Proceedings World Geothermal Congress 2015
Melbourne, Australia, 19-25 April 2015

Renewability Assessment of the Reykjanes Geothermal System, SW-Iceland
Gudni Axelsson1),2), Andri Arnaldsson3), Jean-Claude C. Berthet3), Chris J. Bromley4), Egill Á. Gudnason1), Sigrún
Hreinsdóttir2),4), Ragna Karlsdóttir1), Ingvar Th. Magnússon1), Karolina L. Michalczewska2), Freysteinn Sigmundsson2)and Ómar Sigurdsson5)1) Iceland GeoSurvey (ÍSOR), Grensásvegur 9, 108 Reykjavík, Iceland; 2) University of Iceland, Saemundargata 2, 101 Reykjavík,
Iceland; 3) Vatnaskil Engineers, Sudurlandsbraut 50, 108 Reykjavík, Iceland; 4) GNS Science, Private Bag 2000, Taupo 3352, New
Zealand; 5) HS-Orka, Brekkustígur 36, 260 Reykjanesbaer, IcelandE-mail: gax@isor.is

Keywords: Geothermal, Reykjanes, renewability, pressure draw-down, gravity changes, surface deformation, modelling
ABSTRACT
Geothermal resources are generally classified as renewable energy sources, but this may be an oversimplification. They are inessence of a double nature, i.e. a combination of an energy current (through heat convection and conduction) and vast stored
energy. The renewability of these two aspects is quite different as the energy current is steady (fully renewable) while the stored
energy is renewed relatively slowly, in particular the part renewed by heat conduction. In addition the relative importance of thetwo components depends on both the geological nature of a system and the rate of energy extraction during utilization. A research
project, supported by the GEORG research fund in Iceland, was recently completed in the Reykjanes high-temperature geothermal
system, which is located on the very tip of SW-Iceland, where the Mid-Atlantic ridge comes ashore. The Reykjanes system has
been utilized on a small scale for decades, but in 2006 a 100 MWe power plant started operation in the field. The associated greatly
increased production has caused drastic changes in reservoir conditions, in particular a considerable drop in reservoir pressure. The
purpose of the project was to evaluate the relative importance of the two renewability aspects (energy current vs. stored energy) for
the Reykjanes geothermal system under the current state of utilization. This was done through compilation of available data, suchas reservoir monitoring data, and collection of new data, mainly micro-gravity and geodetic data. Consequently these data were
jointly interpreted, partly by simulating data (e.g. gravity-change data) by an up-to-date numerical reservoir model of thegeothermal system. In addition repeated TEM-resistivity surveying was used to try to follow the growth of a steam-zone in theshallower parts of the geothermal system. The results of the project do e.g. indicate that during the period 2008 – 2010 the renewalof reservoir fluid through recharge was of the order of 30 – 50%.
1. INTRODUCTION
Utilization of geothermal resources plays a fundamental role in the energy economy of Iceland (Ragnarsson, 2013). Hightemperature (> 200°C) volcanic resources found throughout the volcanic zone of the island are used for electricity generation while
lower temperature (< 150°C) resources found outside the volcanic zone are used in direct applications, mainly for space heating(satisfying 90% of the current need). About 2/3 of the primary energy used in Iceland comes from geothermal resources. The
combined installed capacity of geothermal power plants is 665 MWe, which is about 24% of the installed capacity in the country,
the rest being hydropower. Because of this key role of geothermal resources, their sustainable utilization and renewability havebeen receiving ever increasing attention.
Geothermal resources are generally classified as renewable energy sources, but this may be an oversimplification. They are inessence of a double nature, i.e. a combination of an energy current (through heat convection and conduction) and vast stored energy(Axelsson, 2011). The renewability of these two aspects is quite different as the energy current is steady (fully renewable) while thestored energy is renewed relatively slowly, in particular the part renewed by heat conduction. In addition the relative importance ofthe two components depends on both the geological nature of a system and the rate of energy extraction during utilization.
A number of long and well documented utilization and response case histories of hydrothermal systems are available worldwide,
many spanning more than 30 years. These are extremely valuable for studying the long-term response and hence production
capacity of geothermal resources, as well as their renewability and possible sustainable utilization. Simple evaluations, andassociated calculations presented by Axelsson (2011), for a few low-temperature (< 150°C) case histories, demonstrate this. The
cases histories presented reveal that the associated geothermal systems can often be classified as either open or closed as regards
production induced recharge. Fluid volumes extracted over several decades range from being much less to being approximately
equal to the estimated pore volumes of the reservoirs of these case histories. O’Sullivan et al. (2010) provide another example of arenewability assessment, based on numerical modelling of the Wairakei geothermal system on the North Island of New Zealand.
A research project, supported by the GEORG research fund in Iceland (georg.hi.is), was recently completed in the Reykjanes hightemperature geothermal system, which is located on the very tip of SW-Iceland, where the Mid-Atlantic ridge comes ashore (seeFig. 1). The Reykjanes system has been utilized on a small scale for decades, but in 2006 a 100 MWe power plant started operationin the field. The associated greatly increased production has caused drastic changes in reservoir conditions, in particular aconsiderable drop in reservoir pressure.
This GEORG research project is the subject of this paper. The main objective of the project was to add significantly to theunderstanding of the nature of geothermal resources, in particular their recharge and mass balance under production, throughunifying analysis and modelling of data from different sources, pertaining to the Reykjanes-Svartsengi geothermal region in SWIceland, with particular emphasis on the Reykjanes geothermal system. Thus the hope was to improve the understanding of therenewability of geothermal resources and evaluate the relative importance of the two renewability aspects (energy current vs. stored
1

