Monitoring of geothermal reservoirs by hybrid gravimetry

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1 Monitoring of geothermal reservoirs by hybrid gravimetry Jacques Hinderer 1, Marta Calvo 1, 3, Basile Hector 1, Umberto Riccardi 2, Gilbert Ferhat 1, Yassine Abdelfettah 1,4 & Jean Daniel Bernard 1 1 Institut de Physique du Globe de Strasbourg UMR 7516 CNRS/Université de Strasbourg, 5 rue Descartes Strasbourg, France 2 Dipartimento di Scienze della Terra, dell'ambiente e delle Risorse (DiSTAR) Universita "Federico II" di Napoli, Italy 3 IGN Madrid, Spain 4 Institut für Nukleare Entsorgung INE, Karlsruher Institut für Technologie (KIT), Germany

$Gravity signal due to geothermal reservoir Vadose zone Density changes are due to: Pore space opening/closing in hydrofracturing or hydroshearing Fluid infiltration Heating/cooling mineralization$

2 Gravity signal due to geothermal reservoir Vadose zone Density changes are due to: Pore space opening/closing in hydrofracturing or hydroshearing Fluid infiltration Heating/cooling mineralization Contributions to surface gravity changes: After corrections for: solid and ocean tides air pressure effect hydrogravimetry (vadose zone + aquifers) newtonian attraction effects geothermal signal deformation effects (poro elasticity?) strong link to geodesy (cgps + leveling)

3 Observational strategy Networks with RG Scintrex CG5 RG Advantage spatial coverage control on RG drift by loop repetition Disadvantage need for a reference point expensive in manpower and time (measurements + processing) 3

4 Data collection: Gravity measurements with a relative spring gravimeter (Scintrex CG 5) weekly repetition of micro gravity survey with imbricated loops around a reference point (GPK1) Time interval: 2 months in 2013 and 2 months in 2014 Number of stations: 11 stations (Soultz) and 2 stations (Rittershoffen) Link at beginning and end of each survey with Strasbourg gravimetric Observatory where continuous gravity observations with a superconducting gravimeter are available One or two measurements of absolute gravity with FG5 at GPK1

5 Location of the gravity stations Soultz Network of 11 stations Strasbourg gravimetric Observatory Rittershoffen 2 stations

Micro gravimetric repetition network at Soultz sous Forêts

PYR1 GPK2 SOULTZ 5 loops 11 stations Loop 1 GPK1 Pyr1 Pyr2

Kutzenhausen church Pyr3 Soultz church GPK1 PYR4 Loop 3

per week in July August 2013 and July August 2014 Loop 4

6 Micro gravimetric repetition network at Soultz sous Forêts PYR2 Spatial distribution of stations KUTZENHAUSEN GPK1 PYR1 GPK2 SOULTZ 5 loops 11 stations Loop 1 GPK1 Pyr1 Pyr2 Kutzenhausen church GPK1 PYR3 FARM HOHWILLER Loop 2 GPK1 Kutzenhausen church Pyr3 Soultz church GPK1 PYR4 Loop 3 GPK1 Soultz church Pyr4 farm GPK1 CHAPEL Time period: Once per week in July August 2013 and July August 2014 Loop 4 GPK1 chapel farm GPK2 GPK1 Loop 5 GPK1 Soultz church Hohwiller church GPK1

7 PYR 3 station Kutzenhausen church

8 Gravity simple and double differences After tide and drift corrections: Sahel Time t, station x x 0 Information on topography/density Time t 0, station x x 0 Information on topography/density Gravity double difference: Spa al: sta on x station x 0 (reference) Temporal: me t time t 0 Information on underground water storage 8

9 An example of the screen of the micro gravity processing code Pygrav New code written by B. Hector (IPGS/EOST) in Python

