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De l’affleurement à l’écoulement des fluides : architecture sédimentaire, hétérogénéité de perméabilité, et effet des faciès sédimentaires dans la modélisation de l’écoulement des fluides : une étude de cas des grès continentaux du Buntsandstein moyen (Trias, Est de la France)

Year: 2024
Type: dissertation
Abstract: At the Upper Rhine Graben, in Eastern France, the Lower Triassic Buntsandstein serves as an important reservoir of groundwater and lithium-rich geothermal brines. The scientific objective of this study is to comprehend the architecture of the Lower Grès Vosgien Formation (LGV), Middle Buntsandstein; how sedimentological processes, at different scales, generate significant permeability heterogeneities; and the effect of such heterogeneity in fluid-flow modelling results. The study employs high-resolution sedimentological characterisation, through vertical profile descriptions, digital outcrop model, and petrographic analysis. Subsequently, permeability measurements are coupled with sedimentological data, to identify different scales of sedimentary controls on permeability distribution. Finally, a realistic 2D hydrostratigraphyc conceptual model is generated as a reference, to evaluate how different scenarios of heterogeneity simplification impact fluid-flow modelling, concerning particle residence time, macro-dispersivity and upscaled anisotropy.Results indicate that 93% of the LGV is composed of sandstones deposited by a braided fluvial system, with evidence suggesting that discharge variability was a main depositional controlling factor of sedimentary facies and heterogeneity distribution. The LGV stacking pattern reveals periods when fluvial processes were absent, and aeolian processes dominated sediment transport and deposition, constituting 7% of the total LGV thickness. The aeolian deposits record indications of persistent water in the system, either due to water table rise or ephemeral floods In the fluvial channel facies association, an inverse relationship between fluvial flow regime conditions and the permeability of its facies is observed. In the wind- and water-laid facies association (WWL), permeability distribution is directly influenced by climatic conditions, where low permeability hybrid sand sheets were deposited in more humid conditions, and high permeability aeolian dunes in more arid conditions. However, despite the high permeability of the AD, the architecture of the WWL implies low connectivity potential. Compaction is the main process influencing the permeability and porosity of the LGV. Samples with a higher proportion of lithoclasts and infiltrated clay recorded a more advanced degree of compaction, whereas samples with a higher percentage of quartz overgrowth exhibited a lower degree of compaction. The diagenetic overprint affected the distinct facies differently, indicating that primary sedimentary processes and architecture govern the distribution of permeability heterogeneity in the LGV. Permeability data from outcrops exhibited median values up to three orders of magnitude higher than those from subsurface data. However, the same facies, and similar trends of relative porosity-permeability responses to the distinct facies, are observed between outcrops and borehole samples, highlighting sedimentology as an indispensable proxy for reservoir heterogeneity studies[...]
Language: English
Open Access status: green
OpenAlex ID: W6980965863
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