Renewability Assessment of the Reykjanes Geothermal System, SW-Iceland. Gudni Axelsson et al. (see next slide) Iceland GeoSurvey (ÍSOR)

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1 Renewability Assessment of the Reykjanes Geothermal System, SW-Iceland (see next slide) Iceland GeoSurvey (ÍSOR)

2 Contributors Iceland GeoSurvey (ÍSOR): Gudni Axelsson, Egill Á. Gudnason, Ragna Karlsdóttir and Ingvar Th. Magnússon Institute of Earth Sciences, University of Iceland: Sigrún Hreinsdóttir, Karolina L. Michalczewska and Freysteinn Sigmundsson Vatnaskil Consulting Engineers: Andri Arnaldsson and Jean-Claude C. Berthet GNS-Science, New Zealand: Chris J. Bromley and Sigrún Hreinsdóttir HS-Orka: Ómar Sigurdsson Financial support by the GEORG Research Fund in Iceland is acknowledged 2

3 Renewability of geothermal resources Geothermal resources are generally classified as renewable This is an oversimplification, classification is too simple In essence of a double nature, i.e. a combination of: a) energy current (through heat convection and conduction) and b) vast stored energy Renewability of these aspects is quite different: a) energy current is steady and fully renewable b) stored energy is renewed relatively slowly by heat conduction Relative importance of the two components depends on both the geological nature of a system and the rate of energy extraction during utilization 3

4 Project purpose Main objective of project was to add significantly to the understanding of the nature of geothermal resources Particular emphasis on their recharge and mass balance under production, i.e. to improve understanding of their renewability Done through unifying analysis and modelling of data from different sources Emphasis on the Reykjanes geothermal system in SW-Iceland Purpose to evaluate the relative importance of the two renewability aspects (energy current vs. stored energy) for the Reykjanes system, in particular, under the current state of utilization 4

5 Project background Based on compilation of reservoir monitoring data, as well as collection and analysis of micro-gravity and geodetic data Consequently the data were jointly interpreted i) by simple modelling and ii) by simulating data by an up-to-date numerical reservoir model of the system Also repeated TEM-resistivity surveying to try to follow the growth of a steam-zone at the top of the geothermal system Photo courtesy of HS-Orka 5

6 Project phases The project aimed to join together the results of several different scientific methods/ disciplines to address the issue in question, in particular the following methods: A) High-resolution 3-D surface deformation monitoring (InSAR and GPS monitoring) B) Micro-gravity monitoring C) Repeated TEM (transient electromagnetic) resistivity surveying D) Reservoir pressure- and temperature monitoring E) Chemical content monitoring F) Dynamic geothermal reservoir modelling, to jointly interpret data 6

7 Reykjanes, Iceland Photo: O. Sigurdsson 7

8 Reykjanes development Characterized by SW-NE striking tectonic and volcanic activity as well as steam-vents, mud-pools and warm ground in an area of about 2 km 2 Reservoir temperature C The reservoir fluid is hydrothermally modified sea-water Development started as early as 1956 with shallow drilling Seven wells drilled during ; deepest well 1750 m Followed by intermittent, small-scale industrial utilization; salt and seamineral production along with fish drying Exploration and development picked up again in 1998 Included drilling of 14 deep production wells A 100 MW e capacity geothermal power plant commissioned in May

9 Reykjanes production history Average yearly mass production from the Reykjanes geothermal system from 1970 up to 2013; the operation of the 100 MW e power-plant started in 2006, while significant reinjection started in

10 Reykjanes pressure decline Pressure monitoring data from wells at Reykjanes, measured at a depth of 1500 m b.s.l.; most of the data-points are measured in production wells during breaks in production while some are measured in observation wells, e.g. RN- 16 at the margin of the main production field 10

11 Reykjanes subsidence Subsidence in Reykjanes (RNES) and Svartsengi (SVAR) estimated from GPS measurements spanning 1992 to 2014; GARD/GASK is shown here for reference 11

12 Reykjanes subsidence (cont.) Average subsidence rate from January 2009 to July 2013 in Reykjanes estimated from the combination of sets of ascending and descending TerraSAR-X InSAR interferograms 12

13 Micro-gravity monitoring Purpose of estimating the mass changes in the geothermal system during the period Hence the renewal (recharge) of the fluid reserves in the geothermal system Gravity surveys conducted during the summers of 2004 (prior to the start-up of the power plant), 2008 and

14 Gravity change modelling The analysis involved three main steps: 1) An estimation of the mass changes in the geothermal system through a Gauss-integral of the observed gravity changes during two periods, and ; 30 50% during the latter period 2) A simulation of the gravity-change anomaly for by two simple mass change models; center of mass change at m depth 3) A calculation of gravity changes at the observation points of the gravity grid on basis of mass changes in the numerical model of the geothermal system see next slides See Gudnason et al. () 14

15 Numerical reservoir model A TOUGH2 model Calibrated by various reservoir data Gravity changes due to mass changes in the model were calculated at the observation points of the gravity grid Modelled anomaly comparable to observed one, not exactly however 15

16 Chemical content No clear indications of major changes in chemical content of produced fluid (i.e. due to colder recharge) have been observed to date in Reykjanes This result can be used to estimate the minimum volume of the Reykjanes reservoir On basis of the fluid volume extracted during the first 8 years of operation of the power plant a volume of about 1.2 km 3 is estimated (assuming a porosity of 10%) Considerably less than the minimum estimated volume of the system, which is of the order of 3 km 3 This result, along with the limited recharge, likely explains why no chemical changes have been observed so far 16

17 Main results During the renewal of reservoir fluid through recharge is estimated to have been of the order of 30 50%, or about 250 ± 60 kg/s on average; the renewal for is expected to have been correspondingly less Rough mass-balance estimates based on the limited fluid renewal in the geothermal system, during the current large-scale utilization, and the small size of the geothermal system, show that reservoir fluid content may be depleted in some decades; this identifies the need for substantial reinjection; associated research is ongoing In spite of the limited size and recharge the energy in-place in the system is enormous; it is estimated that only a small fraction (2%) will have been extracted after 100 years under current extraction and recharge conditions 17

18 Conclusions/recommendations Simple as well as finite-element modelling of observed deformation can further constrain the mass change in, and the renewability of, the Reykjanes system Gravity change data should be used as a direct calibration parameter in numerical reservoir modelling, when possible The ultimate goal is to set up one all-embracing model to simulate gravity change, deformation and chemical data, along with all reservoir data, in a fully coupled manner Interpretation of the repeated TEM resistivity soundings indicates some shallow changes due to the growth of steam cap of the Reykjanes system, supporting the contention that resistivity methods may be a useful monitoring tool; in this case it didn t yield quantitative results 18

19 Thank you for listening Photo courtesy of HS-Orka 19