Example_Geothermal Energy Utilisation in Iceland

Advanced and

Alternative

Energy Systems

LVA302.067

Geothermal Energy Utilisation in Iceland

Martin Knoglinger

Student of Vienna University of Technology Study of literature for the

Department of Mechanical Engineering, Institute for Energy Systems and Thermodynamics

Abstract

This essay summarises the Geothermal Energy Utilisation in Iceland. It describes the geology of Iceland linked

to geothermal heat coming from volcanic systems generally. Furthermore mechanics and behaviour of

geothermal reservoirs are explained. It gives an overview about used technologies to explore and utilise

geothermal heat for district heating and electricity production especially in Iceland. Explanations are made

with many figures for a clear understanding.


Martin Knoglinger Page 2

Content

1 Introduction ..................................................................................................................................... 3

2 Exploring of Reservoirs and their Assesment afterwards ............................................................... 8

2.1 Exploring of reservoirs ............................................................................................................. 8

2.2 Assessment of reservoir by volumetric model ...................................................................... 12

3 Geothermal reservoir engineering ................................................................................................ 13

4 Monitoring ..................................................................................................................................... 20

5 Temperature gradients and temperature/pressure profiles in geothermal wells ........................ 21

6 Thermodynamic processes to use geothermal steam for electricity production ......................... 28

7 Environmental impact of geothermal power plants ..................................................................... 33

8 Conclusion ..................................................................................................................................... 35

List of figures ......................................................................................................................................... 36

Reference .............................................................................................................................................. 39



1 Introduction

Geothermal energy originates from the heat retained within the Earth’s core since the original

formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the

surface. The global heat loss of the earth is around 40 Mio MW which is about 500 times of the

present extracted heat of exploitation [1]. The heat transfer from the core to the surface is based on

conduction and convection.

Fig. 1: Temperature distribution within the earth [1].

In certain cases geothermal reservoirs are linked to volcanic systems. In order to utilise geothermal

heat, this places have favourable geological conditions to extract this energy because of higher

temperature gradients. Fig. 3 shows all plate systems at the earth’s surface, whereby the borders are

usually linked to volcanic activity. Iceland is located on the middle oceanic ridge as well on a hot spot

as it is shown in Fig. 3 and Fig. 3 . The ridge separates the North American- from Eurasian Plate and

these plates moving apart.

Fig. 2: The mantle plume of the hot spot is responsible for rising ductile material from the mantle and also magma

coming from beneath ridge. Geologists assume a plume diameter of around 100-150km in the upper portion below the

earth’s surface. The specific location on the ridge as well on the hot spot formed Iceland 20 Mio years ago [2].



Fig. 3: Plate boundaries and location of Iceland within the plate system [3].

In geothermal energy utilisation are different methods available. These methods are based on

geothermal systems. Conventional systems differ from Enhanced Geothermal Systems (Hot dry rock).

The following chapters only describe the utilisation from conventional geothermal systems.

Geothermal energy can be used for district heating or driving turbines to produce electricity. It can

be considered as renewable energy source [3]. This is not exactly true, but this is a proper definition

for human time scale. The demand of energy is increasing on the world while fossil and nuclear

energy sources as well are limited. Fig. 4 and Table 1 should explain the importance of geothermal

energy additionally.

Table 1: Table shows proven reserves and resulting depletion time evaluated in 2005 [3].

middle oceanic ridge



Fig. 4: Worldwide technical potential of renewable energy sources [3].

Fig. 4 shows the great potential of geothermal energy. If we could harness only 0.1% of this energy it

would satisfy the present consumption for 10000 years [3].

The geothermal activity especially in case of Iceland is explained below.

The plates drift apart and create fissures. They open some space which are refilled with basaltic

magma again coming from deeper located magma chambers, Fig. 5, Fig. 6 and Fig. 7. This is not

continuously, in fact it happens more accidentally over a time period. The allowable shear stresses of

the rocks are exceeded in a certain time which results in seismic activity. Such (micro) earthquakes

are always linked to discontinuous movements. However, the folowing figures show the behaviour

along the ridge and also the geothermal distribution in Iceland.

Fig. 5: The crustal model of Pálmason: A rock piece solidifies on the surface, slowly sinks down because of the weight of

successive lavas. At the same time, the piece is carried out of the volcanic zone. When the piece was dug down, the pores

close because of outer pressure and alteration and precipitation of secondary minerals takes place [3].

Lines of same age

Movement of solidified basaltic lava after it

reached the surface due to eruption activity



Fig. 6: Geothermal fields in Iceland: The marked places are the installed Power Plants for electricity production in 2009.

The latest produced rocks are along the ridge and the oldest rocks are at the east and west coast of the country [3].

Fig. 7: Guðmundsson 1990: Structure of volcanic system. Magma chambers are formed due very high density of dykes

which band together. Magma reservoirs are located below. They are the origin magma source. During volcanic eruptions

the magmatic material from magma chambers flows up to earth’s surface [2].

Fig. 8: The mantle plume is responsible for the hot spot “Iceland”. It fills up magma chambers located above [2].

Dykes

fissures

mantle plume

magma chamber

magma reservoir/intrusion

magma chamber

mantle

mantle

asthenosphere



There are two different kind of reservoirs basically:

� Low temperature reservoirs (<150°C at one km depth)

� High temperature reservoirs (>150°C at one km depth)

Usually the high temperature reservoirs are linked to volcanic activity and more chemical

constituents are dissolved in the hot water of the reservoir. Following figures show principle sketches

(conceptual models) of the fluid mechanics behaviour within such systems.

Fig. 9: Low temperature fields: In convective systems the heat source is the hot crust at depth in tectonically active areas,

with above average heat flow. Here the geothermal water has circulated to considerably depth (>1km), through mostly

vertical fractures, to mine the heat from rocks by convection [4].

Fig. 10: Conceptual model of high temperature geothermal system: Volcanic systems are in one way or another

associated with volcanic activity. The heat sources for such systems are hot intrusions or magma. They are most often

situated inside, or close to, volcanic complexes such as calderas and/or spreading centres. Permeable fractures and fault

zones mostly control the flow of water in volcanic systems [4].

Geothermal heat utilisation in a conventional way always requires at least three points: Water in the

reservoir, Heat due to temperature which also implies pressure and permeability. Point one regards

to conventional geothermal system. It is not necessary to reinject (compare EGS/Hot dry rock) some

fluid back to reservoir, because the system has already some hot water/steam which can be

extracted. The aim of geothermal utilisation is to extract some heat (point two). Permeability

describes how easily the water can flow through a geothermal system. High permeability due to

fractures is also required in order to extract the energy successful of a reservoir.



