GEORG Geothermal Workshop 2016 SESSION A1, Upstream

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

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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

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
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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
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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
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Photo courtesy of HS-Orka

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
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7. Reykjanes, Iceland
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Photo: O. Sigurdsson

8. Reykjanes development
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 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 km2
 Reservoir temperature 280 – 350°C
 The reservoir fluid is hydrothermally modified sea-water
 Development started as early as 1956 with shallow drilling
 Seven wells drilled during 1968 – 1970; deepest well 1750 m
 Followed by intermittent, small-scale industrial utilization; salt and sea-
mineral production along with fish drying
 Exploration and development picked up again in 1998
 Included drilling of 14 deep production wells
 A 100 MWe capacity geothermal power plant commissioned in May 2006

9. Reykjanes production history
Average yearly mass
production from the
Reykjanes geothermal
system from 1970 up to
2013; the operation of the
100 MWe power-plant
started in 2006, while
significant reinjection
started in 2009
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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
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Reykjanes subsidence
Subsidence in Reykjanes (RNES) and Svartsengi (SVAR) estimated from GPS
measurements spanning 1992 to 2014; GARD/GASK is shown here for reference

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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

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 Purpose of estimating the mass
changes in the geothermal system
during the period 2006–2010
 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 2010
Micro-gravity monitoring

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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,
2004–2008 and 2008–2010; 30 – 50% during the latter period
2) A simulation of the gravity-change anomaly for 2008–2010 by two
simple mass change models; center of mass change at 1300 – 1700 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. (WGC 2015)

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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

16. Chemical content
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 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 km3 is estimated (assuming a porosity of 10%)
 Considerably less than the minimum estimated volume of the system, which is of the
order of 3 km3
 This result, along with the limited recharge, likely explains why no chemical changes
have been observed so far

17. Main results
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 During 2008 – 2010 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
2006 – 2008 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

18. Conclusions/recommendations
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 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

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Photo courtesy of HS-Orka
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