Geothermal energy utilization by revitalization of

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UNIVERSITY OF MISKOLC

Faculty of Earth Science and Engineering

Institute of Environmental Management

Geothermal energy utilization by revitalization of abandoned

oil wells in the foreground of a mountainous region

Thesis

Author:

Bretán Dávid

Degree program:

Hydrogeological engineering MSc

Department supervisors:

Dr. Szűcs Péter

Professor

Miklós Rita

Assistant research fellow

Date of submission: 2020.07.01



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Table of contents

INTRODUCTION

1. Introduction, thesis work scope................................................................................................. 1

LITERATURE REVIEW

2. Geothermal energy and utilization ............................................................................................ 2

3. Direct use of thermal energy ..................................................................................................... 4

3.1. Heat pump systems ............................................................................................................ 5

3.2. Heat collection systems in boreholes ................................................................................. 8

4. Review of deep geothermal heat exchanger systems .............................................................. 10

4.1. National application practices .......................................................................................... 10

4.1.1. Application of deep heat exchangers ........................................................................ 10

4.1.2. Previous studies on depleted hydrocarbon wells ....................................................... 12

4.2. Viability of Enhanced Geothermal System (EGS), Deep Borehole Heat Exchanger

(DBHE), Deep-Well Circulation System (DWCS) ................................................................. 14

4.3. Configurations of Ground Coupled Heat Pump (GCHP) ................................................. 16

4.4. Geothermal well systems ................................................................................................. 18

4.4.1. Deep Geothermal Doublet System (DGDS) ............................................................. 18

4.4.2. Deep Single Well Circulation System (DSWC) ....................................................... 20

5. Characteristics of a (karstic) aquifer system ........................................................................... 22

5.1. Physical properties of the aquifer ..................................................................................... 22

5.2. Thermal properties of an aquifer ...................................................................................... 27

SAMPLE AREA INFORMATION

6. Description of the research area .............................................................................................. 28

6.1. Location of the sample area ............................................................................................. 28

6.2. Geological and thermal properties ................................................................................... 29

6.3. Subsurface modeling of the target area ............................................................................ 33

6.4. Technical parameters of the potential wells ..................................................................... 38

6.4.1. Technical parameters of well W-23 .......................................................................... 40





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METHODOLOGY OF SUBSURFACE MODELING

7. Groundwater modeling of the research area ........................................................................... 46

7.1. Basic concept of modeling ............................................................................................... 46

7.2. GMS model buildup and parameters ................................................................................ 47

7.2.1. Boundary conditions ................................................................................................. 48

7.2.2. Lithological units and hydrogeological parameters .................................................. 49

7.2.3. Groundwater exploitation .......................................................................................... 50

7.2.4. Thermal characteristics ............................................................................................. 51

7.2.5. Observation wells arrangements ............................................................................... 54

RESULTS OF SUBSURFACE MODELING

8. Results of transport modeling ................................................................................................. 55

8.1. Groundwater modeling..................................................................................................... 55

8.2. Heat transport modeling ................................................................................................... 58

FINAL CONCLUSIONS

9. Conclusion .............................................................................................................................. 62

10. Future plans ........................................................................................................................... 64

11. Summary ............................................................................................................................... 65

Acknowledgement ...................................................................................................................... 67

References ................................................................................................................................... 68

Internet sources ........................................................................................................................... 78

List of figures .............................................................................................................................. 78

Annex .......................................................................................................................................... 81



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1. Introduction, thesis work scope

Hungary is a country in an inactive volcanic area, but with a favorable

geothermal potential recognized both at European and international level. There are

many types of extraction and utilization of geothermal energy, of which a large group is

energy recovery. However the development of the sector is slow due to high initial

investment demand and the long planning phase. The latter one could be shortened by

some standardization methods however the initial costs, especially for the drilling phase

still remain high. These prices could be cut in half by sparing the drilling operation. One

of the aims of a research module within the project called PULSE, named GINOP-

2.3.2-15-2016-00010 „Development of enhanced engineering methods with the aim at

utilization of subterranean energy resources‟ project of the Research Institute of Applied

Earth Sciences of the University of Miskolc in the framework of the Széchenyi 2020

Plan is to take steps to remedy this problem by examining the function change of

abandoned former oil production wells in an area for geothermal heat recovery in regard

of their recent status. The examined sample area is located in the foreland of a

mountainous region where numerous unused hydrocarbon wells can be found. The

exact location of the area is not cited, for reasons of better implementation solving

future problems. The project is carried out by the University of Miskolc - its main goal

is to increase the efficiency of non-conventional energy recovery by engineering means.

