| Title | Thermo-Hydro-Mechanical Models of the Natural Circulation at Soultz-sous-Forêts and Rittershoffen EGS Sites (France) |
|---|---|
| Authors | B. VALLIER, V. MAGNENET, J. SCHMITTBUHL, C. FOND |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | Deep geothermal energy, thermal conductivity, permeability, thermo-hydro-mechanical model, hydro-thermal convection |
| Abstract | Numerical models of the deep geothermal reservoirs in the Upper Rhine Graben (URG) have been developed over the last few decades. However, there is still a need for models that can integrate most thermal, mechanical and large-scale geophysical data. We are developing here a two-dimensional model based on a finite element approach. It includes the thermo-hydro-mechanical coupling (THM) and the properties of the brine as a function of the temperature and pressure of the fluid. A representative elementary volume of 100 m is assumed to homogenize the complexity of the small-scale fault network. An inversion to obtain large-scale rock properties has been performed using deep temperature logs and regional stress analyses. This modelling approach is applied to the cases of the Soultz-sous-Forêts and Rittershoffen (France) geothermal sites and the nearby industrial project Rittershoffen. Our study brings new insights on the extension of hydrothermal convection cells through depth, on the interpretation of the temperature gradient at shallow depth. It supports a weak influence of the lithological transition between the sediments and granitic basement on hydrothermal circulation unlike previous studies. We also show the significant effect of brine viscosity on hydrothermal circulation. Lateral temperature variability at depth in the URG is shown to be in accordance with the forecasts of this simple model. In Rittershoffen, the bottom of the hydraulic cap rock is shallower than the discontinuity of the thermal gradient such as in the Soultz case. The comparison between the two geothermal models shows many similarities in terms of rocks properties, decoupling of hydraulic and thermal cap rocks and spatial temperature variability. In both cases, the inverted properties of large-scale rocks correspond to laboratory measurements. Predictions of gravity measurements of the modeled hydro-thermal circulation are proposed. |