Axelsson et al.
energy) for the Reykjanes system, in particular, under the current state of utilization. This was done through compilation ofavailable data, such as reservoir monitoring data, and collection of new data, mainly micro-gravity and geodetic data. Consequently
these data were jointly interpreted, partly by simulating data (e.g. gravity-change data) by an up-to-date numerical reservoir modelof the geothermal system. In addition repeated TEM-resistivity surveying was used to try to follow the growth of a steam-zone in
the shallower parts of the geothermal system.

Figure 1: A simplified map of the Reykjanes geothermal area, located on the very southwest tip of Iceland, showing wells,
tectonic faults and fractures as well as recent volcanism.

This paper is organized as follows. This introduction is followed by a discussion of the renewability of geothermal resources.
Subsequently the Reykjanes geothermal system is briefly introduced along with its utilization. Following that the GEORG project is
described and its main results presented through four subchapters.
2. RENEWABILITY OF GEOTHERMAL RESOURCES
Geothermal resources are distributed throughout the Earth’s crust with the greatest energy concentration associated withhydrothermal systems in volcanic regions at crustal plate boundaries. Yet exploitable geothermal resources may be found in mostcountries, either as warm ground-water in sedimentary formations or in deep circulation systems in crystalline rocks. Shallow
thermal energy suitable for ground-source heat-pump utilization is available world-wide and attempts are underway at developing
enhanced geothermal systems (EGS) in places where limited permeability precludes natural hydrothermal activity. Saemundsson etal. (2009) discuss the classification and geological setting of geothermal systems in considerable detail.
The renewability of geothermal resources is an issue that has received considerable attention lately, as well as the question whether
to classify them amongst the renewable energy resources or the non-renewable ones. They are generally classified as renewablebecause they are maintained by a continuous energy current. Geothermal energy has for example been classified as renewable by
the European Parliament and the Council of the European Union (2009). This has been disputed by some scientists, for example in
Iceland, on the grounds that geothermal energy utilization actually involves heat-mining.
The key to understanding the nature of geothermal resources, including their renewability, as well as estimating their long-term
response to utilization, production capacity and possible contribution to sustainable energy development, is comprehensive andfocussed research, often based on extensive exploration and monitoring data. Axelsson (2011) e.g. points out the extremely
valuable information contained in well documented and monitored utilization case histories. A number of such case histories forhydrothermal systems are available, many spanning more than 30 years. These are also particularly valuable for studying therenewability and possible sustainable utilization of geothermal resources. The Reykjanes geothermal system, which is the subject of
this paper, is an example of a system having a highly informative response case history.
Geothermal resources are normally classified as renewable energy sources, because they are maintained by a continuous energy
current and how enormous the energy content of the Earth’s crust is compared to the energy needs of mankind. This is inaccordance with the definition that the energy extracted from a renewable energy source is always replaced in a natural way by anadditional amount of energy with the replacement taking place on a time-scale comparable to that of the extraction time-scale(Stefánsson, 2000). In addition they simply don’t fit well with non-renewable energy sources like coal and oil, for example becauseof much more limited greenhouse gas emissions.
2Axelsson et al.
But this classification has been disputed, as mentioned above. In addition to the discussions in Iceland mentioned above various
authors have referred to geothermal energy utilization as “heat mining”, for example Sanyal (2010). Axelsson (2011) claims that
this dispute simply arises from a need to force a complex natural phenomenon into an inadequate classification scheme. The claimthat geothermal resources are non-renewable has, moreover, been used as an argument against increased geothermal development.
The foundation for increased geothermal utilization worldwide is, however, improved understanding through amplified research.
Classifying geothermal resources as renewable may also be an oversimplification. This is because geothermal resources are inessence of a double nature, i.e. a combination of an energy current (through heat convection and conduction) and stored energy(Axelsson, 2011). The renewability of these two aspects is quite different as the energy current is steady (fully renewable) while thestored energy is renewed relatively slowly, in particular the part renewed by heat conduction. During production the renewablecomponent (the energy current) is greater than the recharge to the systems in the natural state, however, because production induces
in most cases an additional inflow of mass and energy into the systems (Stefánsson, 2000).
The double nature of geothermal resources is strikingly apparent through different assessments of the geothermal resources of
Iceland carried out in the 1980’s by different investigators. Bödvarsson (1982) estimated the size of the total heat flow through thecrust while Pálmason et al. (1985) estimated the amount of thermal energy stored in the crust. Stefánsson (2000) combined theresults of the two studies in a unified presentation, however, 15 years later.
