In the last years, hydraulic fracturing (HF) has reached maturity, becoming a fundamental aspect of hydrocarbon productivity enhancement and an important component of well costs. Interest on HF has also increased following unconventional resources exploitation, where commercial hydrocarbons rates cannot be otherwise achieved. This work reviews and compares various methods to model HF using conventional reservoir simulators.
Three different simulation solutions were investigated in this paper: i) explicit fracture representation through very detailed local grid refinement (LGR), ii) implementation of embedded discrete fracture models (EDFM) in conventional corner point geometry (CPG) grid and iii) definition of equivalent fractured wells (EFW) where the artificial fractures are assumed under hydraulic equilibrium with the wellbore.
The most common HF modelling methodology consists in the explicit fracture representation by means of LGR. This approach, however, is computationally expensive for full-field models where a large number of HF wells is implemented. Moreover, fracture orientation is constrained by the geometry of the grid, while the actual orientation is due to well trajectory and stress regime. To overcome these issues, an alternative solution was proposed, based on the implementation of Embedded Discrete Fracture Model methodology (Li and Lee (2008)). EDFM allows using a coarser grid resolution with respect to conventional LGR representation. Moreover, it accounts for fracture representation regardless grid orientation. EDFM enabled a smoother and faster process for both calibration and forecast steps, facilitating the implementation of HF wells in coarse simulation grids. The modelling could be further optimised assuming that hydraulic equilibrium between well and fracture is achieved in a negligible time-scale. This allowed using the equivalent fractured well methodology, where fractures are incorporated in the well itself as additional connections. EDFM estimates of fracture-to-matrix connectivity can be used as input for EFW.
This paper presents a comprehensive review of the various tools adopted to model HF in reservoir simulation and, for the first time, a thorough comparison of their effectiveness and efficiency in a real case study. Explict representation by means of LGR, EDFM and EFW were used to simulate a 3-stage hydraulically fractured horizontal well in a tight gas condensate reservoir, including well-test and long term forecast simulation cases. The benefits of each solution are compared in terms of accuracy, computational efficiency and ease of implementation. Numerical results indicated that EDFM and EFW represent powerful solutions for investigating the benefit of HF campaigns in unconventional reservoirs. Moreover, a novel combination of EDFM/EFW with moderate LGR has been investigated in order to achieve an optimal compromise between efficiency and accuracy. Eventually it provides with useful best practices/recommendations for general HF well simulation applications.