Geometric model simplification - Implications for wave propagation and DFN multiphase flow

Pierre Anquez and Nathalie Glinsky and Mustapha Zakari and Paul Cupillard and Guillaume Caumon. ( 2019 )
in: 2019 Ring Meeting, ASGA

Abstract

This contribution is a collection of two papers: 1. To efficiently simulate flow in of porous media advected by discrete fractures, a possible option is to approximate the fracture geometry by a numerically convenient solution. In this paper, we compare different approaches to achieve this goal in two-dimensional domains, namely: a fracture projection strategy, which discretizes the fractures on the edges of a background mesh; a contraction strategy, which merges the fracture elements (lines, extremities of intersections) to ensure minimal angles and distances values in the simplified model; an expansion strategy, which meets the same criteria by moving the fracture elements, hence by preserving the model connectivity. We explain some adjustment of the latter method as compared to already published method. We propose three data sets on which we make numerical simulations of reservoir water flooding. For each model, we compare the oil production and the saturation maps for the reference and simplified models. We observe in one test case that the connectivity changes implied by the first two methods have negligible impact on the simulation results. However, overall results suggest that the expansion strategy which preserves topology generally provides the most accurate solution. 2. In this article, we study site effects in the lower Var valley basin which is a very dense urbanized area located near the city of Nice (South-East of France). Vertical plane wave propagation in a 2D heterogeneous profile is simulated using a Discontinuous Galerkin method. The SW-NE profile in study crosses the basin at the Nice prefecture. The simulations are based on an unstructured mesh made of triangles. In presence of thin layers and tangential contacts inside the basin, the quality of some mesh elements is very poor. In particular, some triangle heights are extremely small, which drastically reduces the overall time step of the simulations. As a consequence, the strong geometric constraints of the model lead to prohibitive computation times (more than 18 days in our case). The contribution of this work is to apply several model simplification techniques in order to modify some areas of the domain and thus to improve the quality of the triangulated mesh. Our objective is to reduce the computation time without significantly changing the physical response of the geological medium. The different model simplification techniques are either manual or automatic; among the automatic ones, some modify the connectivity between materials in the basin. Five simplified meshes are generated and the impacts of the simplifications are analyzed in comparison to the reference initial model. Both the time traces and the transfer functions obtained on the surface of the basin are considered. The results show that the simplification procedures, in particular automatic modifications of the model, resulting in a significant gain in computation time and a negligible impact on the ground motion response.

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BibTeX Reference

@INPROCEEDINGS{AnquezRM2019,
    author = { Anquez, Pierre and Glinsky, Nathalie and Zakari, Mustapha and Cupillard, Paul and Caumon, Guillaume },
     title = { Geometric model simplification - Implications for wave propagation and DFN multiphase flow },
 booktitle = { 2019 Ring Meeting },
      year = { 2019 },
 publisher = { ASGA },
  abstract = { This contribution is a collection of two papers: 1. To efficiently simulate flow in of porous media advected by discrete fractures, a possible option is to approximate the fracture geometry by a numerically convenient solution. In this paper, we compare different approaches to achieve this goal in two-dimensional domains, namely: a fracture projection strategy, which discretizes the fractures on the edges of a background mesh; a contraction strategy, which merges the fracture elements (lines, extremities of intersections) to ensure minimal angles and distances values in the simplified model; an expansion strategy, which meets the same criteria by moving the fracture elements, hence by preserving the model connectivity. We explain some adjustment of the latter method as compared to already published method. We propose three data sets on which we make numerical simulations of reservoir water flooding. For each model, we compare the oil production and the saturation maps for the reference and simplified models. We observe in one test case that the connectivity changes implied by the first two methods have negligible impact on the simulation results. However, overall results suggest that the expansion strategy which preserves topology generally provides the most accurate solution. 2. In this article, we study site effects in the lower Var valley basin which is a very dense urbanized area located near the city of Nice (South-East of France). Vertical plane wave propagation in a 2D heterogeneous profile is simulated using a Discontinuous Galerkin method. The SW-NE profile in study crosses the basin at the Nice prefecture. The simulations are based on an unstructured mesh made of triangles. In presence of thin layers and tangential contacts inside the basin, the quality of some mesh elements is very poor. In particular, some triangle heights are extremely small, which drastically reduces the overall time step of the simulations. As a consequence, the strong geometric constraints of the model lead to prohibitive computation times (more than 18 days in our case). The contribution of this work is to apply several model simplification techniques in order to modify some areas of the domain and thus to improve the quality of the triangulated mesh. Our objective is to reduce the computation time without significantly changing the physical response of the geological medium. The different model simplification techniques are either manual or automatic; among the automatic ones, some modify the connectivity between materials in the basin. Five simplified meshes are generated and the impacts of the simplifications are analyzed in comparison to the reference initial model. Both the time traces and the transfer functions obtained on the surface of the basin are considered. The results show that the simplification procedures, in particular automatic modifications of the model, resulting in a significant gain in computation time and a negligible impact on the ground motion response. }
}