The renewability of different types of geothermal systems (see classification by Saemundsson et al., 2009) is quite diverse. This isbecause the relative importance of the energy current compared with the stored energy is highly variable for the different types. In
volcanic systems the energy current is usually quite powerful, comprising both magmatic and hot fluid inflow. In convective
systems of the open type, i.e. systems with strong recharge, the energy current (hot fluid inflow) is also highly significant. But the
inflow can either originate as hot inflow from depth or as shallower inflow, colder in origin. In shallow inflow situations the inflow
is heated up by heat extraction from hot rocks at the outskirts of the system in question. The renewability of such systems is thensupported by the usually immense energy content of the hot rocks of the systems. In convective systems of the closed type, i.e. withlimited or no recharge, the renewability is more questionable. The energy extracted from the reservoir rocks through reinjection insuch situations is only slowly renewed through heat conduction, but again the energy content of the systems is usually enormous.
They can, therefore, be considered slowly renewable in nature.
Sedimentary systems, which are mostly utilized through doublet operations, are comparable to the closed convective systems as theenergy current is usually relatively insignificant compared to the stored energy. Their renewability is, therefore, mainly supported
by heat conduction and hence is relatively slow. The same applies to EGS- or hot dry rock systems. Both these types can thus also
be considered slowly renewable. In most such cases the stored energy component is extremely large because of the large extent andvolume of the systems.
Sustainable development involves meeting the needs of the present without compromising the ability of future generations to meettheir needs. Geothermal resources have the potential to con-tribute significantly to sustainable energy use worldwide in coming
decades as well as to help mitigate climate change. Sustainable geothermal utilization has received ever increasing attention overthe last decade (see e.g. Axelsson, 2010). The discussion has nevertheless suffered from a lack of a clear definition of what it
involves and from a lack of relevant policies as well as a lack of research on the subject. The word “sustainable” has in addition
become quite fashionable and different authors have used it at will. The terms renewable and sustainable are moreover often
confused. The former simply refers to the nature of a resource, while the latter should refer to how it is used.
3. UTILIZATION OF THE REYKJANES GEOTHERMAL SYSTEMThe Reykjanes geothermal system is located on the southwest tip of the Reykjanes peninsula in SW-Iceland, where the MidAtlantic spreading ridge comes ashore. It is characterized by tectonic and volcanic activity with NE-SW striking faults and fissures.
The nature and utilization of the system is e.g. described by Sigurdsson (2010) and Fridriksson et al. (2010). Below 1 km depth thereservoir temperature is in the range of 280 – 310°C, while temperatures as high as 350°C have been measured in deep wells. The
reservoir fluid is hydrothermally modified sea-water. Surface manifestations are prominent in the area, including steam-vents, mudpools and warm ground. They cover an area of approximately 2 km2 and extensive drilling (see below) appears to indicate that the
productive part of the geothermal system is of approximately the same size.
Development of the Reykjanes geothermal system started as early as 1956 with the drilling of a shallow (~160 m) well. Drilling
continued during 1968 – 1970, when 7 wells were drilled, with the deepest one drilled to a little more than 1750 m depth. One morewell was added in 1983 and during this period some intermittent, small-scale industrial utilization was active at Reykjanes. This
included salt and sea-mineral production along with fish drying. In 1998 exploration and development of the geothermal resources
at Reykjanes picked up again due to plans for large scale electrical generation. This involved comprehensive geological andgeophysical exploration as well as the drilling of 14 deep production wells, later followed by the construction of a 100 MW e
capacity geothermal power plant.
The power plant was commissioned in May 2006, which was manifested in a drastic increase in mass extraction, from about 50 to
800 kg/s (see Fig.2). The production increase has caused a great drop in deep reservoir pressure (see Fig. 3), which amounted toabout 35 bar in the main production sector after 3 years, and about 40 bar after 7 years of production. The pressure draw-down isconsiderably less at the margin of the present production sector (see e.g. well RN-16 in Fig. 3), yet still considerable. To counteract
the pressure draw-down reinjection of separated brine was initiated in 2009, as can be seen in Fig. 2 (difference in total and net
production). Other changes in the geothermal system have ensued as a consequence of the pressure drop (Fridriksson et al., 2010).
These include the development of a steam cap above about 1000 m depth in the main production sector, which has also resulted inincreased discharge enthalpy of many production wells.
The increased enthalpy as well as the drilling of a few shallow wells, aimed at extracting saturated steam from the steam cap, hasresulted in continuously decreasing production (see Fig. 2). In 2013 the total production was only about 70% of what it was in3

