Enhancing Oil Recovery in Ultra-Low Permeability Reservoirs Refracturing: Sweet Spot Evaluation and the Re-Pressurization Plus Infill-Fracturing Strategy

The non-uniform production contribution caused by insufficient reservoir stimulation during initial fracturing significantly constrains the lifecycle and estimated ultimate recovery (EUR) of horizontal wells. Refracturing is therefore urgently required to reconstruct fracture networks and activate undeveloped reserves. In this study, a coupled geomechanics-matrix-fracture-seepage model is developed based on the Unconventional Fracturing Model (UFM) to characterize formation energy evolution and residual oil distribution. Simulation results indicate that initial fracturing creates a limited pressure diffusion radius (5–30 m), resulting in a “strong near-well, weak far-field” pressure distribution and inefficient residual oil utilization. To address this, a synergistic strategy is proposed, integrating “re-pressurization of existing fractures” for energy replenishment with “infill fracturing” for activating bypassed reserves. This strategy significantly outperforms conventional refracturing, increasing the predicted cumulative oil production by 55.86%. Parameter optimization indicates that maintaining a pumping rate of 10–12 m3/min and a fluid intensity of 1700–1900 m3/stage, while optimizing proppant ratios for conductivity, maximizes recovery. This work provides theoretical guidance for sweet spot evaluation and refracturing design in ultra-low permeability reservoirs.