2 Exploring of Reservoirs and their Assesment afterwards

2.1 Exploring of reservoirs

Exploring of reservoirs is basically done by different methods.

High temperature fields Low temperature fields

Geological mapping Geological Mapping

Resistivity Resistivity

Chemical methods Heat flow

Seismic measurements Seismic

Gravity Ground magnetic

Airborne magnetic Soil temperature

Exploration drilling Chemical Geothermometers

Production drilling Exploration drilling

Production drilling

Table 2: Different methods to exploring of geothermal high- and low temperature reservoirs [3].

The aim of resistivity method is determining the resistivity of different layers. Rocks (layers) have

their own electrical characteristic as well geothermal reservoirs. Evaluating of measured resistivity

allows creating models to describe

the geological conditions within

the earth. Measuring of rock

resistivity is needed and essential

but it is also important to evaluate

these data properly otherwise

wrong models are the result. The

geological structure of Iceland is

already well known, and Fig. 11

shows a typical Icelandic resistivity

structure of high temperature fields. This

is just a principle sketch and real

geological structures are usually much more

complex. The resistivity of rocks depends on:

• Porosity

• Temperature

• Fluid salinity

• Type of pore fluid (water, oil, steam)

• Type of rock

• Alteration

• Boiling

200-300°C

Fig. 12: Ions and electrons are responsible for electrical current in

pores of rocks [3].

Fig. 11: Principal sketch of the resistivity structure in Iceland [3].



Three different methods are available to explore a new field by resistivity measurements.

� DC-method

� TEM-method (magneto-telluric soundings)

� MT-method (transient electromagnetic)

The DC-method is described shortly below. This method was developed from Schlumberger to

explore oil reservoirs. The geothermal companies took it over to explore their geothermal water

systems. It is based on dipole-dipole profiling. Different current impulses are transmitted into the

earth, while voltage is measured by other poles located in between the transmitter poles on the

surface. Evaluation of the measured data gives a resistivity profile as it is shown in examples below.

Fig. 13: DC-method to determine the resistivity of different layers below the earth’s surface [5].



Fig. 14: Example of resistivity profile of Reykjanes peninsula (high temperature field). Proper interpretation of such

profiles by geologists is essential for exploring [3].

Fig. 15: Resistivity model: IDDP-Icelandic deep drilling project. One of the boreholes was drilled in Krafla [3].

Wm … resistivity width

depth



Further measurements like micro earthquakes (Fig. 16) provide additional information to evaluate

geological structures. Icelandic rocks (as well minerals) are mainly from basaltic magma. It has its

melting point at around 700-800°C. Earthquakes stops were the material becomes ductile. In case of

the Icelandic crust the border of plastic to solid material is at 12 km depth. Magma chambers have

usually low values of resistivity and therefore high conductivity (derived due to the high density of

material in chamber). Fig. 16 shows the link between earthquakes measurements and resistivity

measurements.

Fig. 16: Depth of distribution of earth quakes as a ratio of maximum numbers of quakes. The other curve shows the

cumulative numbers of earthquakes with depth as ratio of total number of quakes. Depth numbers are in km below sea

level [3].

Other technologies are also used from geologists to get some information about the geological

structure as it is shown in Table 2. All this gathered information are used from geologists to map the

earth’s interior as good as possible but the truth turns out not before drilling of wells. If the

geological mapping promises a high probability of a geothermal reservoir at a certain place below the

surface, the energy content of this reservoir must be well estimated. Afterwards people decide about

the best place to drill which implies to extract the geothermal energy efficiently.

Different techniques are available to assess the energy content of the reservoir:

� Deep temperature estimates (chemistry)

� Surface thermal flux

� Volumetric methods

� Geothermal modelling:

� Simple models (often analytical)

� Lumped parameter models

� Detailed numerical models

Only the volumetric model is explained below.



2.2 Assessment of reservoir by volumetric model

The volumetric method is used for first step estimation of reservoirs. The main drawback is

that the dynamic response of a reservoir to production is not considered, such as pressure

response. Reservoirs with same heat content may have different permeabilities and

recharge. Therefore, it does not consider the different production potentials.

The basic steps of this model are listed below:

� Calculate the thermal energy (heat) stored in certain volume of rock.

� The heat in a predefined volume is calculated with respect to given reference

temperature (f. ex. mean annual temperature) and other given prerequisites

� Estimate the electricity production

Fig. 17: Shows, how to calculate the stored energy in the system [3].

Fig. 17 explains the calculation of the stored energy in the system. The volume can be estimated

from geophysical (e. g. resistivity) and geological data. If electrical generation potential is to be

assessed two methods are possible:

a. By assuming a certain efficiency (h=0.08-0.1) and reference temperature (Treference) according

to the method of generation (about 120-130°C for conventional steam turbines)

b. By estimating the likely enthalpy of the fluid that will be produced the steam fraction (X) at

surface pressure conditions (inlet pressure) can be estimated. To generate 1MWel about

2kg/s of high-pressure steam are required.

Finally the equations to calculate the power output are shown in Fig. 18.



Fig. 18: Equations to calculate the electrical power from a reservoir by volumetric method [3].

In order to obtain the recoverable energy content two factors are required:

� Consequently surface accessibility (A) must be estimated, i. e. what proportion of volume can

be accessed by drilling from surface.

� A recovery factor (R) is incorporated, a factor which indicates how much of accessible energy

may be technically recovered, often assumed in the range of 0.05-0.20.

For individual systems Monte Carlo method is often applied. It involves assigning probability

distributions to different parameters and estimating system potential with probability. Again, using

the volumetric method gives only a rough estimation about the production potential as explained

above. Geothermal models are more accurate and are often used to describe the behaviour of a

reservoir in more details.

3 Geothermal reservoir engineering

Purpose and role of reservoir engineering:

� Describe physical nature and behaviour

of geothermal systems

� Estimate geothermal reservoir properties

� Simulate production response

� Estimate production potential

� Key role in resource management

After the drill bit tapped the geothermal water

reservoir the well must be tested to experience

the properties.

Fig. 19 Drilling operation in Iceland [3].



It is essential to know the properties for production response. Therefore the most significant

properties are explained shortly below.

