A New Aquifer Simulation Tool for Coupled Flow, Heat Transfer, Multi-Species Transport and Chemical Water-Rock Interaction

Abstract

SHEMAT is a new finite difference code for simulating steady-state and transient processes in liquid geothermal reservoirs in two and three dimensions. It is particularly well suited to predict the long-term behavior of heat mining installations in hot aquifers with highly saline brines. It can handle a wide range of time scales, both of technical and of geological processes.

Flow, heat transfer, multi-species transport, and heterogeneous geochemistry are mutually coupled and simulated in sequence. Heat transfer is non-linear because fluid thermal properties and matrix thermal conductivity depend on temperature. Due to the coupling of fluid density both to temperature and to the concentrations of dissolved mineral species the model is also well suited to simulate density driven flow.

Dissolution and precipitation of matrix minerals are calculated by an extended version of the geochemical modeling code PHRQPITZ (Plummer et al., 1988). It uses the Pitzer virial coefficient approach for activity-coefficient corrections and now permits to calculate geochemical reactions in brines of low to high ionic strength and temperatures of 0 - 150 ?C.

Chemical water-rock interaction can change the structure of the pore space and the porosity. This, in turn, may change the transport properties of the reservoir. This is considered in the model by updating permeability in respect to porosity changes brought about by precipitation and dissolution of minerals via a three-term power series of porosity. This new relationship between porosity and permeability is derived from combining the Kozeny-Carman equation (Carman, 1956) for a porous medium with a fractal model for the pore space structure and its change due to chemical water-rock interaction.

Model verification is illustrated by comparison of simulation results with standard benchmarks. Model validation is demonstrated by comparison of simulation results with permeability changes during a core flooding laboratory experiment at high temperature and pressure. The performance of different numerical transport schemes is compared for the case of advection-dominated transport. Geothermal scenarios simulated include a doublet installation for heat mining and the complex hydrothermal regime in the Rhine graben. The first scenario is characterized by a complex interaction of heat transfer, transport, and chemical reactions as the cooling front is propagating from the injection well through the reservoir to the production well. The second scenario is distinguished by a characteristic distribution of temperature and heat flow density along an E-W transect across this rift structure.