The eight GEOTREF PhD has been defended by Margaux Raguenel, the 25th September 2019, in the Ecole Nationale Supérieure de Géologie (ENSG) In Nancy: Modeling the thermo-hydrolic behavior in fractured reservoirs with unstructured meshes: application to the geothermal Reservoir of Basse-Terre, Guadeloupe.
Abstract
The quantification of natural geological resources (oil, gas, water, ore depo-sits, geothermal energy, etc) calls for a precise understanding and description of the geological heterogeneities and physical and chemical processes that influence their formation, their settings, their preservation and their recovery.
In order to understand the physical behavior of subsurface reservoirs, 2D, 3D and 4D geological models must be developed as basis for numerical simulations. A numerical model needs the integration and homogenization of various and multi-scale data. This implies hypothesis, choices and simplifications to represent the complexity of geological objects and physical processes with a limited number of parameters. In the case of high temperature geothermal studies, one must face both the geo-metric and structural complexity of volcanic environments and the resolution of coupled physical processes. This calls for the use of several developments made in different research domains, with different tools. However, interoperability between all these theoretical and numerical tools remains a challenge. This work tackles this issue by preserving the geometrical complexity of geological models thanks to unstructured grids and tools which allows realistic physical simulations.
New strategies are proposed to preserve the geological characteristics of the model, by honoring field data and the organization of geological structures during physical simulations. Several tools have been developed to identify and represent these structures that play a fundamental role in physical processes, such as layers, faults or model boundaries. An adapted numerical representation (allowed by the RINGMesh library which focuses on using geological concepts in the model description) preserves these elements from the geomodeler (SKUA-GocadTM) to the physical simulator (CSMP++). These tools are part of an integrated workflow, going from geological modeling to physical simulations, i.e. from geometrical characterization of the main geological structures to the quantification of reservoir resources.
The developed methodologies are first benchmarked with analytical solutions and applied on synthetic cases. They have then been used in a case study to understand the evolution of thermo-hydraulic processes in the geothermal reservoir of Basse-Terre, Guadeloupe (in the frame of the GEOTREF project). A first study has been led in 2D to assess the physical realism of three possible geological scenarios designed in regard of the available field data, such as temperature profiles and MT survey. This study reproduces the observed temperature profiles in two of the studied scenarios and seems to exclude the hypothesis of strong lateral heat transfers at the island scale.
A second study has then been realized in 3D, using a progressive model complexification approach to test the impact of heterogeneities on the global physical behavior of the reservoir. It appears that the thermal behavior of the designed model is mainly influenced by the topography of the area, rather than the introduction of permeable heterogeneities, such as fault corridors. However, impermeable features such as sliding surfaces have not been added and a major uncertainty about the heat source position, shape and size remains and could be further studied.
These works have allowed (1) to build of a numerical environment that facilitates the tests regarding the impact of geological heterogeneities on the physical behavior of reservoirs, (2) to open the path toward a better understanding and characterization of the thermo-hydraulic behavior of a geothermal reservoir, and especially toward the quantification of the influence of the geometry and connectivity of heterogeneities on physical processes.