� Rock properties:

• Permeability

• Porosity

• Density, compressibility, heat

capacity, thermal expansion, thermal

conductivity

� Fluid properties:

♦ Density, compressibility, heat

capacityy, thermal expansion,

thermal conductivity, viscosity

� Reservoir storage capacity

Most significant are permeability, porosity and storage capacity. They control the response of a

reservoir and its production potential. They also have great variability compared to other rock

properties with limited variability. The porosity is defined as pore space volume divided by total

material volume. The matrix porosity controls the fluid reserves. The permeability describes how

easily fluid flows through rocks. Thus it reflects flow resistance. The permeability can be described by

Darcy’s law, Equ. 1, whereby q describes the mass flux in [kg/s m²] through the reservoir,

Equ. 1: Darcy’s law [6]

υ describes the kinematic viscosity [m²/s] and dp/dx is the pressure gradient in [Pa/m]. The

permeability k [1 D (Darcy) = 10-12m²] varies in a great range. Fractures control the permeability

essential. The storativity coefficient is the mass of fluid that is stored (released) by a unit volume for

a unit pressure increase (decrease) in [kg/m³Pa] [6].

Equ. 2: Storativity [6]

Following figure explains the storativity.

Fig. 21: Storage is the ability of a reservoir to store and release fluid in response to changes in reservoir pressure [6].

Fig. 20: Two-phase flow discharge

measurements[6].



The storativity controls how fast pressure changes occur in reservoirs during production, or mass

extraction.

Fig. 22: Link between pressure drop and mass production [6].

The properties were described before simply. In order to obtain these parameters well test are made

before each well is connected to the power plant. For instance, the steps and tasks of pressure

transient tests are listed below:

� Pressure state of well and reservoir are disturbed (by mass extraction and injection) and pressure

response is measured.

� Purpose to asses characteristics of wells and estimate reservoir properties

� Various kinds of tests exists (few hours to many months)

� Interpretation and analysis are based on pressure diffusion equation, Equ. 3.

Equ. 3: Conservation of mass leads to the Pressure Diffusion Equation. It is a basic differential equation describing

pressure changes and fluid flow, in geothermal and other hydrological systems [6].

The pressure diffusion equation has exactly

the same mathematical form as the heat

conduction equation. Therefore these are

comparable processes, whereby pressure

diffusion is more than 1000000 times faster

than heat conduction [6]. These processes

may often treat separately.

Different models are available to simulate

reservoirs.

� Horizontal confined aquifer

� Horizontal unconfined reservoir

� …

Some of them are shown in Fig. 23. The

pressure diffusion equation can be applied

in each model. The interpretation of well

tests is based on the pressure diffusion

equation.

Fig. 23: Different available models [6].



In order to explain the link between model and well test, further explanations are described below.

Computerized matching of model response

Fig. 24: Example of pressure data from a step-rate injection test [6].

It is important to keep in mind that results from applied pressure diffusion equation are model

dependent, i. e. not actual well/reservoir parameters. Different models yield different values. The

task is to choose most realistic model which fits the data of the well test as close as possible.

Fig. 25: Models which fit well test data as close as possible reflects the real reservoir properties as good as possible [6].

An example of a real well test with fitting computerized model is figured below, Fig. 26.

Water was pumped into

the reservoir and a certain

pressure level was kept

over a couple of hours

Mass extraction out of the reservoir

causes a pressure decline



Fig. 26: Well test and computerized model data: Water level (table) reflects the pressure response in the reservoir[6].

Once more well tests should finally deliver properties like permeability, porosity, storativity and

furthermore fluid properties. Testing of well is followed by monitoring during production. Other

kinds of models are used to interpret test data, in particular from longer tests. Models like

� lumped parameter models or

� numerical models

are used to predict reservoir behaviour (pressure, temperature) in the future. During the production

the pressure decreases in the reservoir. In case of an open system the pressure stabilizes after a

while. Production potential of a geothermal system is

primarily determined by pressure (water level) decline

due to production, but also by energy content

(temperature, enthalpy). There are two kinds of

reservoir natures as it is shown Fig. 27.

+ Closed system with limited or no recharge:

Production potential is limited by lack of water,

but not lack of energy.

+ Open system where recharge equilibrates with

the mass extraction: May be partially colder

water, which causes temperature to decline.

Potential is limited by energy content.

The pressure decline is controlled by size of system, permeability of rock, storage capacity and water

recharge generally. The models itself are not described in this essay but one example is shown how a

reservoir behaviour can be predicted by using such models (see Fig. 28 and Fig. 29).

Fig. 27: Different reservoir systems[6].



Predictions how the reservoir behave at a certain mass extraction is essential, because “large scale

mass extraction” can cause following effects on geothermal systems.

� Pressure decline within system

� Flow from hot springs (and wells) decline

� Discharge of steam vents often increase

� Increased recharge from outside effects cooling of reservoir

� Chemical changes (sometimes detrimental)

� Surface subsidence

� Changes in micro seismic activity

Fig. 28: Production and pressure response history of Ahuachapan in El Savador [6]

Fig. 28 shows the production response of the reservoir in Ahuachapan. The pressure stabilises after a

couple of years. The stabilisation was caused because of the reinjection as well good recharge to the

system. The fluctuation of mass extraction is caused by seasonal periods (winter, summer).

Fig. 29: Predicted pressure changes in the Ahuachapan reservoir for three future production scenarios. The points show

the real measurements in the reservoir. A model was created based on this production history. This model was used to

simulate the pressure response in the upcoming decades [6].



Again, geothermal modelling has two essential roles:

� To obtain information on the conditions in a geothermal system as well as on the nature and

properties of the system

� Predict the response of the reservoir to future production and estimate the production

potential of the system (resource assessment). Estimate the outcome of different

management actions (production rates).

Detailed numerical modelling is the most powerful modelling method, but it is time consuming and

costly. In many cases it is accurate enough to use simpler models. The models can also calculate the

temperature response of the future as it is shown in Fig. 30.

Temperature predictions are made by

geothermal modelling. Different values of

mass extraction cause different

temperature responses. Thus the

temperature response has usually same

behaviour as pressure. The production

history of pressure and other parameters

is obtained by monitoring. It is essential

for predictions and to describe the

characteristics of a reservoir. Reinjection

is often decided by detecting significant

pressure decline in a reservoir. It involves

returning some (even all) of the water

produced back into the geothermal system after energy extraction.

The main purposes of reinjection are listed below:

� Disposal of waste water from power plants and return-water from direct applications for

environmental reasons

� Additional recharge to supplement natural recharge, which often is limited (i.e. closed

system). Therefore it is essential for sustainable utilisation of geothermal systems that have

limited recharge

� Pressure support to reduce pressure decline due to mass extraction

� It enhances thermal extraction from

reservoir rocks

� It reduces or avoids surface subsidence

� Targeted reinjection can enhance or

revitalize touristic attractions like

surface features (hot springs and

fumaroles)

Fig. 31 shows water level prediction for the

Hofstadir geothermal system. It demonstrates

the beneficial effect of reinjection.

