A Robust Embedded Discrete Fracture Modeling Workflow for Simulating Complex Processes in Field-Scale Fractured Reservoirs

Abstract

Traditionally, fractured reservoir simulations use Dual-Porosity, Dual-Permeability (DPDK) models that can idealize fractures and misrepresent connectivity. The Embedded Discrete Fracture Modeling (EDFM) approach improves flow predictions by integrating a realistic fracture network grid within a structured matrix grid. However, small fracture cells with high conductivity that pose a challenge for simulators can arise and ad hoc strategies to remove them can alter connectivity or fail for field-scale cases. We present a new gridding algorithm that controls the geometry and topology of the fracture network while enforcing a lower bound on the fracture cell sizes. It honors connectivity and systematically removes cells below a chosen fidelity factor. Furthermore, we implemented a flexible grid coarsening framework based on aggregation and flow-based transmissibility upscaling to convert EDFMs to various coarse representations for simulation speedup. Here, we consider pseudo-DPDK (pDPDK) models to evaluate potential DPDK inaccuracies and the impact of strictly honoring EDFM connectivity via Connected Component within Matrix (CCM) models. We combine these components into a practical workflow that can efficiently generate upscaled EDFMs from stochastic realizations of thousands of geologically realistic natural fractures for ensemble applications. We first consider a simple waterflood example to illustrate our fracture upscaling to obtain coarse (pDPDK and CCM) models. The coarse simulation results show biases consistent with the underlying assumptions (e.g., pDPDK can over-connect fractures). The preservation of fracture connectivity via the CCM aggregation strategy provides better accuracy relative to the fine EDFM forecast while maintaining computational speedup. We then demonstrate the robustness of the proposed EDFM workflow for practical studies through application to an improved oil recovery (IOR) study for a fractured carbonate reservoir. Our automatable workflow enables quick screening of many possibilities since the generation of full-field grids (comprising almost a million cells) and their preprocessing for simulation completes in a few minutes per model. The EDFM simulations, which account for complicated multiphase physics, can be generally performed within hours while coarse simulations are about a few times faster. The comparison of ensemble fine and coarse simulation results shows that on average, a DPDK representation can lead to high upscaling errors in well oil and water production as well as breakthrough time while the use of a more advanced strategy like CCM provides greater accuracy. Finally, we illustrate the use of the Ensemble Smoother with Multiple Data Assimilation (ESMDA) approach to account for field measured data and provide an ensemble of history-matched models with calibrated properties.

Embedded Discrete Fracture Models

“…This method is explained in more detail and used to model flow in unconventional oil and gas reservoirs in Chapter 10. Hui et al [18] have further simplified and optimized the EDFM preprocessing algorithm, thus allowing users to handle a much larger number of fractures. Additionally, various authors have coupled EDFM with geomechanical simulations for fully coupled simulations [35,38,50].…”

“…Vo et al [45] compared DFM and dual-permeability models and identified a range of deficiencies in terms of the dual-permeability method's ability to account for viscous displacement and spontaneous imbibition. Hui et al [18] used a DFM method to simulate waterflooding in a full-field model in which the fracture network is simplified via a method known as edge collapse. In this simplification, short fracture segments with hanging nodes are removed to reduce the degrees of freedom involved in the simulations.…”

Data-Space Inversion With a Recurrent Autoencoder for Naturally Fractured Systems

“…These nine DFNs are intended to 'span' the range of uncertainty in the spatial variability and connectivity of the fracture system. Please see [16] for a more detailed description of the workflow used to grid and simulate the ensemble of EDFMs.…”

“…The field has over 10 years of primary production history, and a multiyear waterflood pilot involving two injectors with tracers is currently being planned to evaluate water injection as a potential full-field improved oil recovery (IOR) strategy. In this study, a large set of prior models (1850) is generated and simulated using an efficient workflow [16]. These models are based on nine discrete fracture network realizations and a range of matrix and fracture parameters (e.g.…”

“…Color bar displays cluster size, with pink being the largest. Only cells belonging to the 10 largest clusters are shown; isolated cells are not displayed (figure from[16]).…”