A Physically Based Three-Dimensional Streamtube Model for Rapid Waterflood-Front Prediction and Sweep-Efficiency Evaluation in Ultra-Low-Permeability Reservoirs

Accurate and rapid prediction of waterflood front propagation and volumetric sweep efficiency remains challenging in ultra-low-permeability reservoirs because of strong heterogeneity, threshold pressure gradients, reservoir anisotropy, complex well-pattern geometry, and layer-dependent flow interference. In this study, an improved 3D streamtube model was developed for waterflood-front tracking and volumetric sweep evaluation in ultra-low-permeability reservoirs. The model incorporates experimentally constrained threshold pressure gradients, anisotropic coordinate transformation, dynamic streamtube flow-rate allocation, Buckley–Leverett-based non-piston displacement, interlayer interference correction, and irregular well-pattern adaptability. A unified calculation framework was established for both injector–producer and injector–fracture streamtube units, enabling 3D integration of layer-specific swept areas into volumetric sweep efficiency. The proposed model was validated against a commercial numerical simulator using a representative well group from Block A of the Changqing Oilfield. The predicted streamtube architecture and sweep-efficiency evolution agree well with numerical simulation results, with an average relative error of approximately 3.1%, while reducing the computational time from 1043 s to 1.42 s for a 30-year simulation. Sensitivity analysis demonstrates that threshold pressure gradient, well spacing, and inter-well connectivity are the dominant controls on sweep efficiency, whereas well-pattern type, interlayer heterogeneity, and reservoir anisotropy exert secondary but non-negligible effects. Field application further reveals a strongly layer-dependent waterflood behavior: the upper sand body preferentially propagates eastward, whereas the lower sand body advances mainly southward, producing a vertically asynchronous and laterally misaligned sweep pattern. These results show that the proposed model provides an efficient and physically interpretable tool for rapid waterflood-front prediction, refined waterflood optimization, and targeted production enhancement in heterogeneous ultra-low-permeability oil reservoirs.