Fig. 30: Temperature predictions of a reservoir [6]

Fig. 31: Low temperature field in Iceland: The fluctuation is

caused by different seasons (winter, summer)[6]



4 Monitoring

Monitoring is also essential to avoid corrosion and scaling. Chemical constituents are dissolved in the

geothermal water of the reservoir. Pressure drawdown due to water up flow in the well causes

changes in the environment conditions and problems like scaling and corrosion are possible. In high

temperature fields become the fluid supersaturated in SiO2 after flashing. In order to avoid

precipitation in the surface equipment appropriate water treatment (pH-value,...), creating

deposition ponds (blue lagoon), scale inhibitors or dilution with steam condensate are essential. The

solubility of silica decreases with lower temperature. Therefore silica scaling can also be prevented

by cooling the water down in the power plant over a certain temperature where silica is still

dissolved in the fluid. This implies it cannot be extracted all the energy from the fluid because the

water has still around 140°C when it flows back to the reservoir by reinjection, but it compromises no

problems with scaling. Other scaling effects can be forced by carbonate dioxide. CO2 is stripping out

while the water flows up to the surface in the well. The pH of the water increases and calcite

deposition can be caused in the flashing zone of the well.

Equ. 4: Chemical reaction of calcite scaling in a well [1].

Carbonate precipitation can be curtailed by operating production/reinjection system at high

pressures or by scaling inhibitors where they are usually injected into production wells at certain

depth. Other constituents can also cause scaling, not only SiO2 and Calcite. Corrosion is also in many

cases a big problem. It can be controlled by inhibitors too. All these mentioned potential problems

can be detected by monitoring and consequences follow by operators of power plants. Fig. 32 and

Fig. 33 should explain the importance of monitoring.

Fig. 32: Corrosion of well head and damaged turbine blades due to corrosion effects [1].

Fig. 33: Calcite scaling [1].



5 Temperature gradients and temperature/pressure profiles in geothermal wells

Heat transfer within the earth is caused by:

� Conduction through solid material

� Convection, i. e. moving masses (water, magma)

� Thermal radiation, but it is very low unless at very

high temperatures

The relation between heat flow and temperature gradient in steady state flow for conduction is given

by:

Equ. 5: Heat flow by conduction [1]

whereby Q is the heat flow [W/m²], T is the temperature [°C], Z is the depth [m] and κ stands for

thermal conductivity [W/m°C]. The value of κ is usually between 1.5 and 4 in rocks. In Icelandic basalt

κ varies between 1.6 and 2 W/m°C. It is almost constant. Therefore temperature gradient can be

used in maps to express the heat flow. Temperature gradients in Iceland are shown in Fig. 34.

Fig. 34: Temperature gradients in Iceland. White areas are not explored yet. The risk to locate power plants in these

areas is too high [1].

Usually in continental areas the temperature gradients are around 30°C/km. In Iceland are higher

gradients present basically due to the hot spot volcanism. The background temperature gradient

decreases with the age of the rock. Therefore high gradients are available close to the rift axis and

low gradients are at the east and west coast of Iceland, where the older rocks are located. Compare



the age of the rock from Fig. 6 with the temperature gradient figured above Fig. 34. The temperature

gradient in Iceland varies between 40 and 150°C/km generally.

High temperature gradients are always the drilling target as well fractured systems. In many cases

promises a high gradient in shallow depth a geothermal system (fractures) below because fractures

transfer the heat of deeper to shallower parts by convection.

Fig. 35: The temperature logs from low temperature geothermal systems [3].

Basically there are two kinds of geothermal wells and this is important to understand pressure and

temperature distribution in wells:

Single phase wells Two-phase wells

Hot water (only liquid) Liquid phase inflow, boiling starts in well

Dry steam Two-phase inflow, boiling starts in reservoir

Table 3: Well types

The low temperature wells of Iceland usually contain liquid hot water and the high temperature wells

have mainly liquid phase inflow into the well and flashing of the fluid in the borehole afterwards.

Temperature logs measure temperature in wells inside the geothermal systems. Most of the work in

temperature logging is centered towards the

definition of a formation temperature. This is the

equilibrium temperature of geothermal water rock

systems. One example is shown in Fig. 37, this is a

well drilled in 1979. It is non productive. There are

only minor changes in repeated logs over 20 years of

logging. The formation temperature was evaluated

by drawing a line through the mean. Otherwise semi

analytical methods (Horner, Albright) are used to

estimate reservoir temperature.

Fig. 36: Temperature logs from well 4 in Hrisey/Iceland. High

vertical permeability in near vertical fractures causes vertical

convection [6]. Compare to Fig. 9.

depth

temperature

convection

conduction



Pressure conditions in geothermal wells will change in the vertical z direction according to Equ. 6.

Equ. 6: Pressure drop in geothermal wells is caused by friction, acceleration and elevation [6].

In a static well (no fluid flow) only the potential term will be nonzero. The pressure gradient in a

hydrostatic well is therefore shown in Equ. 7.

Equ. 7: Hydrostatic pressure in static well [6].

Fig. 38 shows different possible pressure behaviours from a high temperature well. The figure

represents the pressure in a well when

the well head valve is closed, and no

production goes on.

Artesian wells:

They flow when wellhead valve is

opened.

Non artesian-wells but gas/steam

pressure on the top:

They generally flow when the valve is

opened, due to boiling deep in the well.

Non artesian wells with no pressure on

head:

The water table is deep in the well. They

must be stimulated (or pumped) to flow.

Pressure communication between the

well and the reservoir is through feed

zones of the well. A feed zone

represents a direct flow path between

the reservoir and the well. This can be a

fracture that a well intersects. The

“best” feed zone of a well reflects the

reservoir pressure at the depth of the feed zone. If the location of the feed zone is known, a pressure

log will only determine the reservoir pressure at that one depth in the well. Therefore it is imperative

to know the location of the best feed zone. The “best” feed zone can be determined during drilling

operation. Pressure logs in a well, while it heats up after drilling (drilling mud cools well during

drilling operation), often show that the pressure is constant at a certain depth in the well. This is at

the depth of the best feed zone. Fig. 38 explains heating up process in the well and the estimation of

the so called pivot point afterwards.

Fig. 37: Pressure distribution in geothermal wells [6].



Fig. 38: Determination of the pressure pivot point due to pressure logs in well B-11. The most permeable feed zone is at

1400m depth and its pressure is about 117 bar [3].

Pressure is a property that is tied directly to the reservoir fluid. The reservoir pressure can have

variations in space and time. Pressure variations in space indicate fluid flow directions and/or

permeability variations. Pressure variations in time indicate time variations in the flow pattern of a

reservoir, changes in recharge or outflow (see chapter reservoir engineering). Forced pressure

variations are caused by production/injection. Pressure monitoring tells something about the

response to exploitation and is important for the field management.