Axelsson et al.
2007, the first full year of production. The formation of the steam cap has also resulted in greatly increased surface activity. Yet theincrease is comparable to natural surges in activity due to periodic earthquake activity. The great pressure draw-down is, inaddition, reflected in gravity changes and surface deformation, both of which will be discussed in detail below. Finally it should be
mentioned that no clear indications of major changes in chemical content of produced fluid have been observed to date inReykjanes.

Figure 2: Average yearly mass production from the Reykjanes geothermal system from 1970 up to 2012 (Gylfadóttir, 2014).
The operation of the 100 MWe power-plant started in 2006, while significant reinjection started in 2009.

Figure 3: Pressure monitoring data from a number of wells drilled into the Reykjanes geothermal system, measured at a
depth of 1500 m b.s.l. (Gylfadóttir, 2014). Most of the data-points are measured in production wells during breaks in
production while some are measured in observation wells, e.g. in well RN-16, which lies at the margins of the main
production field.

The great pressure draw-down in the Reykjanes geothermal system is noteworthy, it being considerably greater if represented as
draw-down production unit (i.e. bar/(kg/s)) than in many other geothermal systems. This can result from the relatively small size ofthe geothermal system and relatively low permeability, but the question is also whether it also results from limited recharge? The
objective of the GEORGE project, the subject of this paper, is to try and answer this question. It should, furthermore, be noted thatthe pressure draw-down continues with time in spite of the somewhat declining production. This clearly indicates that recharge tothe geothermal system is considerably less than the production and that the system is partly closed. The reader is referred to thediscussion on open vs. closed systems by Axelsson (2011). The project under discussion tries to estimate to what degree the system
is closed.

4

Axelsson et al.
4. RENEWABILITY ASSESSMENT OF THE REYKJANES SYSTEM
4.1 Project Description
The renewability of geothermal resources hasn’t been systematically addressed and the issue is not yet fully understood. It is highly
dependent on the boundary conditions of geothermal systems, and in particular on their inflow, or recharge, in both the natural stateand during production. The inflow is believed to be a combination of shallow inflow, which is colder than the average reservoir
fluid, and deep hot recharge. The recharge is reflected in the mass balance of geothermal reservoirs during long-term utilization,
which in turn is reflected in e.g. pressure changes, gravity changes and surface deformation. Another side of the mass balance is thedevelopment, or growth, of steam-zones in high-temperature geothermal systems during utilization. The purpose of the GEORG
project discussed here was to develop methods to study the recharge and mass balance and apply them to the Reykjanes-Svartsengi
geothermal region in Iceland, with particular emphasis on Reykjanes.
The project involved the following participants:






Iceland GeoSurvey (ÍSOR), the main geothermal research institute in Iceland, provided expertise on geophysical methods,
geothermal reservoir physics and reservoir modelling.
The Institute of Earth Sciences at the University of Iceland (IES-UI), which has long-standing experience in volcanologicalresearch, provided expertise in geophysical and geodetical methods.
GNS Science of New Zealand, which has long-standing experience in volcanological, geothermal and environment research,
provided support and advice.
HS Orka is the power company utilizing Svartsengi and Reykjanes; it has extensive experience in geothermal exploration andgeothermal resource management.
Vatnaskil Consulting Engineers, which is an engineering firm specializing in water resource and environmental modelling(geothermal reservoirs, groundwater, surface runoff and river hydraulics as well as pollution and environment), is responsible
for the numerical modelling of the geothermal systems involved.

