Effect of CO₂ pre-injection on fracture propagation morphology in shale reservoirs under CO₂ hybrid fracturing

Supercritical CO₂ fracturing, as a waterless fracturing technology, is attracting increasing attention in the shale oil reservoir development industry. In recent years, a novel CO₂ hybrid fracturing method has been proposed to integrate the advantages of both CO₂ fracturing and hydraulic fracturing. However, the specific effects of different pre-injection CO₂ conditions on the physical properties, mechanical characteristics, and crack propagation behavior of shale reservoirs remain unclear. This study utilized Chang-7 shale samples from the Ordos Basin and conducted CO₂ hybrid fracturing experiments under simulated high-temperature and high-pressure reservoir conditions, employing a self-developed experimental apparatus. Quantitative analysis of fracture propagation patterns under the influence of CO₂ pre-injection was performed based on CT scanning results. The findings reveal that: (1) Among different fracturing fluid systems, conventional hydraulic fracturing exhibits the highest breakdown pressure, pure CO₂ fracturing is intermediate, while CO₂ hybrid fracturing significantly reduces the breakdown pressure by 36.2% compared to hydraulic fracturing. (2) Employing CO₂ hybrid fracturing not only effectively increases fracture dimensions (length, width) but also substantially enhances fracture network complexity. (3) The CO₂ pre-injection soaking time significantly influences fracture morphology, with both fracture dimensions and structural complexity showing marked increases as soaking time extends. (4) Increasing formation pore pressure promotes the activation of bedding planes with relatively weaker mechanical strength, leading to significant enhancements in fracture length and complexity, but simultaneously restricts the widening of fracture apertures. The outcomes of this research provide a theoretical foundation for optimizing the design of operational parameters in CO₂ hybrid fracturing for shale oil reservoirs.