Influences of hydraulic boundary conditions on the deep fluid flow in a 3D regional model (central Upper Rhine Graben)

Abstract

Knowledge of groundwater flow is of high relevance for groundwater management or the planning of different subsurface utilizations such as deep geothermal facilities. While numerical models can help to understand the hydrodynamics of the targeted reservoir, their predictive capabilities are limited by the assumptions made in their setup. Among others, the choice of appropriate hydraulic boundary conditions, adopted to represent the regional to local flow dynamics in the simulation run, is of crucial importance for the final modelling result. In this work, we systematically address this problematic in the area of the central part of the Upper Rhine Graben. We quantify how and to which degree different upper boundary conditions and vertical cross-boundary fluid movement influence the calculated deep fluid flow conditions in the area under study. Robust results, which are insensitive to the choice of boundary condition, are: (i) a regional groundwater flow component descending from the graben shoulders to rise at its centre and (ii) the presence of heterogeneous hydraulic potentials at the rift shoulders. Contrarily, results affected by the chosen boundary conditions are: (i) calculated flow velocities, (ii) the absolute position of the upflow axis, and (iii) the evolving local flow dynamics. If, in general, the investigated area is part of a supra-regional flow system—like the central Upper Rhine Graben is part of the entire Upper Rhine Graben—the inflow and outflow across vertical model boundaries need to be considered.

Hydrogeologic and Thermal Effects of Glaciations on the Intracontinental Basins in Central and Northern Europe

“…We close the system of PDEs (Equations 1-3) by imposing an initial pore pressure and temperature distribution. Previous studies aiming at quantifying the effects of the boundary condition on the system's hydraulic regime opted to parameterize the initial setting as being in equilibrium with respect to the imposed boundary forcing (Frick et al, 2015(Frick et al, , 2019aHaacke et al, 2018;Koltzer et al, 2022). The assumption at play here is to neglect the system's memory from its past evolution.…”