Comparing Three DFN Simplification Strategies for Two-Phase Flow Applications
in: ECMOR XVII, pages 1-21, European Association of Geoscientists \& Engineers
Abstract
Numerical flow models based on Discrete Fracture Methods (DFM) represent a fractured porous rock using an unstructured mesh where fractures are a subset of the elements faces. This allows for a high degree of geometric accuracy, but it also raises numerical challenges: the mesh must honor both small and large scale geometric features while keeping tractable and stable computations. For these reasons, we previously proposed a new geometric approximation method, which can be applied before meshing. The aim of this paper is to compare the flow impact of different geometric approximations of irregular and complex two-dimensional fracture networks. We present and validate a Control-Volume-Finite-Element DFM-based water flooding model and three fracture approximations strategies. The first strategy (A) projects fractures on the edges of an initial background mesh. The two others (B and C) rely on graph theory to analyze and modify a boundary representation of the fracture network according to minimal angles and mesh sizes criteria. Strategy B modifies the boundary representation using a contraction approach where flagged fracture elements (lines, extremities of intersections) are merged. Strategy C uses an expansion approach which moves the problematic fracture elements away one from another, hence preserving the model connectivity (we also present some adjustments as compared to the already published method). The approximation strategies A, B and C are applied to three reference data sets with respectively: two crossing fractures; highly connected fractures; anisotropic disconnected fractures. For each model, we compare the oil production and the saturation maps to the reference model. These tests show that the connectivity changes implied by the strategies A and B only have a small impact on the flow solution. Nonetheless, the expansion strategy C which preserves the fracture network topology provides the most accurate solution in all test cases.
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BibTeX Reference
@inproceedings{anquez:hal-03977429,
abstract = {Numerical flow models based on Discrete Fracture Methods (DFM) represent a fractured porous rock using an unstructured mesh where fractures are a subset of the elements faces. This allows for a high degree of geometric accuracy, but it also raises numerical challenges: the mesh must honor both small and large scale geometric features while keeping tractable and stable computations. For these reasons, we previously proposed a new geometric approximation method, which can be applied before meshing. The aim of this paper is to compare the flow impact of different geometric approximations of irregular and complex two-dimensional fracture networks. We present and validate a Control-Volume-Finite-Element DFM-based water flooding model and three fracture approximations strategies. The first strategy (A) projects fractures on the edges of an initial background mesh. The two others (B and C) rely on graph theory to analyze and modify a boundary representation of the fracture network according to minimal angles and mesh sizes criteria. Strategy B modifies the boundary representation using a contraction approach where flagged fracture elements (lines, extremities of intersections) are merged. Strategy C uses an expansion approach which moves the problematic fracture elements away one from another, hence preserving the model connectivity (we also present some adjustments as compared to the already published method). The approximation strategies A, B and C are applied to three reference data sets with respectively: two crossing fractures; highly connected fractures; anisotropic disconnected fractures. For each model, we compare the oil production and the saturation maps to the reference model. These tests show that the connectivity changes implied by the strategies A and B only have a small impact on the flow solution. Nonetheless, the expansion strategy C which preserves the fracture network topology provides the most accurate solution in all test cases.},
address = {Online Event, France},
author = {Anquez, P. and Zakari, M. and Caumon, G.},
booktitle = {{ECMOR XVII}},
doi = {10.3997/2214-4609.202035112},
hal_id = {hal-03977429},
hal_version = {v1},
month = {September},
pages = {1-21},
publisher = {{European Association of Geoscientists \& Engineers}},
title = {{Comparing Three DFN Simplification Strategies for Two-Phase Flow Applications}},
url = {https://hal.science/hal-03977429},
year = {2020}
}