Accurate and efficient numerical simulation of naturally fractured reservoirs is challenging. One of the main challenges is to model the flow behavior in a complex fracture network of multi-scale natural fractures. Though conventional dual porosity and dual permeability(DP/DK) models are accurate and efficient in simulating the ideal fractured reservoirs, the DP/DK models are not adequate for modeling these complex networks of multi-scale fractures. Therefore, it is critical important to develop a new approach to model the complex fracture network with a wide range of fracture-length scales and fracture topologies.
In this paper, we develop a coupled discrete fracture and discrete dual continuum model to simulate the fluid flow in fractured systems. Large-scale fractures(LF) are modeled explicitly using a discrete fracture model, called UDFM, and the local-grid refinement is used to accurately handle the transient-flow regime around LFs. Sparse small-scale fractures(SF) are modeled using a discrete dual continuum approach, called EDFM, which is different from the conventional DP/DK models. This coupled model includes three domains: matrix, discrete LFs, and discrete SFs domains. Moreover, an upscaling technique, applying EDFM on the detailed realization of discrete SFs to generate an appropriate discrete dual-permeability model, is also developed. The upscaling technique is suitable for cases where a detailed prior model for the complete fracture network is available.
The simulation studies of model verification demonstrate the applicability, and robustness of the coupled model for fractured oil reservoirs. We examine naturally fractured reservoirs with multiple configurations in the presence of numerous pre-existing fractures. The simulations of depletion development show that LFs have a noticeable impact on the pressure profile. However, the effect of discrete SFs is weak but can't be ignored. In addition, the LFs and SFs both have a remarkable effect on the water saturation field during the water flooding process.
This study further improves the accuracy and efficiency of modeling irregular and no-ideal fracture geometries.