The thesis work has been made in this aspect, trying to find the most adequate way to

take advantage of the area‟s potential geothermal sources and assess its feasibility.

Based on the geological information and modeled temperature dataset calculations have

been carried out to explore the possibilities of a potential geothermal well-triplet (one

production,-two injection) system. In the first part of the thesis the assessment of

geological and hydrogeological conditions of the area will be presented, which outputs

are used to improve the accuracy of subsequent modeling. According to the subsurface

information the area‟s hydrogeological and thermal properties were assessed and

assumed, also for a more precise execution of the groundwater and heat transport

modeling. In regard of the known and gathered information of the current wells, a

proposition is made for a modification on the existing ones by visualization layouts. In

the second part of the present work the long-term feasibility of groundwater exploitation

and heat transfer phenomena will be discussed through groundwater flow modeling tool

within several operation time periods.



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2. Geothermal energy and utilization

The Earth‟s internal heat, refers to geothermal energy comes from a

combination of nuclear reactions, radioactive decays of U, Th, K (80%) in the rock

bodies and from the high temperature magma at great depths (20%) as a residue of

planetary accretion (Arndt, 2011). The Earth's internal energy is directed toward the

Earth's surface by the propagation of heat (outward heat flow), of which there are three

known types: conduction, heat flow (convection), and temperature difference due to

heat radiation. During heat transfer energy is propagated by attaching vibrations of

matter particles. The asthenosphere is characterized by so-called thermal convection,

which means the displacement of heat energy, carried by a flow of solid, liquid or

gaseous materials. The heat radiation occurs through the emission and absorption of

electromagnetic energy. (Barbier, 1997, Szőnyi, 2006).

In solid rocks, the energy propagates in the form of conduction, which basics

described by Fourier‟s equations. With his experiments he proved that if a column with

a given height has the temperatures of T2 on its lower and T1 on its upper section, and

T2>T1, then the amount of heat on its surface (F) under unit of time (t) (Völgyesi, 2002):

(1)

where

: the thermal conductivity of the material

Fourier-equation quantifies the heat flow density for a unit surface area in a differential

form, that a unit amount of heat (Q) flowing through under a unit of time (t) is

proportional to the heat gradient (T) and thermal conductivity ( )(Völgyesi, 2002):

(2)

This heat energy is expressed by the earth's heat flux, which is a measure of the amount

of heat flowing through a unit of land over a period of time (Völgyesi, 2002).

Geothermal energy could be found everywhere on the planet in different depths,

however not classified as a renewable energy source. The reason is its abundance, and

as a term of renewability, only if the rate between extraction and natural transfer is

equal can be considered as one (Hochstein, 1990). Its flux has uneven distribution on



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the surface which is a value average of 65mW/m2 on continents, and 101mW/m

2 in

oceans. Based on tens of thousands of measurements, the global average is 87mW/m2

according to (Pollack et al. 1993). Geothermal gradient is used as a measure of

temperature increase by the depth beneath the Earth‟s surface. Its value varies between

10 and 60 °C/km depending on the heat production at a certain depth, the dynamics of a

system and the conductivity of the rock bodies (Arndt, 2011). The basic equation for the

calculation of geothermal gradient explained as (Forrest et al., 2007)

(3)

Therefore at the depth of 2000 m, calculated with a 65-75 °C formation temperature and

an estimated average surface temperature of 15 °C a gradient of 25-30°C/km could be

expected (Szőnyi, 2008).* There are special cases when high temperature systems occur

at relatively shallow depths in a form of local thermal anomalies. Usual occurrence (as

in the case of Hungary) is where the earth crust is thinner than the average, but in other

cases it can be formed by the decomposition of radioactive elements enriched granite

bodies surrounded by thermal insulator rocks (Forrest et al., 2007).

