Research on the Mechanism of Complex Fracture Network Formation in High-Angle Natural Fractured Reservoirs

Natural fracture activation through hydraulic fracturing can effectively enhance oil recovery. However, in ultra-deep reservoirs of the Tarim Basin, the combination of high tectonic stress, significant horizontal stress anisotropy, and high-angle natural fractures (typically >60°) complicates fracture network activation, thereby hindering efficient hydrocarbon exploitation. To overcome these challenges, we performed mechanical analysis, laboratory experiments, and numerical simulations to systematically investigate hydraulic fracture propagation patterns and their controlling factors in ultra-deep reservoirs. Inspired by current challenges in the field, large-scale poly-axial experiments were performed to simulate hydraulic fracturing in high-angle fractured formations. Based on the experiments, a fully 3D discrete lattice method based hydraulic fracturing numerical model was developed and optimized to simulate hydraulic fracture propagation in formations with high-angle natural fracture. Finally, systematic investigations were carried out on the spatial propagation patterns of hydraulic fractures and their influencing factors in Tarim’s ultra-deep reservoirs. The dip angle of natural fractures significantly influences fracture network formation. When dip angles are below 50°, the main hydraulic fracture tends to cross multiple natural fractures without activating them. Fracture morphology becomes relatively stable when natural fracture dip angles exceed 60°. Under constant dip angle conditions, smaller azimuth angles promote shear activation of natural fractures. For high-angle natural fractures (dip angle >60°), azimuths below 45° generate shear fracture areas 2–5 times larger than tensile fracture areas, significantly impeding main fracture propagation. Under constant azimuth conditions, fracture morphology becomes insensitive to dip angle variations beyond 60°. In high-angle natural fracture systems—where hydraulic fractures are preferentially captured by natural fractures, and where narrow apertures and poor connectivity prevail—we propose a multi-stage temporary plugging strategy to enhance fracture width and main fracture propagation through proppant-packed fracturing. This approach demonstrably improves stimulation effectiveness in ultra-deep reservoirs.