10 Simple differences: mainly topography and density structure (milligal range)

12 Double differences: sensitive to water distribution and deformation (µgal range)

14 Uncertainty in the double difference gravity observations in our network 2013 Station Survey 1 (2/7/2013) Survey 2 (8/7/2013) Survey 3 (16/7/2013) Survey 4 (23/7/2013) Average SD Station Survey 1 (1/7/2014) Survey 2 (15/7/2014) Survey 3 (23/7/2014) Survey 4 (5/8/2014) Survey 5 (11/8/2014) Survey 6 (18/8/2014) Average SD Strong improvement in 2014 versus 2013 due to instrument change Small uncertainties in the 2014 surveys ranging from 2.6 to 6.6 µgal

15 To be compared to very high micro gravimetric work done on a water catchment in West Africa and walking distance Hector et al. 2014

16 Results for Rittershoffen (ECOGI) Gravity decrease due to production test experiment?

Strong impact of calibration factor of the gravimeter If there is a change α of 10 4 in the scale factor between two surveys, the simple difference change between 2stations is α.

17 Strong impact of calibration factor of the gravimeter If there is a change α of 10 4 in the scale factor between two surveys, the simple difference change between 2stations is α. g m : Max in our network at Soultz = 17 milligal x 10 4 = 1.7 µgal still small effect An example of scale factor changes of a Scintrex CG3 used in the Bouillante (Guadeloupe) gravity survey Stability increases with time Debeglia & Dupont 2007 Our Scintrex CG5 used in 2013 was calibrated on a 323 milligal calibration line between sea and mountain (1740 m) in Greece with AG FG5 Δg(CG5)/ Δg(FG5) = / = 1 + ε with ε =

18 Impact of ground vertical deformation Gravity changes with height: Free air gradient = 0.31 µgal/mm Bouguer gradient (takes into account the gravity effect of a crustal layer of density 2.6) = 0.20 µgal/mm Ex: 1 cm elevation change leads to 2 µgal gravity decrease (without any direct mass transfer contribution) Precise control of elevation of the points of the micro gravimetric network required

19 How to link surface gravity changes to geothermal activity? 0. Idealised case : Point source approach g G M/d 2 1 µgal change due to a mass of 150 kg at 1 m distance or 0.15 MT at 1 km depth 1. Bouguer slab approach Relation between mass loss and gravity change (Allis 2001): g (µgal) 42 M / A where M(in T) is the mass change occurring over an average area A (in m 2 ) 2. Realistic 3D computation with prismatic bodies Takes into account geological model and any reservoir shape Needs an estimate of the density changes within the reservoir

A case study: water injection at 2000 m depth

seismic and borehole data 6 layers 30 x 20 x 5 km 3

20 A case study: water injection at 2000 m depth Modeled surface gravity 20 l/sec during 100 days in a reservoir 1 km x 1 km x 50 m less than 1 µgal Geological model frombaillieux2012 derived from seismic and borehole data 6 layers 30 x 20 x 5 km 3 Bouguer slab: 7.1 µgal Point source: 0.3 µgal Y. Abdelfettah 2014

21 Summary Set up of a micro gravity network (11 stations in 5 loops at Soultz and 2 stations at Rittershoffen) Efficient processing of all the gravity data using Pygrav code in Python (local development) Strong improvement in the quality of the results in 2014 wrt 2013 with uncertainties in the gravity double differences of less than 5 µgal Relative stability of the Soultz network and possible (small) signal at ECOGI stations in august 2014 Importance of the calibration stability of the gravimeter Importance of the vertical control of the gravity stations First results of realistic modeling suggest weak gravity signals at the surface

22 Thank you for your attention!

Università degli Studi di Napoli Federico II, Dipartimento di Scienze della Terra Napoli, Italy

Università degli Studi di Napoli Federico II, Dipartimento di Scienze della Terra Napoli, Italy Three years of monitoring using leveling and hybrid gravimetry applied to geothermal sites of Soultz-sous-Forêts and Rittershoffen, Rhine Graben, France Gilbert FERHAT 1, Nolwenn PORTIER 1, Jacques HINDERER