Monitoring of a geothermal reservoir by hybrid gravimetry; feasibility study applied to the Soultz-sous-Forêts and Rittershoffen sites in the Rhine graben
© Hinderer et al. 2015
Received: 31 March 2015
Accepted: 4 August 2015
Published: 7 September 2015
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.
Gravimetry is generally used as a prospecting method for underground structures at various scales (volcanoes, geothermal, gas and oil reservoirs, mineral resources, stratigraphy) and contributes to the static imagery in addition to other methods like magneto-tellurics (e.g., Volpi et al. 2003, Newman et al. 2008; Geiermann and Schill 2010) or seismics (Concha et al. 2010; Sanjuan et al. 2010). Time-lapse gravimetry can also be a monitoring tool of any underground or surface mass redistribution and has many applications in volcanology (magmatic chamber evolution), hydrology (water storage changes in the critical zone), and geothermics.
Gravity has the potential to obtain valuable information on water storage changes and water flows using non-destructive observations of a geothermal reservoir with spatial resolution ranging from meter to kilometer.
Moreover, new instruments are available, like the portable superconducting gravimeter iGrav (Warburton et al. 2010) or will be available soon, like the cold atom absolute gravimeter (Bidel et al. 2013; Wu et al. 2014; Merlet et al. 2010) that will even improve in the near future this potentiality.
Characteristics of the different gravimeters involved in hybrid gravimetry
Small drift (1–2 μGal/year)
High-precision continuous monitoring
Absolute reference + long-term evolution
Relative spring (RG)
Large drift (tens or hundreds of μGal/day)
Prospection + repetition network
a permanent gravimeter which allows a precise continuous monitoring of the time-varying gravity at a reference station located on the investigated site; in order to be able to retrieve the long-term behavior, one uses generally a superconducting gravimeter (SG) rather than a spring meter because of its very small instrumental drift (a few μGal/year) and better precision (0.1–0.01 μGal) (Hinderer et al. 2007);
a ballistic absolute gravimeter (AG) that allows to control the long-term gravity changes by repeated parallel recording over short periods of time with the SG (Sugihara and Ishido 2008; Jacob et al. 2008), as well as to check the calibration stability of the SG;
a spring relative gravimeter (RG) to repeat observations on a micro-gravimetric network around the reference station by successive loops in order to gain more insight into the space-time changes in the investigated region (Naujoks et al. 2008; Gehman et al. 2009; Jacob et al. 2010; Hare et al. 2008; Davis et al. 2008).
In this feasibility study, we do not have any SG measuring continuously on site but rather use a link to a SG in operation in the Strasbourg Gravimetric Observatory 40 km away. This impacts clearly the absolute accuracy of our local network even if we performed two AG measurements on our reference station GPK1 showing no gravity variation over a period of about 6 months (see “Absolute gravity measurements at the reference site GPK1” section).
Hybrid gravimetry is often coupled to other types of measurements like precise positioning, hydrometeorology (rain), or hydrology (piezometry, soil moisture) and brings new insights in several research topics. Progress has been made in hydrogravimetry in the study of water storage changes using mainly SG and RG measurements (Longuevergne et al. 2009; Creutzfeldt et al. 2010; Naujoks et al. 2010; Pfeffer et al. 2011; Hinderer et al. 2012; Hector et al. 2015). Similarly, new results were obtained from hybrid gravimetry in volcanology using mostly AG and RG measurements (Greco et al. 2012; Hautmann et al. 2010; Bataglia et al. 2008) that allow to determine also absolute changes in the local network which were unknown in previous studies based only on RG observations (e.g., Jousset et al. 2000). Finally, many studies in geothermics are now available using the concept of hybrid or super-hybrid gravimetry (Nishijima et al. 2000; Oka et al. 2010; Sofyan et al. 2010; Sugihara and Ishido 2008; Takemura et al. 2000).
There are several causes leading to density changes of geothermal origin like pore space opening/closing in hydrofracturing or hydroshearing, fluid infiltration, heating/cooling, as well as mineralization (De Vivo et al. 1989; Schultz et al. 2012) but the main causes for gravity changes are due to fluid injection and/or withdrawal (Allis et al. 2001; Hunt and Bowyer 2007; Hunt and Graham 2009).
In Fig. 2, the conical shape of influence (footprint) is due to the fact that the lateral extension of a layer has to increase with depth to produce the same surface gravity effect (Hector et al. 2014).
The density (and mass) changes of geothermal origin will then alter surface gravity by direct Newtonian attraction but also possibly indirectly via the vertical surface displacement generated by poro-elasticity. It is therefore of primary importance to always combining gravity observations with precise geodetic measurements (leveling, GPS, and SAR); in principle, the best option is to co-locate both gravity and leveling stations, simultaneously measured for the control of the vertical deformation and the study of the time variable vertical gravity gradient which is highly sensitive to the sub-surface mass redistribution (Hunt et al. 2002).