After the determination of the reservoir pressure

and reservoir properties by well testing, discharge

measurements follow to gather some information

about the dynamic behavior of the reservoir as

consequence of production. As mentioned before, in

Icelandic fields is usually the water still liquid when it

enters the well. In two phase flow wells flashes the

water in a certain depth when it rises up to surface.

A mixture of liquid water and steam is the consequence at the well head. The mass extraction of the

Fig. 39: Discharge measurements of a two-phase well [6]

Pivot point:

Intersection



reservoir causes a pressure drop in the reservoir which describes Darcy’s law. The pressure drop in

the well is calculated by different models, for instance by Friedl or Chisholm and Laird model. How to

calculate pressure drops from reservoir up to the wellhead is well explained in [7].

The discharge measurements should confirm all unsure parameters taken by assumptions to

calculate the pressure drop in the well using a certain model. The flashing depth is obtained by

pressure drop calculations on the other hand. Important flow parameters are:

� Q = total mass flow [kg/s]

� T = temperature of the fluid discharged [°C]

� H = discharge enthalpy [J/kg]

� Non condensable gas content [%] and dissolved solids

� P0 = wellhead pressure [bar]

� Depth to the water table in pumped wells [m]

� The pressure drop from the reservoir into the well during discharge

The main purpose of discharge measurements is the “back pressure curve” or also called

“productivity curve”. Two phase flow characteristics are shown in Fig. 41.

Hot water well (low temperature fields)

characteristic is based on Equ. 8,

Equ. 8: Equation describes water level (pressure) versus

flow rate [6].

where Q is the flow rate, H0 is water level at zero flow, B*Q is linear draw down from reservoir to

well (Darcy’s law, Equ. 1) and C*Q² (calculated by Equ. 6) is the turbulent pressure drop at the inflow

point and in the well. Two-phase flow wells differ from hot water well curves. The shape of the curve

is more rounded because of flashing in the well. The boiling changes the properties of the fluid in the

Fig. 40: Two phase flow characteristics. Water fed wells and boiling in the well afterwards is usually the case in Iceland. The well flows after the valve was opened. In this manner exists a maximum discharge pressure. Two-phase fed well means the boiling of the water would already happen in reservoir [6].

Fig. 41: Hot water well characteristics [6].



well continuously. Other parameters, like the casing (borehole) diameter, influence the shape of the

curves generally. A bigger casing diameter allows more mass flow rate at same pressure level.

Furthermore pressure and temperature of two-phase flow wells are described in more details

because they are usually present in Iceland. The temperature of two-phase flow wells is lowered

along the saturation curve, Equ. 9.

Equ. 9: Equation for temperature decline after boiling of water in two phase flow wells

Fig. 10 and Fig. 42 show how the temperature behaves in volcanic geothermal systems. There is no

big difference compared to geothermal wells which are connected to reservoirs. After boiling starts

in a well the temperature lowers according to Equ. 9.

Fig. 42: Conceptual model of a volcanic geothermal system from White 1973: Curve 1 is the reference curve for the

boiling point of pure water. Curve 2 shows the temperature profile along a typical circulation route from recharge at

point A to discharge at point E [3].

Fig. 43 shows measurements of temperature and pressure during discharge. The boiling curve of

water is also sketched in the figure. This is also a typical two phase flow well characteristic, where the

water starts to flash in a certain depth in the well. The pressure slope changes after boiling, because

the density of the water column in the upper portion becomes lighter due to the bubbles in the

water. As mentioned before, the curve of the temperature also changes suddenly because of boiling.

The temperature and pressure curves always depend on the geothermal system. Each system has its

own characteristic, Fig. 43. The water flow in a vertical geothermal well is isenthalpic.

Thermodynamic derivation of this characteristic is available in [7].


Martin Knoglinger

Fig. 43: Temperature and pressure measured during discharge of well. Well KJ9, Krafla, Iceland

causes the sudden

Fig. 44: Temperature logs of different wells in Iceland. Each system has its own characteristics. The temperature logs

from Vestmannaeyjar and Akranes show the background gradient. There wasn’t drilled in a convective cell. The logs from

Reykjavik are from active geothermal systems. There the heat has been transferred by convection from deeper part to

the shallower part. Krafla and Nesj

liquid

mixture steam/water

flashing


: Temperature and pressure measured during discharge of well. Well KJ9, Krafla, Iceland

sudden change in slopes of temperature and pressure curve [6]

: Temperature logs of different wells in Iceland. Each system has its own characteristics. The temperature logs

eyjar and Akranes show the background gradient. There wasn’t drilled in a convective cell. The logs from


the shallower part. Krafla and Nesjavellir represent geothermal systems in an even more complex manner

temperature

pressure

boiling curve

temperature follows

boiling curve


Page 27

: Temperature and pressure measured during discharge of well. Well KJ9, Krafla, Iceland. Flashing in the well

[6].

: Temperature logs of different wells in Iceland. Each system has its own characteristics. The temperature logs

eyjar and Akranes show the background gradient. There wasn’t drilled in a convective cell. The logs from


in an even more complex manner [3].

boiling curve



6 Thermodynamic processes to use geothermal steam for electricity production

After the well was tested all properties are available of the reservoir and the well itself. The power

plant can be connected to the well, but there is one additional step to do before. At the well head is

a mixture of water and steam with certain pressure level available. The well head pressure is

measured but can be determined by calculations as well what is shown in [7].

Fig. 45: Example how to calculate the pressure on the well head using Equ. 1 and Equ. 6 taken from [7]. The reservoir

pressure is given by cold water column and depth of reservoir.

Separators separate the mixture of water and steam in their single phases. Afterwards the steam is

going to the turbines to drive them. A typical process flow sheet is shown in Fig. 46. If electricity

production is combined with district heating, the efficiency of the global process will be higher.

Fig. 46: Possible flow sheet of geothermal power plant [8].



In Iceland are mainly used Single-flash power plants as well Double-flash power plants. These are the

most common conventional types for electricity production of geothermal heat on the world. Single

flash and Double flash technology explanations were taken from [7] for this essay.

Single flash technology

Fig. 47: Flow sheet of single flash power plant [7].

