Study on Mechanisms of Breakdown and Fracture Propagation in Multi-Lithology Interactive Reservoirs of Inter-Salt Shale Oil

In view of the problems of strong heterogeneity, complex bedding, unclear fracture propagation mechanisms, and great difficulty in fracturing stimulation in multi-lithology interactive reservoirs of inter-salt shale oil in the Qianjiang Sag of the Jianghan Basin, typical multi-lithology interactive reservoirs in this area were taken as the research objects. Relying on physical simulation experiments of true triaxial hydraulic fracturing and following the three similarity criteria of injection rate, in-situ stress, and energy, a systematic study was conducted on the control mechanisms of lithologic combination, elastic modulus, interface strength, fracturing fluid type, injection rate, and viscosity on fracture initiation and propagation. Combined with the gray correlation analysis, the weights of the main controlling factors were clarified, and field tests were carried out for verification. The results indicate that compared with conventional shale reservoirs, inter-salt shale oil reservoirs feature an additional salt-rock corrosion propagation mechanism, forming a total of six types of fracture propagation modes. Fractures easily penetrate from layers with high elastic modulus to layers with low elastic modulus, and low-strength lithologic interfaces easily induce fractures to turn and consume energy. Supercritical CO₂ and alcohol-based fracturing fluids can effectively inhibit salt-rock corrosion and reduce breakdown pressure; a small injection rate and low viscosity are conducive to forming complex fracture networks, while a large injection rate and high viscosity can promote fractures to penetrate and extend across layers. The influence weights of various factors are in the order of fracturing fluid and rock interaction > elastic modulus > injection rate > interface strength > fracturing fluid viscosity. The field application of the fracturing mode of “CO₂ pre-pad + multi-cluster closely spaced high-intensity fracturing + step-up injection rate + high viscosity for stable pressure + shut-in imbibition” can effectively avoid the risk of wellbore salt crystallization and expand the stimulated reservoir volume controlled by fractures, yielding a significant production increase in the test well. The research results provide theoretical support and process references for the efficient fracturing development of multi-lithology interactive reservoirs of inter-salt shale oil.