Time-lapse gravimetry helps to monitor the behavior of a geothermal reservoir, in its natural state with no anthropic perturbation but mostly when transient effects due to stimulation by injection/withdrawal occur. One key point is the long-term evolution of the reservoir (Sofyan et al. 2011).
In this paper, we present the first monitoring results obtained for the Soultz and Rittershoffen (NE Alsace, France) geothermal sites. The methodology, the data processing, and forward modeling as well as the results obtained between July 2013 and August 2014 are mainly presented and discussed. The PyGrav code we developed to optimize the data processing and to reduce the data uncertainties is also presented.
In this section, we first introduce the micro-gravimetric network that was set up on the Soultz and Rittershoffen geothermal sites as well as the measurement protocol. We present then the absolute gravity observations, which were done with FG5#206 AG at the reference site of the network, as well as the continuous series at the same site obtained from a Scintrex CG5 gravimeter during a 34-day time span. We introduce also the precise geodetic positioning we use to control the vertical deformation. We finally discuss the approach we follow to model the gravity effects of any geothermal reservoir.
The Soultz geothermal site is the first EGS (Enhanced Geothermal System) demonstration site producing electricity in France. Several wells from 2200- to 5000-m depth have been drilled, stimulated, and circulated within deep naturally fractured granite (Genter et al. 2010). The injection well (GPK1) was drilled to a depth of 3600 m and production well (GPK2) even deeper (5000 m) allowing initially two-well hydraulic circulation. Later on, other injection wells were added to form a multi-well system to monitor, measure, and manage the geothermal system during exploitation (Genter et al. 2013).
Description of the loops of the Soultz gravity network
GPK1 – Pyr1 – Pyr2 – Kutzenhausen church – GPK1
GPK1 – Kutzenhausen church – Pyr3 –Soultz church – GPK1
GPK1 – Soultz church – Pyr4 – farm – GPK1
GPK1 – chapel – farm – GPK2 – GPK1
GPK1 – Soultz church – Hohwiller church – GPK1
At each measurement point, the CG5 is first leveled and the operator waits 15 min to allow the instrument to become quiet after transportation. If needed, it is again precisely leveled before a sequence is launched of 5–10 consecutive cycles of 90-s duration each depending on the convergence of the results of each cycle (mean gravity after 90 s). Prior to the measurements, the long-term drift is removed with a linear fitting, and the residual drift is checked to be less than 4 μGal/h. Thus, if 3 consecutive measurements are within a 1–3 μGal range and no residual drift is observed, the measurements are stopped.
In 2014, being informed that a production test would occur in August 2014, we have added 2 more stations around the Rittershoffen geothermal site where the ECOGI experiment takes place. With a geological context similar to the Soultz-sous-Forêts project, this geothermal project is dedicated to an industrial use for heat application (24 MWth at 160 °C). The first well was drilled in 2012 and a second one in spring 2014, both to a depth close to 2500 m.
One station is very close to the site (old bridge) and the second one in the nearby village (Betschdorf). A denser network like the Soultz one with 10–15 stations will be established in the future (still this year) for a better monitoring of the ECOGI site.
Each survey starts and ends from Strasbourg Gravimetric Observatory enabling us to connect the local network of Soultz and Rittershoffen to a known reference which is monitored by both continuous (SG) and absolute (AG) instruments. There is hence one tie per survey (i.e., per week) between Strasbourg and Soultz. There is a weekly repetition of this survey during the summer months (July and August) in 2013 and 2014 leading to 14 surveys over a period of 4 months. The variability of the 2014 weekly amplitude of the J9-GPK1 ties using CG5 RG is found to be of the order of 5–7 μGal; this value has to be compared to the difference in the absolute values at GPK1 using FG5 AG between April and October 2013 which is 0.3 ± 3.4 μGal (see “Absolute gravity measurements at the reference site GPK1” section). In fact, since we have continuous SG measurements at our reference station J9, we also computed the difference between the April and October 2013 J9-GPK1 ties using both SG and AG measurements which leads to a value of 3.7 ± 3.4 μGal since there is a 4.0-μGal gravity increase at J9 station from the SG data corrected for the same effects (tides, air pressure, polar motion) as the FG5.
Absolute gravity measurements at the reference site GPK1
Absolute gravity determinations at the reference site GPK1
9 80 910 145.0
9 80 910 145.3
It turns out that our reference site GPK1 seems to be very stable with no significant change in gravity in the 6-month interval (within the uncertainty of 3.4 μGal inferred from our two AG observations). This stability has to be checked again in the future.
