The interaction between natural fractures (NF) and hydraulic fractures (HF) governs the stimulated reservoir volume and the complexity of the fracture network in black shale. This study aims to reveal the HF-NF interaction mechanism and establish a fracture-network propagation pattern for the normal-pressure shale gas of the Longmaxi Formation in the basin-margin transition zone of southeastern Chongqing, which is characterized by intense structural deformation, large horizontal stress difference, well-developed high-angle fractures and low pressure coefficient. Based on a review of HF-NF interaction research, multi-scale NF characterization was carried out by integrating core, image-logging and seismic data, combined with numerical simulation of artificial fracture-network propagation, to analyze the coupled control of approach angle, horizontal stress difference, fracture net pressure, interface strength and NF scale on the interaction mode. (1) HF-NF interactions can be classified into crossing, arrest (capture) and activation (dilation/shear), discriminated mainly by the combination of approach angle and stress difference and modulated by interface strength and net pressure; (2) small-scale NFs (3-5 m) weakly constrain the network and HF mostly crosses them, producing a nearly linear network insensitive to angle and spacing; (3) large-scale NFs (12-18 m) are readily captured or activated, inducing branches and complex networks at moderate angles and near-well positions, with the affected area increasing with angle and decreasing with spacing; (4) the NF-scale-angle-spacing coupling controls the evolution from linear to complex networks. A pattern diagram reflecting “interaction mode-scale/angle/spacing coupling-network geometry” is established, providing a geological-engineering basis for optimizing intensive-cutting, high-proppant volumetric fracturing of normal-pressure shale gas.
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