Sedimentological control on permeability heterogeneity, and its effects in fluid-flow modelling: a case study of the Middle Buntsandstein sandstones, Upper Rhine Graben, Eastern France

Sedimentary processes govern fluid-flow heterogeneities in porous media in several scales, therefore, their understanding is a common practice in the petroleum industry. However, hydrogeologists have lagged behind when it comes to discretising porous sedimentary aquifers in flow, heat and transport models. At the Upper Rhine Graben, in Eastern France, the Lower Triassic Buntsandstein Group serves as an important reservoir for groundwater and lithium-rich geothermal brines. The main objective of this study is to assess the architecture of the Lower Grès Vosgien Formation (LGV), Middle Buntsandstein, and how sedimentological processes, at different scales, generate significant permeability heterogeneities. It is implemented a 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 hydrostratigraphic conceptual model is generated as a reference, allowing the evaluation of 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, comprising 7% of the total LGV thickness. The aeolian deposits record signs of persistent water in the system, either due to water table rise, or ephemeral floods, primarily contributing to the sedimentary facies association with the lowest permeabilities of the LGV, exceeding 3 orders of magnitude lower than the fluvial deposits. Despite representing only 7% of the LGV total thickness, the aeolian deposits exhibit lateral extensions that extrapolate outcrop scales (hundreds of metres), representing significant vertical flow baffle zones.

Fluid-flow simulations demonstrate that model simplifications, whether through assigning deterministic permeability values (mean), or stochastically distributing permeabilities, unconstrained by realistic sedimentary architectures, have a direct impact on macro-dispersivity (both vertical and horizontal), vertical mean residence time, and upscaled anisotropy results. Nevertheless, the results for horizontal mean residence time show no significant effect when simplifying the observed sandstone heterogeneities.