| Title | Hybrid Gravity Monitoring of a Geothermal Reservoir |
|---|---|
| Authors | Hinderer, J; Calvo, M; Abdelfettah, Y; Ferhat, G; Riccardi, U; Hector, B Bernard, J.-D., Littel, F |
| Year | 2016 |
| Conference | European Geothermal Congress |
| Keywords | gravity, leveling, monitoring, reservoir |
| Abstract | Time-lapse gravity is a monitoring tool to investigate underground mass redistributions and hence helps to follow a geothermal reservoir both in its natural state or undergoing man-made stimulations and therefore optimizing its utilisation. We first recall the concept of hybrid gravimetry, which is the optimal way to combine different types of instruments and techniques of measurement. In particular, hybrid gravimetry uses a reference station where gravity is continuously recorded with a relative gravimeter (spring or superconducting) and regularly checked with absolute gravity measurements, as well as repetitions of a micro-gravimetric network of several satellite stations in the vicinity of the reference one. We investigate the feasibility of the hybrid gravity technique applied to two geothermal reservoirs in northern Alsace, France, namely the Soultz-sous-Forêts site which is the first EGS (Enhanced Geothermal System) demonstration site producing electricity in France and the Rittershoffen site where the ECOGI experiment dedicated to an industrial use for heat application (24 MWth at 160 °C) takes place. We first show model predictions for the gravity changes both at the surface and as a function of depth using a very simple source linked to a simulated geothermal activity. We show the temporal gravity variations (double differences) that have been observed on the micro-gravimetric network of Soultz in the absence of any geothermal production and infer a detectability threshold for the gravity monitoring that can be achieved by precise observations and appropriate corrections. We finally point out the importance of the precision of the height changes that are needed to correct gravity measurements for the effect of the ground deformation and we present what can be inferred from high precision leveling. |