Hybrid gravity monitoring of a geothermal reservoir: a case study in northern Alsace, France

Time-lapse gravity is a monitoring tool to investigate underground mass redistributions and hence to monitor a geothermal reservoir both in its natural state or undergoing man-made stimulations. 

We will review 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 at least one reference station where gravity is continuously recorded with a relative gravimeter (spring or superconducting) and regularly checked with absolute gravity measurements, as well as repeat surveys of a micro-gravimetric network of several satellite stations in the vicinity of the reference one. This combination of instruments gives access to the temporal changes of the absolute gravity value at each station.

Different problems arising with this method will be investigated: time stability of the reference station, time variation of the micro-gravimetric network related to changes in the calibration factor of the spring gravimeters and importance of measuring the height changes of the gravity stations.

This hybrid gravity approach will be applied to a case study on two geothermal reservoirs in northern Alsace, 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.

The temporal gravity variations observed on these two sites in 2014 and 2015 during no geothermal activity will be shown first and help us to infer a detectability threshold that can be achieved by precise observations and appropriate corrections. We will also show preliminary results on the possible impact of the ECOGI onset of production in spring 2016.

Besides we will present the results of the analysis of a long series (2001 to 2012) of water level changes in a borehole well above the Soultz reservoir and how the tidal results can be used in terms of knowledge of poro-elastic properties of the reservoir.

A final part will be dedicated to the modelling of surface and borehole gravity changes due to the geothermal activity taking into account both attraction and elastic deformation effects.