2D or not 2D: Are two dimensions enough to ...

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XVI International Conference on Computational Methods in Water Resources (CMWR-XVI) Ingeniørhuset

2D or not 2D: Are two dimensions enough to accurately model convective fluid flow through faults and surrounding host rocks?
Author:Michael Kuehn <m.kuehn@geophysik.rwth-aachen.de> (Applied Geophysics, RWTH Aachen University)
Conny Zeeb <phoenix.conny@gmx.de> (Hydrology, University of Tübingen)
Klaus Gessner <kgessner@cyllene.uwa.edu.au> (The University of Western Australia, School of Earth and Geographical Sciences)
Presenter:Michael Kuehn <m.kuehn@geophysik.rwth-aachen.de> (Applied Geophysics, RWTH Aachen University)
Date: 2006-06-18     Track: Special Sessions     Session: Multi-Disciplinary Approaches To Reactive Transport Simulation In Aquifer Systems

In many studies of water-rock interaction, convective fluid flow has been invoked to explain diagenetic processes, metamorphism, or metal precipitation. Fluid convection in faults is increasingly recognised as an important mechanism for fluid flow, heat transfer, and mass transport in hydrothermal systems, particularly in consolidated and crystalline rocks. Convection is influenced not only by heat transport processes within the fault but also by lateral heat transfer to and from the surrounding rock mass. There is often a close spatial relationship between major ore deposits and regional scale faults. Most numerical studies simulate free convection in 2D only. This is because fluid patterns are more easily recognised with less complicated geometries, less computational time is required, or because some computer codes are restricted to two dimensions. Using the finite difference simulation code SHEMAT, a series of numerical simulations of thermally driven fluid flow have been carried out to investigate the difference in the fluid flow patterns in 2D and 3D models for the same geological architecture. SHEMAT solves coupled problems involving fluid flow, heat transfer, species transport, and chemical water-rock interaction on a Cartesian grid. In SHEMAT, the different flow, transport, and reaction processes can be selectively coupled. The results of this study show that 2D and 3D models of convection in hydrothermal systems produce significantly different results. In many cases 2D models represent an oversimplification, and conclusions reached from such investigations are likely to be irrelevant. In the case of planar high permeability regions, such as faults and permeable stratigraphic units extending along strike, 2D and 3D modelling outcomes vary significantly. Hence 3D models are absolutely essential to describe the flow field in these cases. An exception is incorporation of an impermeable basement, resulting in 2D convection patterns identical to observed 3D fluid flow fields, but only if vertical fault permeability equals horizontal host rock permeability. Conceptual exemptions are 2D models of high permeability regions with close to radial or linear symmetries, such as damage zones between fault jogs or at fault intersections, giving reasonable results in 2D.