The project has aspired to join together the results of several different scientific methods or disciplines to address the issue inquestion, 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 and(f) dynamic geothermal reservoir modelling.
Combining surface elevation and gravity data has been used to estimate the mass balance of geothermal systems during production,
resistivity surveying has been used for exploration and chemical data has been used to study processes in geothermal systems andtheir recharge. Detailed numerical modelling of geothermal systems has also been used extensively to simulate the response ofgeothermal reservoirs to production. The innovative aspect of this project involves joining together the results of the different
methods through unified modelling of aspects (a) through (e).
Extensive repeated geodetic measurements have been collected in the study areas for decades. These include conventional opticallevelling, precise GPS geodetic measurements, satellite radar interferometric observations utilizing InSAR satellite data, andelectronic distance measurements prior to advent of GPS. The project aimed at combining all available data to infer 3Ddeformation fields. This includes geodetic data that ÍSOR has collected in the geothermal region as well as data on deformation
over broader area collected by IES-UI and collaborators. Derivation of a precise 3D-deformation field was based on joint
interpretation of InSAR images together with GPS derived 3D displacement vectors at the surveyed sites. The Reykjanes region isaffected by plate movements and is volcanically and tectonically active. Therefore the data analysis included the separation of
deformation due to these causes from the deformation caused by geothermal energy production.
High resolution micro-gravity surveying has been conducted in the region by ÍSOR and its predecessors at regular intervals during
the last three decades. The project aimed at interpreting such data jointly with the surface deformation data. The project also applied
repeated TEM resistivity surveying at a few selected locations in Reykjanes, surveyed before large-scale production started, with
the intention of monitoring the growth of a steam zone in the region.
The data analysis and interpretation of the project involved different kinds of modelling. This included simple modelling of thegravity changes observed on basis of the mass changes in the reservoir (see below), simple modelling of the surface deformation
observed and one-dimensional inversion of TEM resistivity survey data. In addition a numerical reservoir model developed by
Vatnaskil for the geothermal systems involved, already available, was employed for forward modelling.
The numerical reservoir model is regional and covers the three geothermal fields of the Reykjanes peninsula: Svartsengi, Eldvörp
and Reykjanes. It covers a total area of about 1102 km2 and is three-dimensional, divided vertically into 15 horizontal layers ofvarying thickness. The model and contains 2025 elements per layer and within an area of approximately 4 km2 centered in the
Reykjanes field, the numerical mesh consists of a relatively dense network of regular elements. The TOUGH2/iTOUGH2 modelling
framework is used for calculating conditions in the model. The present model, which is based on older models dating back to as
early as 1982, has been under development since 2009; it’s constantly being updated as new data become available. The model
extends vertically from +50 m a.s.l. down to -3450 m a.s.l., with its bottom being well below the deepest wells in Reykjanes. The
model models the initial temperature and pressure conditions in the Reykjanes system (Svartsengi as well) by simulating estimatedinitial temperature and pressure profiles for all deep wells within the model area as well as simulating the pressure decline observedin the reservoirs since production from them started (see e.g. Fig. 3 for Reykjanes).
5

Axelsson et al.
The results of the items discussed above will be presented and reviewed briefly below. It should be emphasized that the project has
intended to address specifically, and add to the understanding of, boundary conditions and their role in the long-term production
response of geothermal systems. Up to now more emphasis has generally been put on detailed information on the production partsof the systems, which has been made available through the drilling and testing of numerous wells.
The approach and methods applied in the project are, to a variable degree, comparable to that employed to studies of geothermal
resources in other geothermal countries, most notably New Zealand, where scientists have taken a pioneering role in thedevelopment of appropriate methods. As recent examples the work of Hunt et al. (2003) involving analysis and modelling ofgravity changes associated with geothermal utilization and that of Brockbank et al. (2011) involving comparable deformation
modelling may be noted. Some related work has also been carried out in Japan with Nishijima et al. (2005) providing an example ofgravity change data interpretation. In addition Bromley (2014) discusses the connection between production-induced subsidence
and induced seismicity. It should, therefore, be noted that an important part of the project focuses on applying methods comparable
to those developed for geothermal fields in New Zealand to the Reykjanes-Svartsengi fields and on comparing the results withresults obtained in New Zealand.
4.2 Deformation
To monitor crustal deformation, in particular the subsidence, around the Reykjanes geothermal power plant, annual and biannual
GPS measurements have been performed by IES-UI on a regular basis during the last decade and InSAR images from the German
TerraSAR-X and TanDEM-X satellites have been collected and analyzed (Michalczewska et al., 2014). In addition ÍSOR has
conducted GPS measurements in the measurement grid used for gravity monitoring (see below) every few years. These data havebeen merged and jointly analyzed to provide a spatial and temporal model of surface deformation in the area. The main results arepresented in figures 4 and 5.

