Experimental study on hydraulic fracture propagation in middle-deep shale gas reservoirs in the southern Sichuan Basin: The effects of vertical heterogeneity

During the hydraulic fracturing of shale gas in the southern Sichuan Basin, China, field monitoring data have shown that hydraulic fracture (HF) propagation is restricted by height and cannot sufficiently cover vertical pay layers. The geomechanical behavior of gas shale in this region creates heterogeneity between the layers, which is responsible for the height restriction of HF propagation and limits the safe and efficient development of shale gas reservoirs in this region. To study vertical HF propagation, we conducted a series of laboratory hydraulic fracturing experiments under true triaxial loading. Three kinds of heterogeneity factors were tested: stress, elasticity, and interface strength. Analysis of fracture morphology and injection pressure curves resulted in several findings: (1) four types of vertical HF shapes emerged: I-shape, restricted I-shape, T-shape, and H-shape; (2) HF was induced by weakly cemented interface(s) with height limitations, while a layer under larger stress (e.g., 6 MPa) restricted fracture propagation in the middle layers, and heterogeneous elasticity was resistant; (3) regarding the geomechanical status of Changning middle-deep gas shale, interface strength may be more significant than fracture height. This study demonstrates the role of vertical heterogeneity in limiting fracture height and provides theoretical support for hydraulic fracturing optimization in this region.