Monitoring of a geothermal reservoir by hybrid gravimetry; feasibility study applied to the Soultz-sous-Forêts and Rittershoffen sites in the Rhine graben

Abstract

The study is devoted to the monitoring of a geothermal reservoir by hybrid gravimetry combining different types of instruments (permanent superconducting gravimeter, absolute ballistic gravimeter, and micro-gravimeters) and different techniques of measurements (both time-discrete and recording data collection). Using a micro-gravimetric repetition network around a reference station, which is regularly measured, leads to the knowledge of the time and space changes in surface gravity. Such changes can be linked to the natural or anthropic activities of the reservoir. A feasibility study using this methodology is applied to two geothermal sites in the Alsace region (France) of the Rhine graben. We show the results in terms of gravity double differences from weekly repetitions of a network of 11 stations around the geothermal reservoir of Soultz-sous-Forêts, separated into 5 loops during July-August 2013 and 2014 as well as preliminary results from 2 stations near Rittershoffen (ECOGI). We point out the importance of a precise leveling of the gravity points for the control of the vertical deformation. A first modeling of surface gravity changes induced by realistic geothermal density perturbations (Newtonian attraction) is computed in the frame of the existing geological model and leads to gravity changes below the μGal level being hence undetectable. However, and for the same case, borehole gravity modeling showed a significant anomaly with depth that can be used as a complementary monitoring method. We show that in the limit of our uncertainties (SD~5 μGal), we do not detect any significant gravity change on the geothermal site of Soultz in agreement with the fact that there was indeed no geothermal activity during our analysis period. On the contrary, the measurements near Rittershoffen show a signal above the noise level which correlates in time with a production test but cannot be explained in terms of Newtonian attraction effects according to our basic numerical simulation.

Uncertainty Quantification and Global Sensitivity Analysis of Subsurface Flow Parameters to Gravimetric Variations During Pumping Tests in Unconfined Aquifers

“…Among the sets of geophysical data which can be of interest, gravimetric measurements are increasingly considered to carry valuable information to effectively complement drawdown data for aquifer characterization. Monitored gravity variations have been shown to embed a remarkable information content and are employed in several applications, including, e.g., geothermal energy (Hunt, ; Hinderer et al, ; Hunt & Bowyer, ; Hunt & Graham, ; Sofyan et al, ) or petroleum engineering (Alnes et al, ; Eiken et al, ; Kabirzadeh et al, ; Katterbauer et al, ; Young & Lumley, ). Local variations in the acceleration of gravity are due to the Newtonian attraction and to deformations created by loads/stresses.…”

New results on the gravity monitoring (2014–2017) of Soultz-sous-Forêts and Rittershoffen geothermal sites (France)

“…To cover an approximately 4 km 2 area around the geothermal plants, thirteen gravity stations (see Fig. 2) have been measured each summer since 2014 for the Soultz-sous-Forêts network and since 2015 for the Rittershoffen network (Hinderer et al 2015). Stations 10, 12 and 13 are common to both networks.…”

“…To report on some successful applications of such methods, we can mention Allis and Hunt (1986) at Wairakei geothermal field in New Zealand, De Zeeuw-van Dalfsen et al (2006) at Krafla geothermal plant in Iceland, Sugihara and Ishido (2008) at Okuaizu and Ogiri geothermal plants in Japan, Nishijima et al (2010) at the Takigami geothermal field in Japan. In France, Hinderer et al (2015) initiated microgravity monitoring at the Soultz-sous-Forêts geothermal plant in summer 2013 with two additional stations near Rittershoffen. In Hinderer et al (2015), only the 2014 gravity double differences were computed because of a faulty gravimeter used in 2013.…”

“…In France, Hinderer et al (2015) initiated microgravity monitoring at the Soultz-sous-Forêts geothermal plant in summer 2013 with two additional stations near Rittershoffen. In Hinderer et al (2015), only the 2014 gravity double differences were computed because of a faulty gravimeter used in 2013. We continue here this previous work which described only Fig.…”

Time-lapse gravity data at Prudhoe Bay: New understanding through integration with reservoir simulation models

“…This ongoing study integrates the known mine workings into the inversion of time-lapse gravity data to delineate the distribution of water within the underground coal mine, and additionally defines predicted rubble zones where water migrates closer to the surface. Hinderer et al (2015) demonstrates a feasibility study of using hybrid gravimetry to monitor a geothermal reservoir at Soultz-sous-Forets and Rittershoffen sites in the Rhine graben.…”