Figure 4: Subsidence in Reykjanes (RNES) and Svartsengi (SVAR) estimated from GPS measurements spanning 1992 to2014. The site Gardskagi (GARD/GASK), outside the affected area, is shown here for reference.

Figure 4 shows the changes in elevation (subsidence) in the Reykjanes and Svartsengi fields during the last two decades while Fig.
5 shows a map of the subsidence in the area, along with the estimated horizontal displacement-field. The rate of subsidence inReykjanes appears to have been fastest in the two years after the power plant started production but has relatively been steady sincethen. This corresponds well with the observed pressure changes in the geothermal reservoir (see Fig. 3).
The InSAR data from January 2009 to July 2013 show a 4 km x 3 km subsidence bowl with over 40 mm/yr subsidence rate in thecenter, which also corresponds well with observed spatial variations in reservoir pressure draw-down. The subsidence is elongatedin the NE-SW direction following the direction of the main fissure swarm of the region, as does the pressure draw-down bowl.
At the time of writing of this paper the surface deformation above the Reykjanes geothermal system hasn’t been modelled. The aim
is, however, to model it with a simple point and ellipsoidal source models and interpret the results jointly with the results of thegravity modelling presented below and the numerical reservoir model of the geothermal system.
4.3 Gravity Changes
An analysis of gravity changes observed in the Reykjanes geothermal field during the last decade was at the core of the project,with the principal purpose of estimating the mass changes in the geothermal system during the period 2006–2010 and hence therenewal (recharge) of the fluid reserves in the geothermal system. This work is described in more detail by Gudnason (2015). The
gravity surveys this work was based on were conducted by ÍSOR during the summers of 2004 (prior to start-up of the Reykjanes
power plant), 2008 and 2010. Figure 6 shows a contour-map of the measure gravity changes in Reykjanes between the middle of2008 and the middle of 2010, as an example. The analysis involved three main steps. First, an estimation of the mass changes in the6Axelsson et al.
geothermal system through a Gauss-integral of the observed gravity changes during two periods, 2004–2008 and 2008–2010.
Secondly, a simulation of the gravity-change anomaly for 2008–2010 by two simple mass change models. Thirdly, a calculation ofgravity changes at the observation points of the gravity grid on basis of mass changes in the numerical TOUGH2-model of thegeothermal system discussed above. It should be mentioned here that Eysteinsson (2000) presents the results of gravity monitoringin the Svartsengi field up to the end of last century, including a Gauss-integral analysis.

Figure 5: Average subsidence rate from January 2009 to July 2013 in Reykjanes estimated from the combination of sets ofascending and descending TerraSAR-X InSAR interferograms (Michalczewska et al., 2014). Horizontal velocities are
estimated from GPS data spanning 2008 to 2013.7.

The results of a Gauss-integration of the gravity-change anomaly for the period 2008–2010 indicates that renewal of the reservoir
mass during that period was likely in the range of 30–50%. The renewal for the period 2006–2008 was probably somewhat less, but
can’t be estimated because of the loss of a centrally located measuring point in 2006. A simulation of the 2008–2010 anomaly by aspherical mass-change volume indicates that the center of the volume is at a depth of 1300–1700 m. Modelling of the gravity
changes on basis of the mass changes in the TOUGH2 numerical reservoir model of the Reykjanes geothermal system result inquite comparable results for the 2008–2010 period. The observed and calculated anomalies are comparable in shape, even though
the calculated one doesn’t extend as much to the NE as the observed one and the calculated maximum anomaly is a little smaller. A
smaller and lower gravity-change anomaly for the 2008–2010 period indicates that the numerical model allows somewhat more
recharge than is the case in reality. These results show that the gravity data can now be used to support the calibration of thereservoir model.
4.4 Other Aspects
TEM-resistivity surveying was conducted in the Reykjanes field in both 1996 and 2004. In association with the project describedhere soundings were repeated in 6 stations in 2009, 2011 and 2012. The purpose was to attempt to monitor the growth of the steam
cap of the geothermal system. Interpretation of the repeated TEM resistivity soundings indicates some shallow changes, particularly
an increase, in resistivity at some of the stations, changes which may likely be attributed to the growth of steam cap of the system.
The interpreted changes don’t yield quantitative results, however, but the results supporting the contention that resistivity methods
may be a useful monitoring tool.
No clear indications of major changes in chemical content of produced fluid have been observed to date in Reykjanes, as alreadymentioned. Such monitoring data can be used as the basis of simple modelling of the contribution, or balance, of fluid of different
origin in the reservoir, i.e. colder recharge vs. hot reservoir fluid. If it is assumed that the fluid recharging the system has a chemicalcomposition different from that of the reservoir, the fact that no changes have been observed contains information on the minimum
volume of the Reykjanes reservoir. On basis of the fluid volume extracted during the first 8 years of operation of the Reykjanes
power plant a volume of about 1.2 km3 is estimated (assuming a porosity of 10%), which is considerably less than the minimum
7

