Accurate modeling of fractured reservoirs is very challenging due to the various scales of fractures. The fracture networks may be too complex to be represented using the equivalent continuum model (ECM) or dual porosity-dual permeability (DPDK) model, yet too computational costly to be modeled using the discrete fracture (DFM) or embedded discrete fracture (EDFM) models. This paper proposes a hybrid model that integrates ECM, DPDK, and an integrally embedded discrete fracture model (IEDFM) to account for multi-scale fractures. The hybrid model is applied to investigate the coupled geomechanics-fluid flow process in fractured reservoirs.
In the hybrid model, small-scale fractures are upscaled into effective matrix permeability tensor using ECM, medium-scale fractures are considered as an individual continuum using DPDK, and large-scale fractures are explicitly represented using IEDFM. The multiphase flow in effective matrix and fracture continua is modeled using the multi-point flux approximation (MPFA), and fluid exchanges between the anisotropic continua and the large-scale fracture control volumes are precisely calculated using the IEDFM. Empirical models are used to calculate the displacement of natural fractures, and analytical models are used to calculate the aperture changes of hydraulic fractures. The overall deformation of a fractured rock is described using an equivalent method. The coupled geomechanics-fluid flow system is discretized by the finite element-finite volume method (FV-FEM) and solved using the fixed-stress split iterative coupling approach.
Several examples are presented to demonstrate the applicability of the proposed method. The hybrid model is first employed to simulate water flooding process in a naturally fractured reservoir with multi-scale fractures. Effects of different scales of fractures, geomechanics coupling and capillary pressure are investigated. A case of producing from horizontal well in a hydraulic fractured tight oil reservoir is then studied, where the hydraulic fractures are modeled explicitly using IEDFM and the stimulation areas around hydraulic fractures are modeled using DPDK. Effects of stimulation area size on the pressure depletion and on the stress evolution process in the reservoir are investigated.