The single flash technology is usually used when the water has temperatures over 160°C at the well

head. The terminology single-flash system indicates that the geofluid has undergone a single flashing

process, i. E., a process of transitioning from a pressurized liquid to a mixture of liquid and vapour, as

a result of lowering the geofluid pressure below the saturation pressure corresponding to the fluid

temperature. The flash process may occur in the production well anywhere from entry point to the

wellhead as a result of the loss of pressure due to friction and gravity head. It is often the case in a

newly developed field that the flashing occurs initially, but with time as the field undergoes

exploitation and the reservoir pressure declines, the flash point may move down the well and even

enter the formation. Sometimes the term “separated steam” is used for this type of plant owing th

the manner in which the steam is obtained for use in turbine. While the actual location of the flash

point can be important in the operation of a power plant, from the point of view of understanding

the thermodynamics of the energy conversion process, it is irrelevant. In Fig. 47 are shown the main

components of a single flash plant. At each production well, PW, there is a assemblage of equipment

to control and monitor the flow of the geofluid from the well to the plant. This equipment includes

several valves, WV, a silencer, S, (simple cyclone separator or huge tank using the gravity effect),

piping and instrumentation (pressure and temperature gauges). It is important to separate the two

phases efficiently prior to the steam being admitted to the turbine. Liquid entrained in the steam can

cause scaling and/or erosion of piping and turbine components. Where the separators are situated at

a distance from the powerhouse, the steam transmission pipelines are fitted with traps to capture

and remove any fine moisture droplets that may have formed in the transmission pipelines and

escaped the steam traps. The moisture remover, MR, is usually located directly outside the

powerhouse. The turbines used in geothermal application must be made of corrosion resistant

materials owing to the presence of gases such as hydrogen sulphide that can attack ordinary steels.

The steam enters the turbines essentially as a saturated vapour at a moderate to low pressure (5 to



10 bars) which is markedly different from the steam conditions normally found at a coal, oil or gas

fired power plant where highly superheated steam is used. As a result, significant amounts of

moisture appear in the steam path of geothermal turbines, particularly in the lowest pressure stages.

The relatively slow droplets strike the back of the leading edge of the blades causing erosion unless

this is reinforced. These critical areas must be protected from damage by cobalt-rich alloy strips.

Turbines for single flash units are typically rated at 25-55 MW and consist of 4-5 stages of impulse

reaction blades. The steam from the turbine is condensed by means of either a surface-type

condenser, C, as shown in Fig. 47 or in a direct contact condenser of either the barometric or low

level type. Most plants now employ surface condensers in which the geothermal steam passes

through the shell side and cooling water passes through the tube side. The maintains physical and

chemical separation between the geothermal steam and the cooling water, and allows more

effective removal and treatment of noncondensable gases. Gases, such as carbon dioxide and

hydrogen sulphide exist with the natural steam and do not condense at the temperatures in the

condenser. Therefore unless they are removed they will increase the overall pressure in the

condenser and lower the turbine power output. Steam jet ejectors with aftercondensers, SE/C,

and/or vacuum pumps are used for this purpose. The cooling water is usually obtained from a cooling

tower that recirculates a portion of the condensed steam after it has been cooled by partial

evaporation in the presence of a moving air stream (items CT and CWP in Fig. 47. This means that

geothermal power plants do not need a significant supply of cooling water, a major advantage in

areas that are arid. A small amount of fresh water is needed, however, to provide for replacement of

tower blow down. Lastly, injection wells for excess condensate and cooling tower blowdown are

used, Fig. 47. Fig. 48 shows the temperature versus entropy diagram of a single flash process.

Fig. 48: Temperatuer-entropy state diagram for single flash plants [7].

The sequence of process begins with geofluid under pressure at state 1, reflects reservoir condition,

close to the saturation curve. The flashing process is modelled as one at constant enthalpy, because

it occurs steadily, spontaneously, essentially, adiabatically and with no work involvement. Kinetic and

potential energy are neglected. The separation process is modelled at one constant pressure, an

isobaric process. The steam mass fraction is given by intersection of pressure level at separation



process and the isenthalpic line due to flashing. Power is produced from 4 to 5 by turbines. All

auxiliary power requirements for the plant (pumping power, fan power, station lighting, ...) must be

subtracted from this to obtain the net power. The expansion process is influenced by the isentropic

efficiency. It must be recognised that the isentropic efficiency of a turbine is affected by the amount

of moisture that is present during the expansion process. This effect should be quantified, for

instance by the so called Baumann rule. Condensing the wet steam is usually done by direct-contact

or surface-type condensers. Cooling towers for geothermal power plants are much larger in cooling

capacity than for conventional fossil or nuclear power plants of the same power rating. One often

sees a 5-cell cooling tower used for 30 MW geothermal flash plants, whereas that same tower could

easily accommodate a 250 MW state of the art combined cycle plant.

Double flash technology

The double-flash steam plant is an improvement on the single flash design in that it can produce 15-

25% more power output for the same geothermal fluid conditions. The plant is more complex, more

costly and requires more maintenance, but the extra power output often justifies the installation of

such plants. The fundamental new feature is a second flash process imposed on the separated liquid

leaving the primary separator in order to generate additional steam, albeit at a lower pressure than

the primary steam.

Fig. 49: Simplified double-flash power plant schematic [7].

The schematic diagram for a double-flash plant is shown in Fig. 49. The design differs from the single-

flash plant in Fig. 47 in that a flasher F has been added and there is a low-pressure steam line from it

to the turbine in addition to the high pressure one from the separator. The cooling tower that

provides the cooling water CW is not shown in Fig. 49. The turbine shown is a dual admission, single

flow machine, where the low pressure steam is admitted to the steam path at an appropriate stage

so as to merge smoothly with the partially expanded high pressure steam. Other designs are

possible, for example, two separate turbines could be used, one for high pressure steam and one for

the low pressure steam. The processes for the double flash plant are shown in Fig. 49, a

temperature- entropy diagram.



Fig. 50: Temperature versus entropy process diagram for double-flash plant with a dual admission turbine [7].

Each process generates a fractional amount of steam given by the quality of the two-phase mixture.

Each flash is followed by a separation process. The optimisation process for a double-flash plant is

more complicated than for a single flash plant because of the extra degree of freedom in choice of

operating parameters.

Further energy conversion technologies, dry steam power plants and binary power plants, are not

discussed in this assay. Dry steam power plants are not present in Iceland yet, because of lower

energy content of reservoirs. Binary systems are usually used for hot water wells (>100°) with low

energy content. In the Svartsengi plant in Iceland binary systems produce electricity within a complex

process system. There is the backpressure steam of a flash turbine used as heat source instead of hot

water. Following table, Table 4, compares the different technologies regarding to different

parameters.

:

Table 4: Comparison of basic geothermal energy conversion systems taken from [7].

Finally the environmental impacts of geothermal power plants are discussed in following chapter.



