The effects of the boundary layer and fracture networks on the water huff-n-puff process of tight oil reservoirs

Application of multi-fractured horizontal wells has led to significant progress in the development of tight oil reservoirs. However, oil production rapidly decreases and the estimated ultimate recovery is still low when the well is in production and approaching depletion. Water huff-n-puff is regarded as a useful measure to improve oil recovery due to oil-water imbibition and drainage. Additionally, complex fracture networks consisting of natural fractures (NF) and hydraulic fractures can significantly enhance oil recovery in the process of water huff-n-puff after multi-stage hydraulic fracturing. However, there is a non-flowing thin film on the solid-liquid wall surface in the pore-throats of the matrix, and the so-called boundary layer effect (BLE) is quite significant at the nanoscale. This effect reduces the effective flow space and effective permeability of the reservoir, leading to production damage. To date, few numerical simulation studies have considered the impact of the BLE and complex fracture networks in water huff-n-puff process simulation studies. In this study, the process of water huff-n-puff is evaluated considering complex fracture networks and formation damage in a matrix. The embedded discrete fracture model (EDMF), validated by local grid refinement (LGR), is applied to describe the distribution of hydraulic fractures and NF. The well performance considering the BLE is analyzed by a vectorization programming. The results show that the BLE reduces production and accelerates the production decline, so the BLE should be considered in reservoir simulations and productivity evaluations of water huff-n-puff processes. A certain degree of natural fracture development is conducive to improving the sweep efficiency of the water huff-n-puff, which could significantly improve oil recovery compared to depletion. By considering the impact of the BLE in water huff-n-puff simulations in tight oil reservoirs, it is possible to accurately evaluate the well performance and provide a theoretical basis for the optimization of water huff-n-puff.