The measurement of the absolute gravity at Soultz and Strasbourg Observatory led also to establish a quick calibration line between these two points which helps to control the calibration factor of the Scintrex gravimeter over time. The amplitude of this calibration line is however modest (26.746 mGal) and smaller than the line of 323.170-mGal amplitude between Chelmos (1740-m altitude) and Temeni (sea level) in the Gulf of Corinth (Greece) which was measured in December 2013 by our FG5 AG. The CG5 gravimeter which was available for our 2013 study was calibrated using this line to a precision slightly better than 10−4. The CG5 used in 2014 is a new instrument acquired a few months before the summer surveys and calibrated by the manufacturer.
It is also important to point out here that the repetition of a micro-gravimetric network has to be done with a calibrated instrument (if possible always the same). Calibration accuracy can be better than 10−4 when a large amplitude absolute baseline is used (Debeglia and Dupont 2002) and this is in general enough for micro-gravimetric surveys; in our network, the largest gravity difference between two stations is about 16 mGal and the calibration error leads then to 1.6-μGal gravity change which is smaller than the mean network loop uncertainty of 5 μGal discussed in “Data processing” section. However, calibration changes with time and can reach 10−3 over a 2-year period (Jacob et al. 2010) emphasizing the fact that a regular check of the stability of the calibration factor is needed.
Continuous relative measurements at GPK1
Precise geodetic positioning
The first term in right hand side of Eq. 1 is usually called the free air correction and amounts to about – 0.31 μGal/mm; the second term is the effect of an infinite Bouguer slab of density ρ. The sum of the two effects is – 0.2 μGal/mm assuming a mean crustal density of 2670 kg m−3.
This is why a precise control of the station elevations is required. This is achieved in our project by high-precision geodetic leveling which should lead to a few millimeter precision on all the points of the network.
To perform a rigorous vertical control, all gravimetric sites are equipped with a leveling benchmark. During May 2014, a large leveling network (~40 km long) connecting the 13 gravimetric sites was observed in 4 main loops and a small loop around ECOGI site (Ferhat et al. 2014). The closure loops show an equivalent precision of 1.5 mm/km for the main loops and 0.5 mm/km for the small loops (Ferhat et al. 2014). This accuracy is large enough to guarantee a vertical precision better than a few millimeter required for gravimetric variation interpretation. From a preliminary investigation based on a repetition of the leveling network 3 times in 2014 on the small loop around ECOGI, it turns out that most of the height changes are less than 1 or 2 mm. Moreover, 2 continuous GPS (cGPS) stations have been installed within the leveling network and 4 cGPS stations around ECOGI site (cf. Fig. 4) to insure long-term stability analysis. Again, the analysis of the vertical component does not show any significant motion exceeding 1 or 2 mm (Heimlich et al. 2013).
Gravity modeling of geothermal effects
Besides our observational approach, we also wanted to estimate the surface gravity changes that might be expected from any deep geothermal activity. If the density changes linked to such an activity are spatially known, one is then able to predict if the surface gravity effects are detectable and even to set up the optimal station positioning. Unfortunately, we do not have here this knowledge and must rely on very simple (simplistic) approximations to compute the order of magnitude of the gravity effects.
Assuming now that the mass anomaly is spread over a surface A (Bouguer slab approach); B is equal to 42 μGal m2 T−1. The gravity change is expressed in μGal (B = 42) if the mass is expressed in tons (T) and the surface in square meters (m2) (Allis and Hunt 1986). In this latter case, as can be proven most simply with Gauss’s law for gravity (La Fehr 1965), the gravity change is independent on the depth but this is only valid if the lateral extension is much larger than the depth.
The possible gravity effect caused by geothermal utilization is assessed using 3D forward modeling and then the misfit is computed between before and after geothermal events (e.g., hydraulic stimulation, production, water injection, etc.). In our approach and in order to simulate the real conditions, the measurement stations are located on the real topography and the reference model is of any 3D complexity. More important, our formalism can be applied to any geothermal context.
The geological model (Fig. 9) for our study zone extracted from Baillieux (2012) is derived from seismic and borehole data and consists in a 6 layer model with dimension ~30 × 20 × 5 km. The geological stratigraphy is simplified to model only the Tertiary, Jurassic, Keuper, Muschelkalk, Buntsandstein, and the Basement horizons. These horizons showed vertical variations up to 500 m when crossing the faults (Baillieux 2012). The sedimentary layers show east dip as well as the top of the basement. The thicker geological unit is mainly the Tertiary which can reach 750 m (±320 m), whereas the other sedimentary units do not exceed 373 m, the thickness of the Buntsandstein for example (Baillieux 2012 and references in there). The geothermal reservoir in the simulated water injection area is located in the granitic basement unit below 1500-m depth.