Geothermal system is a geological setting with the properties of a heat source,

migration pathway and the heat storage in a form of a permeable rock and/or high

temperature fluid. The volume of rocks from which heat can be gathered is called

geothermal reservoir. In terms of its physical state the fluid stored in the reservoir may

be water, steam or a mixture of these (Hochstein, 1990). In some reservoirs, water is

replenished by precipitation. The hot water and steam can appear as a natural surface

manifestation in a form of springs or geysers, but most commonly wells needed for

recovery. Today wells can be drilled to more than 3 meters depth to tap the geothermal

reservoirs and transport energy due to the water to the surface for utilization (Kiruja,

2011). For the elements of a geothermal system, only the heat source must be natural,

and the other two elements of the system can even be artificial (transport medium, crack

system). In artificially influenced geothermal systems, the source of heat is natural, but

the reservoir, carrier fluid or both are man-made. Geothermal systems are characterized

for their enthalpy which primarily means the temperature of the stored liquid and

geologic strata. According to enthalpy low (<100 °C) medium (100-150 °C) and high



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(>150 °C) systems can be distinguished (Muffler et al Cataldi 1978). So its usability

from one side is temperature, but geology dependent on the other.

Geothermal energy or heat can be harvested in different ways depending on the

geological conditions of the area. For heat recovery – except for heat pumps – it is

inevitable to extract water or steam from underground. The ways of utilization can

happen by heat pumps, in a form of direct heat supply or cogeneration of electricity and

heat (Szőnyi, 2008). Sufficient efficiency for power generation with current technology

requires water of at least 120 °C. The range of these temperatures can be found at a

depth of 2500-3000 m, in limited reservoirs in Hungary. Therefore most of the thermal

energy used efficiently in large quantities is utilized directly for heat supply (residential

heating, domestic hot water production, greenhouse heating, crop drying, etc.) because

low enthalpy waters can be recovered at more than 70% of the country‟s territory except

the mountainous regions (Horváth, 2011; Liebe, 2001). The manner which will be

detailed in this work is the direct use of thermal energy which refers for any other

application apart from electricity generation.

Innovative technologies are now offering new perspectives based on the fact

that if only geothermal fluid is extracted then some heat supplies remain idle (Szőnyi,

2008). These relatively new techniques based on the so far untouched part of ground

heat called Heat Pumps (Rybach, 1981). So far ground-coupled heat pumps systems in

Hungary implemented in shallow depth (<300m), therefore low enthalpy medias,

whereas deep geothermal sources starting at a depth of 300-500 m not yet commercial.

In the last decade, the direct use of ground source heat pumps had the most significant

spread in thermal energy utilization worldwide. They are also one of the fastest growing

sources of renewable energy represent their category (Rybach, 2005).

3. Direct use of thermal energy

Geothermal heat pumps (GHPs) are present since the late 1940s and they have

come a long way to this day. Heat pumps are devices utilizing ambient heat, based on

the principle of a refrigerator accordingly to the 1st and 2

nd law of thermodynamics. The

process considered as a heat transfer between a source and a sink accordingly to the

Carnot-process. The harnessed heat serves the heating needs in winter, cooling in

summer, air conditioning in domestic use and also suitable for hot water production



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depending on the media‟s temperature. By reversing the flow direction, we get the so-

called ground-charging function of a heat pump which provides a heat storage for

further use, depending mainly on the natural and artificial reservoir properties

(Grassley, 2010).

3.1. Heat pump systems

The main components of a heat pump system are the heat source, the extraction

(collection) system and the heat pump itself connected to the system which utilizes heat

further. The extraction of heat could happen in open or in closed pipe circumstances.

The enclosed-loop systems can be arranged in a horizontal, vertical and pond/lake way

(Fig. 1). Meanwhile open-loop arrangement uses a well or surface water body source as

the heat exchange media circulating in the heat pump system, therefore it is both highly

quality and quantity dependent. Quality from the side of total dissolved substances

(TDS) and quantity from the resource distribution itself. Choosing the most suitable one

depends on the climate, subsurface conditions, available space and costs. Each

approaches can serves both residential and commercial needs (Int. 1).

Figure 1 Different heat pumps setups in case of a shallow heat source [1]

Further due, considering the initial target geology of the current work will be dealing

with closed pipe networks. In this case the pipes – usually made of PVC - are running

into the ground with a secondary (commonly lower boiling point) liquid circulating in

them. The material of the pipe is heat and depth dependent, at greater depths pipe made



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of steel are used. The heat exchanger transfers heat between the refrigerant of the heat

pump and the liquid circulating in the closed loop. A closed-loop system can operate

through a direct exchange where a heat exchanger is not included whilst instead pumps

the refrigerant through the buried tubing. The operation principal of a heat pump system

is detailed below (Fig. 2) (Int. 1).