7 Environmental impact of geothermal power plants

Environmental concerns are:

� Surface disturbances

� Physical effects – fluid withdrawal

� Noise

� Thermal pollution

� Protection

� Social and economic effects

Surface disturbance includes excavation, construction, roads, landslides, scenery, and

changes in surface activity.

Fig. 51: Hellisheidi, biggest power plant in Iceland. Surface disturbance is due to pipes. Thermal expansions of pipes cause

tensions. Therefore the pipe constructions are never only straight [1].

Fluid withdrawal can cause subsidence, lowering of groundwater table, gravity changes and

seismicity. Subsidence of the surface can be a great problem. Wairakei power plant in New

Zealand caused subsidence of almost half meter a year because of production from the

geothermal field.

Area Total subsidence [mm] Time [years] Rate [mm/year]

Wairakei 7600 19 400

Svartsengi 130 12 11

Larderello 1850 63 29

Table 5: Subsidence in different areas [1].



Subsidence can be lowered as well avoided by reinjection after the geothermal fluid has

undergone the process in power plant. Further physical effects because of fluid withdrawal

are seismicity and lowering of ground water table.

Fig. 52: Geothermal sounds compared to familiar sounds [1].

Fig. 52 compares geothermal sounds caused by drilling, well testing and power plant operation with

familiar sounds of normal life.

Gravity change is also a common side effect of geothermal exploitation. Surveys are carried out

before and during the utilisation (monitoring). Before utilisation because to identify the pristine

state of the field and during utilisation to observe the environmental effects of the utilisation.

Fig. 53: Negative gravity changes due to production from Svartsengi geothermal field. The recharge of the field is 60% [1].

Air pollution is one of the consequences of geothermal heat utilisation. Major offenders are gases

like CO2 and H2S. Power production causes 1.9 times less CO2 gas emissions than from fossil fuels [1].

Carbon dioxide emissions can be considered as a transfer but not formation of the gas. Hydrogen

sulphide is lethal in large quantities, but the quantity is usually small so that unpleasant smell is the

main worry. Complaints and regulations regarding abatement related to pre-production activity. For



instance sulphur can be oxidised to SO2, what still cause less emissions compared to power

production from fossil fuels. H2S abatement is also possible by reinjection of geothermal water.

Treatment by Claus Selectox process or bacteria is a further possibility too.

8 Conclusion

In 2005 was the installed capacity by geothermal power plants 9064MWel on the world [3]. The

installed power in Iceland is 574 MWel [3]. This is around 20% of electricity production from the

country and 80% is produced by hydro power [2]. The government plans to increase the capacity of

geothermal plants as well hydro power plants in the upcoming future [9]. The locations of existing

and planned power plants in Iceland are shown in Fig. 6. On the other hand geothermal heat is used

to heat buildings, heat swimming pools, dry fishes as well melt parking areas in Iceland. Geothermal

can be seen as renewable source [3], and therefore it should be exploited. It could help to decrease

the greenhouse gas effect because it causes less emission than fossil fuels.

Fig. 54: From fossil fuel to geothermal: the environmental benefit. The left pictures show the capital city Reykjavik in

1933, before geothermal space heating, and in 2008 from almost same view [1].



List of figures

Fig. 1: Temperature distribution within the earth: ................................................................................. 3

Fig. 2: The mantle plume of the hot spot is responsible for rising ductile material from the mantle

and also magma coming from beneath ridge. Geologists assume a plume diameter of around 100-

150km in the upper portion below the earth’s surface. The specific location on the ridge as well on

the hot spot formed Iceland 20 Mio years ago. ...................................................................................... 3

Fig. 3: Plate boundaries and location of Iceland within the plate system .............................................. 4

Fig. 4: Worldwide technical potential of renewable energy sources ...................................................... 5

Fig. 5: The crustal model of Pálmason: A rock piece solidifies on the surface, slowly sinks down

because of the weight of successive lavas. At the same time, the piece is carried out of the volcanic

zone. When the piece is dug down, the pores close because of outer pressure and alteration and

precipitation of secondary minerals takes place..................................................................................... 5

Fig. 6: Geothermal fields in Iceland: The marked places are the installed Power Plants for electricity

production in 2009. The latest produced rocks are along the ridge and the oldest rocks are at the east

and west coast of the country. ................................................................................................................ 6

Fig. 7: Guðmundsson 1990: Structure of volcanic system. Magma chambers are formed due very high

density of dykes which band together. Magma reservoirs are located below. They are the origin

magma source. During volcanic eruptions the magmatic material from magma chambers flows up to

earth’s surface. ........................................................................................................................................ 6

Fig. 8: The mantle plume is responsible for the hot spot “Iceland”. It fills up magma chambers located

above. ...................................................................................................................................................... 6

Fig. 9: Low temperature fields: In convective systems the heat source is the hot crust at depth in

tectonically active areas, with above average heat flow. Here the geothermal water has circulated to

considerably depth (>1km), through mostly vertical fractures, to mine the heat from rocks by

convection. .............................................................................................................................................. 7

Fig. 10: Conceptual model of high temperature geothermal system: Volcanic systems are in one way

or another associated with volcanic activity. The heat sources for such systems are hot intrusions or

magma. They are most often situated inside, or close to, volcanic complexes such as calderas and/or

spreading centres. Permeable fractures and fault zones mostly control the flow of water in volcanic

systems .................................................................................................................................................... 7

Fig. 11: Principal sketch of the resistivity structure in Iceland ................................................................ 8

Fig. 12: Ions and electrons are responsible for electrical current in pores of rocks ............................... 8

Fig. 13: DC-method to determine the resistivity of different layers below the earth’s surface ............. 9

Fig. 14: Example of 2-dimensional resistivity profile of Reykjanes peninsula (high temperature field).

Proper interpretation of such profiles by geologists is essential for exploring .................................... 10

Fig. 15: 3-dimensional resistivity model: IDDP-Icelandic deep drilling project. One of the boreholes

was drilled in Krafla. .............................................................................................................................. 10



Fig. 16: Depth of distribution of earth quakes as a ratio of maximum numbers of quakes. The other

curve shows the cumulative numbers of earthquakes with depth as ratio of total number of quakes.

