3D Fracture Network Morphologies and Breakdown Pressure in Supercritical CO2 Fracturing of Tight Sandstones: Effects of High Temperatures and In Situ Stress Differentials

Supercritical CO₂ (SC-CO₂) fracturing is a key technology for the development of unconventional tight reservoirs. With an increase in the extraction depth, both the temperature and the in situ stress of the reservoirs rise rapidly. Understanding and characterizing the effects of high temperature and in situ stress on fracture propagation during SC-CO₂ fracturing are crucial for optimizing this technique. However, few SC-CO₂ fracturing experiments under true triaxial loading conditions with temperatures up to 200 °C have been conducted. In this study, true triaxial fracturing tests were conducted on artificial tight reservoir sandstone specimens to explore the influence of different high temperatures and in situ stress differentials on SC-CO₂ fracturing behaviors. Using computed tomography and the U-net artificial intelligence algorithm, the 3D morphology of fracture networks and the breakdown pressure were quantitatively analyzed. Experimental results show that thermal shock induced by the temperature difference between the fracturing fluid and the rock matrix significantly affected the breakdown pressure and enhanced the complexity of fracture networks. Compared to conventional hydraulic fracturing, SC-CO₂ fracturing generated more complex and tortuous fracture networks, reducing the breakdown pressure by 16–24%. As the temperature increased, the number of branch fractures within the SC-CO₂ fracture network substantially increased, boosting fracture complexity by 8–9%, approximately. In contrast, as the horizontal stress differential increased, the complexity of SC-CO₂ fractures gradually decreased by 4–6%. These findings provide important insights into the mechanisms of fracture network propagation in SC-CO₂ fracturing of unconventional tight reservoirs at different depths.