Hybrid Coupled Discrete-Fracture/Matrix and Multicontinuum Models for Unconventional-Reservoir Simulation

Abstract

Summary Unconventional reservoirs are the focus of considerable attention as a primary energy source. Numerical simulation is a core kernel of reservoir-engineering work flows for reservoir evaluation, optimization, and management. Accurate and efficient numerical simulation of unconventional reservoirs is challenging. There is substantial physical complexity involving a number of tightly coupled mechanisms in the modeling of these reservoirs. The complexity is further amplified by the multicontinuum nature of the stimulated formation, and the complex fracture networks with a wide range of fracture-length scales and topologies. To adequately capture the effects of the multiscaled fracture system, we develop two alternative hybrid approaches that are aimed at combining the advantages of multicontinuum and discrete-fracture/matrix (DFM) representations. During the development of unconventional resources, geological and geophysical information may be available in some cases to suggest a prior characterization, whereas in many other cases, this prior model may be incomplete and limited to hydraulic fractures. The two hybrid approaches could be used for different applications depending on the available characterization data and different requirements for efficiency and accuracy considerations. The first hybrid model couples an embedded-discrete-fracture model (EDFM) with multiple interacting continua (MINC) into EDFM/MINC, which simulates the fracture network characterized by stimulated-reservoir-volume (SRV) concept. This optimized model can reduce the computational cost that is associated with the widely applied logarithmically spaced/locally refined (LS/LR) DFM technique, while improving the flexibility to model the complex geometry of hydraulic fractures. The MINC concept allows the hybrid model to handle the extreme contrast in conductivity between the small-scale fracture network and the ultratight matrix that results in steep potential gradients. For the second class of hybrid model (unstructured DFM/continuum), the primary fractures are described by use of DFM with unstructured gridding, and the small-scale fractures are simulated by continuum-type approaches in a fully coupled manner. Optimized local-grid refinement is used to accurately handle the transient-flow regime around primary fractures. An upscaling technique that applies EDFM on the detailed realization of the discrete-fracture network by use of the target unstructured grid to generate an appropriate 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. Simulation studies demonstrate the applicability of the developed hybrid-fracture models. Model verification is conducted against several reference solutions.