Mechanism of reservoir breakdown and fracture propagation of inter-salt shale oil with muitiple lithologic layers

This study addresses the unclear fracture behavior and unsatisfactory fracturing effects in the multi-lithologic-layers interactive reservoirs of salt-interbedded shale oil. Using the Qianjiang Formation reservoir in the Jianghan Basin as a research object, true triaxial hydraulic fracturing experiments were conducted to investigate the fracture breakdown and propagation mechanisms. The results show that fracture propagation patterns can be classified into six types: through-and-turn, through-without-turn, turn-without-through, without-turn-without- through, dissolution-and-through, and dissolution-without-through. The core mechanism of fracture propagation lies in the dynamic coupling between the lithology interface and the stress field. The sensitivity ranking of the influencing factors is as follows: fluid-rock interaction > elastic modulus > injection rate > interface strength > fracturing fluid viscosity. The study also reveals that for every 10 MPa increase in elastic modulus difference, fracture height and length increase by 0.58 cm and 0.43 cm, respectively. For every 1 MPa increase in interface strength, fracture height increases by 0.28 cm, while fracture length decreases by 1.81 cm. Compared to water-based and alcohol-based fracturing fluids, SC-CO₂ fracturing fluid reduces the fracture pressure by 32% and 24%, respectively. For every 1 mL/min increase in single-cluster injection rate, fracture length increases by 0.06 cm. Additionally, for every 10 mPa·s increase in fracturing fluid viscosity, fracture height and length increase by 0.20 cm and 0.07 cm respectively. This study provides a theoretical basis for the efficient development of multi-lithologic-layers interactive reservoirs in salt-interbedded shale oil and offers important guidance for on-site fracturing operations.