Figure 2 Operational flow chart of a heat pump [2]

The circuit absorbs the heat within which leads to the evaporation of the refrigerant.

(The temperature if it is necessary can be set by pressure valves to match the heat

sources.) Then the power intake happens, when an (electric) compressor compresses the

vapour which results in a superheated vapour. The vapour condensed back into liquid

form while releases its heat to the “sink”. The cooled liquid‟s temperature decreased

below zero degree through the expansion valve. In the last step turns back into vapour

in the evaporation chamber then the process starts from the beginning (Aittomäki et al.,

2008).

The operation itself not directly transforms the gathered heat into a useable

form. Thus an external energy, raise the heat from a lower temperature level to a higher

one. The required external energy input called electricity, which in this aspect means

work. Regular heat pump systems usually operate with lower temperatures as the source

of heat situated at a shallower depth. Therefore the so-called “lift” is higher than in case

of a deeper relatively higher temperature system. The coefficient of performance (COP),

describes the heat pump‟s efficiency which means the ratio of heat output to effective

power intake based on the magnitude of the lift.



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As mentioned earlier the energy conservation law (1st law of thermodynamics)

in case of a heat pump could be described as (Laitinen et al., 2014):

Qcond = Qcomp + Qevap (4)

where

Qcond: condensing heat

Qcomp: power of compression

Qevap: evaporation heat

The COP which gives the efficiency of a heat pump expressed as (Laitinen et al., 2014):

(5)

where

Qpump: electricity consumption of the pump

The temperature-entropy diagram, or T-S diagram used in thermodynamics to visualize

heat transfer phenomena or cycle. It well describes the basic working principle of a heat

pump mentioned above (Fig. 3).

Figure 3 T-S diagram of a heat pump [3]

According to the 2nd

law of thermodynamics and the T-S cycle diagram (temperature-

entropy) the specific COP can be evaluated as (Borgnakke et al., 2009):

(6)



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where

Tcond: condensing temperature [K]

Tevap: evaporation temperature [K]

The previous function describes an ideal condition but in reality there are occurring

irregularities e.g.: internal heat transfer losses, pressure drops etc. In practice the

following one is used (Borgnakke et al., 2009):

COPHP CAm

(7)

where

CA: Carnot non ideality factor (0,45-0,55)

m: mechanical efficiency of the compressor (0,90-0,95)

Due to the losses, the heat source and application temperatures had to be compensated

by ΔT=5-10 °C (e.g: condensing temperature=application temperature +ΔT) before

replaced in Eq. (7) (Lund, 2019).

COP of a system depends on the difference between source and sink. Therefore

considering the utilization of a deeper resource (40-60 °C) refers to a yield of orders of

magnitude higher than in case of a regular heat pump. Due to the source of these deep

heat exchanger systems situated at a greater depth, the required energy intake will also

be greater which results in further losses like temperature and frictional pressure losses

experienced by the fluid circulating in the pipe network and formation at a given mass

flow rate.

3.2. Heat collection systems in boreholes

As it was mentioned above the temperature of the source highly affects the

performance and the range of usage. The most common sources in case of a ground

source heat pump can be soil, water bodies and also bedrock. Hereinafter the

introduction of open-loop type heat collector systems will happen in a detailed manner,

where the transfer medium is in connection with its environment. Mainly due to the

geology of the examined area, the focus is on the heat extraction, with partially

bothering the structures of the rock bodies, creating an artificial circulation in them.



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The most significant difference between a horizontal and a vertical system

setup beside the arrangement is the depth. For a vertical setup the term of borehole heat

exchanger is used. Usually their depth does not reach more than 200 m, but in some

cases it runs up to ~3000 m. The establishment costs increasing with the depth and also

the hardness of the ground/rocks. In case of a deep-thermal source the pipes run a long

way vertically into the ground to reach the required depth. Therefore to tap a heat

source vertically first a borehole (if there is no existing ones) should be drilled to

establish a heat pump system (Silwa et al., 2015). Two main types of collectors used in

boreholes are considered in this section (Fig. 4-5). Current research of energy harvest by

reinvestment in depleted wells based on a secondary circulation in a steel U-pipe or a

coaxial arrangement. The coaxial pipe setup is more preferable mainly because of the

larger surface area for heat transfer and also for the hydraulic properties at greater

depths. Thermal insulation of the pipes also has a crucial effect on the operation. At

shallower regions, only partial insulation could lead to non-favorable heat losses, which

means a significant amount when the pipes run deeper. Therefore the insulation of the

pipes is inevitable on the whole length of the pipes (Macenić and Kurevija, 2018).