Axelsson et al.
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.
Based on the production and reinjection history of the Reykjanes geothermal system, along with the results of the modelling of theobserved gravity changes, some simple mass and energy balance modelling can be performed. This hasn’t been completed at thewriting of this paper, but rough estimates show that due to the limited recharge and small reservoir volume the reservoir fluid
content may be depleted in some decades, unless reinjection is increased. Plans in that direction are already underway. In contrast,
the energy in-place in the system is enormous and it is estimated that only a small fraction (2%) of that will have been extracted
after 100 years under current extraction and recharge conditions, in spite of the limited size and recharge of the system.

Figure 6: Observed gravity changes in Reykjanes from 2008 to 2010 (from Gudnason et al., 2015). The black dots denote
fixed gravity monitoring stations.

5. CONCLUSIONSThis paper has reviewed and presented the results of a research project involving the Reykjanes high-temperature geothermal
system, which is located on the very southwest tip of Iceland, supported by the Icelandic GEORG research fund and carried out
during 2009 – 2014. The purpose of the project was to assess the renewability of this particular geothermal resource, specifically toevaluate the relative importance of the two components of its renewability, i.e. the steady energy current (through heat convection
and conduction) and the vast stored energy, which is renewed quite slowly, under the current state of utilization. This was done
through compilation of available data, such as reservoir monitoring data, and collection of new data, mainly micro-gravity andgeodetic data. Consequently these data were jointly interpreted, partly by simulating data (e.g. gravity-change data) by an up-todate numerical reservoir model of the geothermal system. In addition repeated TEM-resistivity surveying was used to try to follow
the growth of a steam-zone in the shallower parts of the geothermal system.
The principal results are the following:

During the period 2008 – 2010 the renewal of reservoir fluid through recharge is estimated to have been of the order of 30 –50%. This corresponds to about 250 ± 60 kg/s on the average. The renewal can’t be estimated for the period 2006 – 2008 due
to lacking data, but is expected to have been less.
This renewal is less than the 60 – 70% renewal which has been estimated for the nearby Svartsengi system (Eysteinsson,
2000), indicating that the Reykjanes system is more closed, i.e. with less natural recharge, despite a quite comparable
geological setting.
The limited fluid renewal in the Reykjanes geothermal system during the current large-scale utilization along with the small
size of the geothermal system identifies the need for greatly increased reinjection. This is already being planned andassociated research is ongoing, e.g. comprehensive tracer testing.
8

Axelsson et al.


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 of that will have been extracted after 100 years under current extraction and recharge conditions.
Deformation modelling has not been completed at the time of writing of this paper, but it can further constrain the
renewability of the Reykjanes resource as well as the likely shape and volume of maximum mass change due to utilization.
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
doesn’t yield quantitative results, however.

Some further evaluation of the data from Reykjanes is recommended as well as further development of the methods employed.
This includes:








Gravity monitoring should be made more comprehensive by both increasing the number of stations and increasing its
frequency (from once every few years to once every one or two years). Steps in that direction are already underway. Sometechnical improvements in the methodology are also being planned.
Deformation monitoring can also be improved, both by making it more comprehensive and by following advances in GPS
and InSAR monitoring technologies.
Gravity change data should be used as a direct calibration parameter in the numerical reservoir modelling, instead of using it
only together with forward modelling as done here.
The simple deformation modelling still to be completed is expected to provide additional constraints on the nature of the
Reykjanes geothermal system. More complex finite element modelling should also be attempted.
Monitoring of pressure changes in marginal wells of the geothermal field should be enhanced (frequency and accuracy) tobetter determine the extent and shape of the pressure draw-down in the Reykjanes geothermal system, and hence help
delineate the systems boundaries and its boundary conditions.
Chemical monitoring data can be used as the basis of simple modelling of the contribution, or balance, of fluid of different
origin in the reservoir (see previous section), i.e. colder recharge vs. hot reservoir fluid, both data on changes in chemical
content and data indicating no changes.
The ultimate goal would, consequently, to set up one all-embracing model to simulate gravity change, deformation andchemical data along with all reservoir data in a fully coupled manner.