It is obvious from Fig. 10 that the predicted gravity change at the surface is very small, below the μGal level, and hence undetectable in micro-gravimetry. We would like to test our modeling with observations; since there is presently no geothermal activity in Soultz, our experiment is merely a “null test” where we check that no gravity change occurs. This leads to a “To” state acting as a reference for the future monitoring during production.
Fully manual or automatic selection is possible according to specific thresholds in tilt, standard deviation, or duration of the gravity observations. Each selected measurement is then corrected for tides and air pressure and the software allows to remove the instrumental drift on all the chosen loops of the network. This is done using the least-square inversion scheme described in Hwang et al. (2002). This first step leads to the gravity simple differences between the reference point and any point of the network; the standard deviation is computed following Hwang et al. (2002) as the square root of the posteriori variance resulting from the inversion scheme. When different repetitions of the network are done, gravity double differences are computed according to Eq. 5; the standard error (uncertainty) on a gravity change between two surveys and for a specific station is the square root of the quadratic sum of respective station standard errors for each survey.
Results and discussion
Average standard deviation (SD) for each survey in 2013 and 2014
The variations in time are now much smaller with amplitude in the range of a few μGals almost never exceeding 10 μGal. These changes are linked to several processes including vertical deformation, underground water redistribution (soil water content + water table), and possibly deeper geothermal contributions.
Almost all observed changes in the Soultz network (stations 1–11) are within the rectangular uncertainty zone and are hence not significant. In other terms, we do not observe any gravity change that exceeds our measurement precision.
The lack of detectable gravity changes indicated by our results for 2014 is in agreement with the fact that during this period, the geothermal activity was completely stopped in Soultz. When this activity will restart as expected in 2016 after major improvements in the central geothermal system, the induced gravity changes should still be small according to our (very) simple modeling and hardly observable by our network and related uncertainty. A more precise computation will be done according to known input parameters like production/injection flow rate and stimulation duration.
Stations 12 and 13 around the Rittershoffen geothermal site show larger changes (reaching 25 μGal) that are largely above our precision level and coincide with the start of well production tests at ECOGI beginning in August 2014. However, we need to have additional measurements to confirm the correlation between gravity and geothermal activity, especially having in mind that the Rittershoffen gravity loop is longer than the other loops near Soultz, which may deteriorate the drift correction of the micro-gravimeter. Moreover, our simple modeling has shown that gravity changes due to reasonable amount of injected mass are below 1 μGal; we must be cautious on the origin of the changes which may be due to more superficial hydrological effects. However, notice that an increase of 10 μGal would require a water table increase of 25 cm (or 25 cm/φ where φ is the porosity). We plan to acquire in the future piezometric data close to our investigated site to estimate this contribution.
Since the successive surveys in 2013 of the Soultz network can be dismissed because of an instrument defect, we basically only rely on the 6 surveys performed in summer 2014 using a new instrument. The time changes of the weekly repetitions of the stations are clearly small and mostly within the uncertainty level of the order of 5 μGal. We have to repeat again these measurements in summer 2015 to check that the changes from 1 year to the next are also small, especially in the lack of geothermal activity. If this is true, we will then have a well-defined reference network to detect the possible gravity changes that might occur when the geothermal plant will be restarted in 2016. The comparison of the uncertainties in the ties between the local reference station (GPK1) and the external reference station (J9 Observatory) shows that the use of absolute measurements at GPK1 combined with continuous SG observations at J9 leads to better results than CG5 RG ties alone. It is also obvious that the ideal case would be to install at GPK1 a permanent SG regularly checked with FG5 measurements as suggested in a true hybrid gravimetric approach.
The only observed significant changes in 2014 close to the ECOGI site in Rittershoffen that are possibly related to the injection tests at the same period rely only on two stations. We plan to densify in the future the network around ECOGI with additional stations to check the stability of the Rittershoffen network in the lack of activity in summer 2015. We also intend to detect the gravity signature of the future tests planned end of 2015 and to monitor the gravity change during the 2016 production period.
The rather large distance of the mass sources in deep geothermal reservoirs (2.5 km for Rittershoffen and 5 km for Soultz) leads to very small surface signals, at least from the purely Newtonian point of view. However, borehole gravimetric modeling showed that a significant signal arises from water injection according to depth, when the source-sensor distance decreases.
This study was supported by Labex G-EAU-THERMIE project (Investissements d’Avenir), France and by the Institute for Nuclear Waste Disposal (INE)- Karlsruhe Institute for Technology (KIT), Germany.
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