Figure 4 U-pipe, coaxial type of collector pipes [4]



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Figure 5 U-pipe, coaxial type of collector pipes 2 [5]

Underground thermal sources serve both the need of heating and cooling. The

extracted heat originated from the rock mass used for space heating, in the summer

period for air conditioning. Therefore the heat content of the aquifer rock can be

extracted by using fully the hot fluid originated form the matrix or in a mixed, enhanced

way of artificial supply. In this manner, in both cases the cooled water is reinjected

above the same aquifer layer through the same well called doublet operation (Satman,

2011). The other way around, the heat extracted from the space due to the latter process

transferred back into the source for enhance natural regeneration. This so-called reverse-

mode of a doublet system refers to Aquifer Thermal Energy Storage (ATES), where the

inverted flow can actively use to store heated water (or waste heat) in a suitable media

(Satman, 2011). This phenomena, applies only to the shallow heat exchangers, while

deep borehole ones may only operate in heating mode. Therefore the heat source can

only regenerate through natural conductional heat transfers from neighbouring rocks. If

groundwater flow is present in the media then the back-heating process could also

happen in a convective way (Silwa et al., 2015).

4. Review of deep geothermal heat exchanger systems

4.1. National application practices

4.1.1. Application of deep heat exchangers

There are several ranges of activities where a deep exchanger can be

implemented both in an efficient and economical way but open-loop or ground water

circulation systems are originally introduced in groundwater remediation activities

(Scholz 2000; Mohrlok et al. 2002; Johnson and Simon 2007). A technique called



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pump-and-treat cleansing of contaminated groundwater implemented as first extracting

the polluted then reinjecting the cleaned water into the aquifer. The idea for using the

same methodology for geothermal heat recovery was rediscovered by Xu and Ryback,

2005. These doublet systems originally worked with two distinct wells, one for

exploitation and the other for injection. Single-well circulation systems only require one

well for the two processes offering a space,-and cost-saving alternative. The newly

installed open systems are spreading more slowly than their closed-loop counterparts

(Rode et al. 2015). The reason is the heat carrier fluid of the operation, the groundwater

which means several requirements in the terms of quantity but even more so in quality

(e.g.: effect of clogging). Possible effects evoked by utilizing deep geothermal fluids are

described by several publications (e.g.: Keith, 1993; Bansal and Steinhagen, 1993;

Vasconcelos et al., 2015). Doublet systems impacts on quality changes are also

investigated by several authors (e.g.: Brons et al. 1991, Possemiers et al. 2014) but has

not been examined scientifically yet (Zeng et al. 2017).

Deep circulation systems today are common for example in the mining

industry during excavation work beside raw material heat could be gathered

simultaneously. This category usually means open systems originated from the

ventilation air or via dewatering of a site (Solik and Heliasz, 2002). The situation stands

for terminated mines too, where both thermal energy and remediation purposes can also

be fulfilled by pumping out the water from the mine shaft. For a closed underground

mine the transfer of water from one shaft to another could serve the function of a heat

carrier media while it heated up by the surrounding rocks (Mutke, 2008). The closed-

loop vertical insulated pipe arrangements for heat utilization could serve the same

purpose (Hopkirk and Rybach, 1994). These kinds of systems usually pre-planned and

installed before closure of excavation.

Poor production rate in hydrocarbon wells leads to abandonment. As in case of

mining act water flooding leads to an increased water-cut, in this sense in the well

which loses its commercial value. Boreholes which are scheduled for closing can be

retrained for a function change of heat extraction. It means a partial closure, sealing of

potential risk zones with cement plugs then installing a heat exchanger pipe into the

borehole (Silwa, 2002; Silwa, 2014). This way the wells can be further utilized for

geothermal energy exploitation (Wang et al. 2018). The existing borehole could cut



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50% of the total costs of the investment, while also providing useful geological,-

physical,- chemical information about the potential reservoir itself (Bu et al. 2012).

