Hydraulic Fracture Cross-Layer Propagation in Multicyclic Sandstone: Experimental Investigation of Driving and Resisting Stress Competition for Reservoir Stimulation

Abstract

Efficient stimulation of heterogeneous sandstone reservoirs requires a comprehensive understanding of cross-layer hydraulic-fracture propagation and its controlling mechanisms. Taking the Jurassic Lianggaoshan Formation in the Sichuan Basin as the geological prototype, 300 mm cubic artificial specimens containing coarse-, transitional-, and fine-grained cycles were prepared by sequential layered casting and tested using large-scale true-triaxial hydraulic fracturing. The coupled effects of interlayer and vertical stress differences on cross-layer fracture propagation were investigated. Under the tested conditions, increasing the interlayer stress difference from 2 to 6 MPa changed the fracture response from continuous, nearly vertical penetration through all depositional cycles to fracture arrest and pronounced diversion along the interface, forming a T-shaped branch and producing large-amplitude sawtooth pressure fluctuations. At a vertical stress difference of 2 MPa, the fracture was arrested at the upper interface, accompanied by extensive lateral diversion and asymmetric downward deflection, whereas a vertical stress difference of 6 MPa enabled continuous penetration through all three depositional cycles and produced a characteristic stepwise pressure response. These contrasting behaviors indicate that cross-layer fracture propagation is governed by competition between the vertical driving force and interfacial resistance. The stepwise pressure response observed during successful cross-layer penetration is consistent with a cyclic mechanism involving inferred fracture-tip blunting, net-pressure accumulation, interfacial breakthrough and reinitiation, and local pressure release. The results provide a mechanistic basis for understanding hydraulic-fracture propagation across heterogeneous depositional interfaces in multicyclic sandstone reservoirs.