Experimental Study on the Mechanism of Cross-Layer Propagation of Hydraulic Fractures in Multilithologic Interbedded Reservoirs

Multilithologic interbedded reservoirs commonly consist of frequent alternations of fine-grained rocks, carbonate rocks, and soluble evaporite interlayers. The contrasts in mechanical properties and fluid–rock interactions tend to induce hydraulic-fracture deflection, height containment, and complex cross-interface propagation. To elucidate fracture initiation and cross-layer connectivity, a self-developed true-triaxial hydraulic fracturing simulation system was used to systematically investigate the effects of lithologic configuration, fracturing-fluid viscosity, injection rate, interface position, and injection-fluid type on fracture morphology and cross-interface behavior. Integrated analyses were performed by jointly interpreting injection-pressure responses and three-dimensional fracture reconstructions. The interactions between hydraulic fractures and lithologic interfaces/natural fractures can be categorized into three modes: (i) deflection with restricted growth, (ii) penetration without activation, and (iii) penetration with synchronous activation. Under water-based fluids, soluble evaporite interlayers predominantly develop dissolution-induced conductive pathways, which reduce stress concentration at the fracture tip and weaken interface strength, thereby promoting activation of interfaces or natural fractures. Moderately increasing viscosity and injection rate enhances cross-layer connectivity while lowering the probability of passive activation of interfaces/natural fractures; however, excessively high injection rates may induce fluid diversion and increase the likelihood of complex fracture growth. The injection-fluid type exerts a pronounced control on breakdown pressure and connectivity patterns: supercritical CO2 yields the lowest initiation pressure, water-based fluids the highest, and alcohol-based fluids an intermediate response. In the pressure curves, attenuation of propagation pressure corresponds to enhanced cross-layer penetration, whereas a sustained pressure increase indicates dominant diversion or restricted propagation. These findings provide experimental support for parameter optimization and fracture-control design in multilithologic interbedded reservoirs in Southwest China and analogous geological settings.