Experimental investigation on cross-layer propagation of hydraulic fractures in shale-sandstone interbedded reservoirs

China's shale oil and gas resources are widely distributed in shale-sandstone interbedded reservoirs, whose complex lithology and strong heterogeneity pose significant challenges to hydraulic fracturing design. To address issues such as the difficulty in controlling fracture height and the challenge of forming an effective fracture network, this study utilizes synthetic rock samples that can represent the characteristics of interbedded reservoirs and investigates the initiation and propagation of hydraulic fractures under different viscosity, injection rate, and construction scheme. By combining real-time monitoring of injection pressure with acoustic emission, the temporal and spatial evolution characteristics of hydraulic fractures as well as the mechanisms of their vertical and horizontal extension are revealed. The results indicate that a higher fracturing fluid viscosity is essential for ensuring the vertical cross-layer propagation of hydraulic fractures, while a lower fluid viscosity facilitates the activation of weak interlayer surfaces, promoting sufficient horizontal propagation along these planes and forming branched fractures. Although a higher injection rate enhances the vertical cross-layer propagation of hydraulic fractures, it also causes greater diversion of the main fracture plane, resulting in simpler fracture morphology and limiting the stimulation effect. Additionally, an alternating injection of high and low viscosity fracturing fluids allows hydraulic fractures to both break through weak interlayer surfaces and achieve uniform horizontal propagation, resulting in a more complex fracture morphology. The findings are expected to provide a scientific basis and practical guidance for optimizing hydraulic fracturing designs in interbedded reservoir conditions.