic-mechanical-damage (THMD) numerical model were applied to elaborate characters of fracture initiation pressures and geometries after using different volumes of Sc-CO2 during pre-injection gel fracturing. The physical and numerical simulation analyses revealed the fracture propagation law that is associated with Sc-CO2 pre-fracturing. The results are as follows: (1) Sc-CO2 pre-injection effectively reduced the fracture initiation pressure by 12.7% as the pre-injection volume increased from 0 to 88.61 mL; (2) greater pre-injection volumes of Sc-CO2 enhanced fracture complexity degree, reflected by that the indexes of fractal dimension, tortuosity, and volume of the induced artificial fractures; (3) the THMD model was successfully validated, which can accurately predict fracture propagation morphology, fracture area ratio, and fractal dimension; (4) both the physical and numerical simulations indicated that, the fracture apertures increased from 0.09 mm to 0.15 mm, as the pre-injection volumes of Sc-CO2 increased from 14.53 to 88.61 mL; (5) fracture propagation exhibits a nonlinear dependence on Sc-CO2 pre-injection volume, characterized by weakly activated, effectively activated, and saturation-activated patterns. These findings provide systematic guidance for optimizing the process parameters of Sc-CO2 pre-fracturing in shale reservoirs.