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SUMMARY The study investigates thermal-, hydraulic- and chemically coupled processes of enhanced geothermal systems (EGS). On the basis of the two existing numerical codes, the finite element program fracture and the geochemical module of chemtough, frachem was developed, to simulate coupled thermal–hydraulic–chemical (THC) processes, accounting for the Soultz specific conditions such as the high salinity of the reservoir fluid and the high temperatures. The finite element part calculates the thermal and hydraulic field and the geochemical module the chemical processes. According to the characteristics of the Soultz EGS reservoir, the geochemical module was modified. (i) The Debye–Huckel approach was replaced by the Pitzer formalism. (ii) New kinetic laws for calcite, dolomite, quartz and pyrite were implemented. (iii) The porosity–permeability relation was replaced by a new relation for fractured rock. (iv) The possibility of re-injecting the produced fluid was implemented. The sequential non-iterative approach (SNIA) was used to couple transport and reactions. Sensitivity analyses proved the proper functionality of frachem, but highlighted the sensitivity of the SNIA approach to time steps. To quantify the frachem results, a comparative simulation with the code shemat was conducted, which validated frachem. Coupled THC processes in a fractured zone in the Soultz reservoir at 3 500 m (T0= 165 °C), which occur as a result of the injection of fluid (Tinj= 65 °C) at one end of the zone and the production at the other end, were modelled for 2 yr. Calcite is the most reactive mineral and therefore the porosity and permeability evolution results from the calcite reactions: near the injection point, porosity and permeability increase and near the production well they decrease. After 2 yr, the system seems to be very close to steady-state. Therefore, mineral dissolution and precipitation during the circulation of the fluid in the reservoir do not represent a limiting factor on the EGS reservoir (at 3 500 m depth) performance at the Soultz site. Finally, the numerical transmissivity was compared to the transmissivity of the 1997 circulation test. The fact that the transmissivity decreases during the circulation test, when thermomechanical effects are factored out, points to geochemical processes in the reservoir such as the precipitation of calcite. These findings highlight the importance of THC coupled EGS reservoir models. The integration of geochemical considerations is therefore indispensable for integrated simulations of EGS systems and predictions of its performance.

Detailed investigations of the temperature distribution in the Rhine Graben indicate a regular pattern of graben-parallel thermal anomalies following major north–south striking faults. These faults, which might imply north–south convection pattern due to higher permeability, have been ignored in conventional models based on 2D east–west striking sections. The present study investigates small-scale temperature anomalies in the Rhine Graben, focusing on its north–south striking major faults and their important permeability structures. Analytical solutions for convective flow in vertical faults were applied to a clearly observable anomalous temperature pattern. Through these calculations the fault geometry, minimal fault permeability and time to convective onset were derived from observed distances between anomalies. Since analytical solutions are limited to simple model geometries, further improvement was achieved through numerical model simulations, which allow the assumption of more complex initial and boundary conditions. Using the finite volume code TOUGH2 we investigated in detail the convective fluid flow pattern along the Cfault in the Landau region through a 3D model. There, regions of alternatively higher and lower temperatures follow each other at an average distance of 1800 m. Focusing on the predominant north–south permeability structure, our model consists of a vertical north–south striking fault and surrounding matrix with fault geometries based on analytical predictions. The observed temperature could be explained by a numerical model indicating fluid velocities in the fault of � 10 � 9 ms � 1 and an anomaly whose minimal age is 77,000 years. The study proves that the observed small-scale graben-parallel temperature anomalies can be explained only by convection systems within north–south striking fault zones. The regional flow regime is therefore significantly influenced by the convective fluid flux in major north–south striking faults. Investigations of temperature anomalies in the Rhine Graben must take into account this north–south flow. Since the presented results correlate with findings from other geothermal anomalies in the Rhine Graben such as the one at Soultz, we suspect this to be a general feature of the major north–south striking faults there. Moreover, since in most grabenlike structures thermal anomalies are associated with the Graben master faults, the present study may provide common clues for investigating other graben-parallel temperature anomalies. � 2003 Elsevier Science Ltd. All rights reserved.

The outdoor radiation exposure of the population in Switzerland from external sources results from cosmic background as well as from natural and artificial ground radiation. The geographical distribution of these components and of the total dose rate are represented on maps consisting of 2 kmx2 km grid cells. The average dose rate on Swiss territory outdoors is 147 nSv/h (1.29 mSv/a). The distributions are then related to the population density distribution by GIS application. The population is exposed to an average dose rate of 108 nSv/h (0.95 mSv/a) per capita which is just below the threshold for man-made dose rate given by national regulation. The lower value (relative to the country average 147 nSv/h) arises from the fact that most of the population lives north of the Alps where the lithology is dominated by rocks of relatively low radioactivity and where the cosmic radiation is low relative to the Alps.