The Predictive Role of Structural Curvature in Dynamic Fractures of Ultra-Low Permeability Reservoirs in the Triassic Yanchang Formation, Ordos Basin

Ultra-low permeability reservoirs in the Triassic Yanchang Formation of the Ordos Basin are often characterized by the presence of natural fractures. During waterflood development, these natural fractures, combined with artificially induced hydraulic fractures, exhibit dynamic fracture behavior. However, due to the lack of seismic and other geophysical data, the prediction and early warning of such dynamic fractures remain challenging. In this study, a typical reservoir in the Chang 6 Member was investigated. Through fine stratigraphic subdivision, an isochronous stratigraphic framework was established, and microstructure maps of the reservoir top, along with structural curvature maps, were generated to analyze the relationship between dynamic fractures and structural curvature. The results show a strong correlation between the distribution of wells with dynamic fractures, structural curvature, and the volume of water injection. Wells exhibiting typical fracture-related water breakthrough are mainly located within or adjacent to zones of high structural curvature. In the northern part of the reservoir, a local linear structural zone with abnormally high curvature hosts a dense distribution of such wells. The dominant direction of waterflooding follows a northeast-southwest orientation, which aligns closely with the direction of the high-curvature zones. Data from tracers and waterflood fronts suggest that communication between injection wells and oil wells experiencing fracture-related water breakthrough occurs via the matrix-fracture system, rather than through large-scale fractures. Based on curvature distribution and reasonable injection pressures, fracture warning strategies for injection wells were proposed, including recommendations to reduce injection volumes and lower injection pressures to prevent fracture initiation. This approach provides a reference for fracture prediction in similar ultra-low permeability reservoirs lacking 3D seismic data.