Simulation of Coupled Fracture Propagation and Well Performance under Different Refracturing Designs in Shale Reservoirs
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AbstractThe successful development of unconventional reservoir relies on the massive hydraulic fractures which provide high conductive paths and large contact area between matrix and wellbore. Since well production always shows a steep decline after the early high flow rate, refracturing can be an economically promising option to compensate for the loss of production and improve the well delivery. Although extensive field cases have been published on refracturing practice, there is a lack of guidelines on how to select and optimize refracturing design.In this paper, a series of synthetic cases are used to evaluate the well performance under different refracturing designs. The key characteristics for refracturing simulation is accurate quantification of the depletion-induced stress and pressure field change. Thus, finite element method is used to solve the coupled reservoir flow and geomechanics model while a cohesive zone model is adopted to simulate the fracture propagation. A viscoelastic model is used to simulate the time-dependent fracture conductivity change due to proppant embedment. The synthetic cases are grouped into two categories: refracturing in existing perforations and in newly created perforations. A systematic sensitivity study is performed on the effects of fracturing spacing, matrix permeability and refracturing time.Numerical results show different fracture configurations for refracturing in depleted and virgin reservoirs and thus demonstrate the importance of accounting for stress and pressure changes during initial production. Refracturing the existing perforation is likely to create a wider but shorter fracture compared to the virgin case because the fracturing fluid in these fractures encounter less resistance for enlarging fracture width rather than fracture length because of reduced pressure and total stress. Refracturing new perforations appears to give better short-term performance than refracturing existing perforations but worse long-term performance. Simulation results indicate that for relatively low permeability reservoir it is favorable to add more fracture area; however, the advantage is diminished while for relatively high permeability reservoir where the SRV might already be depleted. When proppant degradation is severe, and fracture conductivity is the limiting factor for production, refracturing existing perforations becomes more attractive. Moreover, simulation results seem to indicate an optimum time window exists for refracturing in this scenario.