Depth numbers are in km below sea level ............................................................................................ 11

Fig. 17: Shows, how to calculate the stored energy in the system ....................................................... 12

Fig. 18: Equations to calculate the electrical power from a reservoir by volumetric method.............. 13

Fig. 19 Drilling operation in Iceland ....................................................................................................... 13

Fig. 21: Storage is the ability of a reservoir to store and release fluid in response to changes in

reservoir pressure. ................................................................................................................................ 14

Fig. 20: Well test: Two-phase flow discharge measurements ............................................................... 14

Fig. 22: Link between pressure drop and mass production .................................................................. 15

Fig. 23: Different available models ........................................................................................................ 15

Fig. 24: Example of pressure data from a step-rate injection test ........................................................ 16

Fig. 25: Model which fits the well test data as close as possible reflects the real reservoir properties

as good as possible ................................................................................................................................ 16

Fig. 26: Well test and computerized model data: Water level (table) of vertical axis reflects the

pressure in the reservoir. ...................................................................................................................... 17

Fig. 27: Different reservoir systems ...................................................................................................... 17

Fig. 28: Production and pressure response history of Ahuachapan in El Savador-pressure stabilization

............................................................................................................................................................... 18

Fig. 29: Predicted pressure changes in the Ahuachapan reservoir for three future production

scenarios. The points show the real measurements in the reservoir. A model was created due to this

production history. This model was used to simulate the pressure response in the upcoming decades

............................................................................................................................................................... 18

Fig. 30: Temperature predictions of a reservoir.................................................................................... 19

Fig. 31: Low temperature field in West Iceland: The fluctuation is caused by different seasons (winter,

summer) ................................................................................................................................................ 19

Fig. 32: Corrosion of well head and damaged turbine blades due to corrosion effects ....................... 20

Fig. 33: Calcite scaling ............................................................................................................................ 20

Fig. 34: Temperature gradients in Iceland. White areas are not really explored yet. The risk to locate

power plants in these areas is too high................................................................................................. 21

Fig. 36: The temperature logs from low temperature geothermal systems ......................................... 22

Fig. 37: Temperature logs from well 4 in Hrisey/Iceland. High vertical permeability in near vertical

fractures causes vertical convection. Compare to Fig. 9. ...................................................................... 22

Fig. 38: Pressure distribution in geothermal wells ................................................................................ 23

Fig. 39: Determination of the pressure pivot point due to pressure logs in well B-11. The most

permeable feed zone is at 1400m depth and its pressure is about 117 bar ......................................... 24



Fig. 40: Discharge measurements of a two-phase well ......................................................................... 24

Fig. 41: Two phase flow characteristics. Water fed wells and boiling in the well afterwards is usually

the case in Iceland. The well flows after the valve was opened. In this manner exists a maximum

discharge pressure. Two-phase fed well means the boiling of the water would already happen in

reservoir ................................................................................................................................................ 25

Fig. 42: Hot water well characteristics. ................................................................................................. 25

Fig. 43: Conceptual model of a volcanic geothermal system from White 1973: Curve 1 is the reference

curve for the boiling point of pure water. Curve 2 shows the temperature profile along a typical

circulation route from recharge at point A to discharge at point E ...................................................... 26

Fig. 44: Temperature and pressure measured during discharge of well. Well KJ9, Krafla, Iceland.

Flashing in the well causes the change in slopes of temperature and pressure curve ......................... 27

Fig. 45: Temperature logs of different wells in Iceland. Each system has its own characteristics. The

temperature logs from Vestmannaeyjar and Akranes show the background gradient. There wasn’t

drilled in a convective cell. The logs from Reykjavik are from active geothermal systems. There the

heat has been transferred by convection from deeper part to the shallower part. Krafla and

Nesjavellir represent geothermal systems in an even more complex manner. ................................... 27

Fig. 46: Example how to calculate the pressure on the well head using Equ. 1 and Equ. 6 taken from

(1). The reservoir pressure is given by cold water column and depth of reservoir. ............................. 28

Fig. 47: Possible flow sheet of geothermal power plant ....................................................................... 28

Fig. 48: Flow sheet of single flash power plant ..................................................................................... 29

Fig. 49: Temperatuer-entropy state diagram for single flash plants..................................................... 30

Fig. 50: Simplified double-flash power plant schematic ....................................................................... 31

Fig. 51: Temperature versus entropy process diagram for double-flash plant with a dual admission

turbine ................................................................................................................................................... 32

Fig. 52: Hellisheidi, biggest power plant in Iceland. Surface disturbance is due to pipes. Thermal

expansions of pipes cause tensions. Therefore the pipe constructions are never only straight. ......... 33

Fig. 53: Geothermal sounds compared to familiar sounds ................................................................... 34

Fig. 54: Negative gravity changes due to production from Svartsengi geothermal field. The recharge

of the field is 60%. ................................................................................................................................. 34

Fig. 55: From fossil fuel to geothermal: the environmental benefit. The left pictures show the capital

city Reykjavik in 1933, before geothermal space heating, and in 2008 from almost same view. ........ 35



Reference

[1] Stefán Arnórsson, Halldór Ármannsson, Ólafur G. Flóvenz. JAR218 - Geothermal Resources -

lecture slides. University of Iceland, Iceland : s.n., 2010.

[2] Halldór Kjartansson, Ari Trausti Guðmundsson. Lebende Erde - Facetten der Geologie Islands.

Reykjavik : Mál og Menning, 2007. ISBN: 9979-3-2778-3.

[3] Guðni Axelsson, Halldór Ármannsson, Ólafur G. Flóvenz, Sverrir Þórhallsson, Halldór Pálsson.

VÉL203F-Geothermal Power Development - lecture slides. University of Iceland, Iceland : s.n., 2010.

[4] Icelandic Geosurvey ISOR, Gudni Axelsson. Production capacity of geothermal systems. Reykjavik,

Iceland : s.n., 2008.

[5] Georgsson, Lúdvík S. Geophysical methods used in geothermal exploration. Reykjavik, Iceland :

s.n., 2009.

[6] Grímur Björnsson, Guðni Axelsson. REY201F - Well logging - lecture slides. University of Iceland,

Iceland : s.n., 2010.

[7] DiPippo, Ronald. Geothermal Power Plants-Principles, Applications, Case Studies and

Environmental Impact. s.l. : BH-Second Edition, 2007.

[8] Magnús Þór Jónsson, Halldór Pálsson, Ragnheiður Inga Þórarinsdóttir. REY203F-Measurements

and System Analysis - lecture slides. University of Iceland, Iceland : s.n., 2010.

[9] Geosurvey, ISOR - Icelandic. www.isor.is. http://www.geothermal.is/publications/publications.

[Online] 2010.

[10] Association, International Geothermal. http://www.geothermal-energy.org/. [Online] 2010.

[11] Skuli Johannsson, Hakon Skulason, Prof. Emeritus Valdimar K. Jónsson. Personal conversations

about geothermal utilisation in Iceland. www.kaldara.com. 2010.

[12] Lúdvík S. Georgsson, Ragna Karlsdóttir. Resistivity methods - DC and TEM with examples and

comparison from the Reykjanes peninsula and Öxarfjördur, Iceland. Reykjavik, Iceland : s.n., 2009.

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