Numerical simulation study on sweet spot identification and parameter optimization of refractured horizontal wells in ultra-low permeability reservoirs

Non-uniform production contribution caused by insufficient reservoir stimulation during initial fracturing significantly constrains well lifecycle and economic ultimate recovery (EUR). Refracturing is urgently required to reconstruct fracture networks and activate undeveloped reserve zones. An geomechanics-matrix-fracture-seepage coupling model is developed using the Unconventional Fracturing Model (UFM), which can revealing formation energy evolution and residual oil distribution characteristics after hydraulic fracturing in ultra-low permeability reservoirs. Building on this foundation, key re-fracturing technologies and parameter optimization are investigated. The study indicates that after the initial fracturing, energy diffusion is limited, and water injection primarily enhances energy near fractures, failing to establish an effective reservoir displacement system and resulting in inefficient residual oil utilization. A ‘Re-pressurization of existing fractures + inter-stage new fracture stimulation’ strategy is implemented to enhance stimulated reservoir efficiency of complex fracture networks. By focusing on fracture half-length, discharge rate parameters are optimized during fracturing to maximize the stimulated volume within the target sweet spot. For multi-stage clustered perforated horizontal wells, proppant and fluid injection volumes are optimized while ensuring adequate fracture conductivity. This study provides theoretical guidance for sweet spot evaluation and parameter optimization in re-fracturing of ultra-low permeability horizontal wells.