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Simulations in the field of geosciences: a software prototype example serving the environment, energy and climate

April 2018

IFP Energies nouvelles has developed FraC [1, 2], a software prototype targeting the fine representation (mesh) of a fracture network, in order to obtain reference simulations for transport or transient flows in geological media.

In the field of geosciences, mass and heat transport process modeling in an underground environment is essential. Its importance in the context of oil production is obvious but it is also invaluable for the study of ground contamination [3]. Commonly, tracers (concentration of a product that can be monitored from the surface) are injected in order to gain a better understanding of fluid/rock interactions or to study underground contamination [3]. But this kind of simulation is also encountered in the field of geothermal energy and as part of safety studies for geological storage facilities (heat, CO2, radionuclides). This involves solving the Advection-Dispersion Equation (ADE), potentially incorporating geochemical processes (adsorption, desorption, solubility, etc.).

Since transport simulation relies on a highly accurate representation of the underground environment, it has been the focus of intensive research for many years. When the underground environment is made up of fracture networks (DFN*), modeling difficulties are made worse by the geometric complexity (Figure 1a,b). Furthermore, modeling the existing flow contrasts within the DFN or between the DFN and the unfractured rock remains a challenging task. Commonly to overcome these difficulties, the geometries are simplified and to use volumetric meshes, associated with equivalent physical properties, calculated to represent the heterogeneities of the environment [4]. Another practice is to build optimized meshes in terms of the number of grid cells [5, 6].

If these approximations are acceptable in a pure diffusion regime, it may not necessarily be the case when tackling ADE-type transport equations, which are more difficult to solve. Few solutions without approximation are available. Nevertheless, the range of validity for approximate solutions, traditionally used in software tools, cannot be established without these reference solutions.

In this context and thanks to post-doctoral residency, a research study has been launched [1]. For such purposes, it was necessary to develop a software prototype to obtain an accurate DFN representation (Figure 1b). Moreover, we make no further assumptions to model intersections between fractures and therefore obtain a conforming mesh [2] that can be coupled directly with open-source external flow simulation codes. Obviously, a simulation time increase is observed, due to both the fine representation of the underground environment and the conformal constraint. Furthermore, by combining this mesh with open-source and open-access tools, such as DUMUX (University of Stuttgart) or CAST3M (CEA), it was possible to obtain ADE reference simulations (Figure 2) for complex DFN geometries, without any modification of the open-source software codes. These results, already presented at international conferences, are the topic of another publication, currently under review [2].

* for "Discrete Fracture Network"

                                               Click on image to expand

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Publications :

  1. T.-D. Ngo, A. Fourno, B. Noetinger. Modeling of transport processes through large-scale discrete fracture networks using conforming meshes and open-source soft-ware. Journal of Hydrology. Vol. 554, pp 6679.
    >> DOI: 10.1016/j.jhydrol.2017.08.052 (2017).
  2. A. Fourno, T.-D. Ngo, B. Noetinger. FraC : A user-friendly conforming mesh method for discrete fracture networks.
    Submitted in J. Comp. Phys (2018).
  3. M. Verscheure, A. Fourno, J.P. Chilès. Joint inversion of fracture model properties for CO2 storage monitoring or oil recovery history matching. Oil Gas Sci. Technol. - Revue d’IFP Energies nouvelles 67 (2), 221–235 (2012).
  4. A. Fourno, C. Grenier, A. Benabderrahmane, F. Delay. A continuum voxel approach to model flow in 3D fault networks : a new way to obtain up-scaled hydraulic conductivity tensors of grid cells. Journal of Hydrology. Vol. 493, pp 68-80.
    >> DOI: 10.1016/j.jhydrol.2013.04.010 (2013)
  5. N. Khvoenkova, M. Delorme. An optimal method to model transient flows in 3D discrete fracture network. Peer-reviewed IAMG 2011 publication.
    >> DOI:10.5242/iamg.2011.0088 (2011)
  6. B. Noetinger. A quasi steady state method for solving transient Darcy flow in complex 3D fractured networks accounting for matrix to fracture flow. Journal of Computational Physics, vol 283:205-223 (2015).


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