ACKNOWLEDGEMENTS
The authors would like to thank HS-Orka for allowing publication of data from the Reykjanes geothermal system, and the GEORG
research fund for financial support.
REFERENCES
Axelsson, G.: Using long case histories to study hydrothermal renewability and sustainable utilization, Geothermal Resources
Council Transactions, 35, (2011), 1393–1400.
Axelsson, G.: Sustainable geothermal utilization – Case histories; definitions; research issues and modelling, Geothermics, 39,(2010), 283-291.
Brockbank, K., Bromley, C.J., and Glynn-Morris, T.: Overview of the Wairakei-Tauhara subsidence investigation program,
Proceedings, 36th Workshop on Geothermal Reservoir Engineering, Stanford University, California, (2011), 7 pp.
Bromley, C.J.: Seismicity and subsidence: Examples of observed geothermal deformation synergies from New Zealand,
Proceedings, 39th Workshop on Geothermal Reservoir Engineering, Stanford University, California, (2014), 7 pp.
Bödvarsson, G.: Terrestrial energy currents and transfer in Iceland, In G. Pálmason (ed.): Continental and oceanic rifts,
Geodynamic Series, 8, Am. Geophys. Union, (1982), 271–282.
European Union: Directive 2009/28/EC of the European Parliament and of the Council 23 April 2009. Official Journal of theEuropean Union, L 140/16, (2009), 47 pp.
Eysteinsson, H.: Elevation and gravity changes at geothermal fields on the Reykjanes peninsula, SW-Iceland, Proceedings, World
Geothermal Congress 2000, Kyushu-Tohoku, Japan, (2000), 559–564.
Fridriksson, Th., Óladóttir, A.A., Jónsson, P., and Eyjólfsdóttir, E.I.: The response of the Reykjanes geothermal system to 100
MWe power production; Fluid chemistry and surface activity, Proceedings, World Geothermal Congress 2010, Bali,
Indonesia, (2010), 7 pp.
Gudnason, E.Á., Arnaldsson, A., Axelsson, G., Magnússon, I.Th., Berthet, J.C.C, and Halldórsdóttir, S.: Analysis and modelling ofgravity changes in the Reykjanes geothermal field in Iceland, during 2004–2010, Proceedings, World Geothermal Congress
2015, Melbourne, Australia, (2015).
Gylfadóttir, S.S.: Svartsengi – Reykjanes; Temperature and pressure monitoring 2013, Iceland GeoSurvey report, ÍSOR-2014/026,
Reykjavík, (2014), 86 pp.
Hunt, T., Graham, D., and Kuroda, T.: Gravity changes at Tauhara geothermal field, Proceedings, 25th New Zealand Geothermal
Workshop, Auckland, New Zealand, (2003), 121–126.
Michalczewska, K., Hreinsdóttir, S., Dumont, S., and Sigmundsson, F.: Crustal deformation on the Western Reykjanes Peninsula
2009 to 2014 mapped by InSAR and GPS measurements, Institute of Earth Sciences report, JH1401, University of Iceland,(2014), 71 pp.

9

Axelsson et al.
Nishijima, J., Fujimitsu, Y., Ehara, S., Kouno, E., and Yamauchi, M.: Micro-gravity monitoring and repeated GPS survey at
Hatchobaru geothermal field, Central Kyushu, Japan, Proceedings, World Geothermal Congress 2005, Antalya, Turkey,(2005), 5 pp.
O’Sullivan, M., Yeh, A., and Mannington, W.: Renewability of geothermal resources, Geothermics, 39, (2010), 314–320.
Pálmason, G., Johnsen, G.V., Torfason, H., Saemundsson, K., Ragnars, K., Haraldsson, G.I., and Halldórsson, G.K.: Geothermal
resource assessment for Iceland (in Icelandic), Orkustofnun (National Energy Authority) report, OS-85076/JHD-10,
Reykjavík, (1985), 134 pp.
Ragnarsson, Á.: Geothermal energy use, country update for Iceland, Proceedings, European Geothermal Congress 2013, Pisa, Italy,(2013), 11 pp.
Saemundsson, K., Axelsson, G., and Steingrímsson, B.: Geothermal systems in global perspective, Proceedings, Short Course on
Surface Exploration for Geothermal Resources, organized by UNU-GTP and LaGeo, San Salvador, El Salvador, (2009), 16
pp.
Sanyal, S. K.: Future of geothermal energy, Proceedings, 35th Workshop on Geothermal Reservoir Engineering, Stanford
University, California, (2010), 6 pp.
Sigurdsson, Ó.: The Reykjanes seawater geothermal system – Its exploitation under regulatory constraints, Proceedings, World
Geothermal Congress 2010, Bali, Indonesia, (2010), 7 pp.
Stefánsson, V.: The renewability of geothermal energy, Proceedings, World Geothermal Congress 2000, Kyushu-Tohoku, Japan,(2000), 883–888.

10