Exploiting heat from depleted or abandoned oil wells can be achieved both by an open

or a closed-circulation system. For converting a borehole into a closed-loop heat

exchanger one well is sufficient as it is operating without direct fluid extraction

(Templeton, 2013; Davis and Michaelides 2009; Templeton et al. 2014; Sanyal and

Butler 2010; Kujawa et al. 2005; Ghoreishi-Madiseh et al. 2012). A closed-loop

system‟s feasibility for reuse of abandoned oil exploration or production bores is proved

by Lund (2019); Wu et al. (2019). Another part of studies cite the application of open-

loop systems. The way of implementation is to use the hydrocarbon reservoir as a

groundwater based thermal one. Usually open-loop circulation systems operate with a

doublet of wells, commonly one extraction and two injection wells. The term Deep

Geothermal Single Well (DGSW) uses a single borehole for both processes (Westaway,

2017). Several studies (Lund et al. 2005; Kujbus, 2007; Wei et al., 2009, Kharseh et al.,

2015) inspect the economical and practical feasibility for a potential use of oil well as a

posterior thermal reservoir. The existing wells also provide the opportunity for

extension. If a well is not promptly suitable for heat recovery (e.g.: not sufficient heat or

low conductivity), the wells can be drilled deeper to reach favorable thermal conditions

at a lower cost. A potential function change of a well can also eliminate the charge of

plugging process, and additional closure costs, which commonly means a higher

investment all together. It is also possible to restore plugged holes for this purpose but it

also involves additional costs (Sui et al., 2018). In the present study the initial depth‟s

properties of the boreholes were not considered viable for a closed-circuit heat pump

system. Henceforth the further assumptions are made in a possibility of two types of

open-loop deep well circulation system.

4.1.2. Previous studies on depleted hydrocarbon wells

The geothermal possibilities of existing hydrocarbon wells through

computational model considering geothermal heat flux was done by Kujawa et al.

(2006). The used setup of coaxial pipes was carried out under outer pipe injection

circumstances (Fig. 5). Calculations included three different flow rates with a partially

insulated inner pipe. This study also suggests a whole length insulation of the inner pipe

to avoid heat loss caused by short-circuiting as mentioned above (Section 3.4).



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Numerical method for examining the heat transfer between the working fluid (in a

coaxial heat exchanger) and the rock formation around was developed by Davis and

Michaelides (2009). The goal was to prove electricity generation potential by using

different working fluids and inner pipe insulation. The initial cost deduction of drilling

by reusing an abandoned hydrocarbon well for geothermal energy exploitation was

studied by Bu et al. (2012). In this research a 50% reduction in investment was proved

compared to the costs of a newly drilled well. A numerical model was developed to

describe the heat transfer between as in case of Davis and Michaelides (2009). But the

paper expressed it as a function of geothermal gradient and fluid-flow rate. The study

also proved that in a steady-state heat-transfer regime at a low outlet temperature and

short distance of wells could be a long lasting system. Cheng et al. (2014a)

experimented with the efficiency of using different working fluids mainly in aspect of

electricity production. The organic materials (R600a, R600, R134a, R290, R245fa,

R143a, propylene) were tested between 1000-6000 m depth range and 40-50°C/km

geothermal gradients with two power generation systems. From one side the

experiments proved that <3000 m boreholes are not suitable for electricity generation

because of low efficiency. On the other hand R134a and R245fa type fluids showed a

higher performance compared to water and other tested fluids. The study-state heat

transfer (constant output temperature) in this case was achieved in 300 days. The

models and set terms of Kujawa et al. (2006) and Bu et al. (2012) were compared to the

heat transfer model of Templeton et al. (2014) by an own buildup. The results of these

studies proved that in a coaxial heat exchanger beside the geothermal gradients and

mass flow rates of the pipes the well-chosen geometry and sizes of the casings have an

important role. The effect of the pipe insulation between the interfaces was studies by

Cheng et al. (2014b). Using R143a fluid and proved that the thickness of the insulating

layer linearly affects the temperature output. A depleted well with an unsealed bottom

was examined by Cheng et al. (2016), assuming the communication between the

injected fluid and the reservoir. The paper both takes into account the properties of the

reservoirs (size, depth, porosity) and the fluid pressure,- velocity. The open-to-reservoir

circumstances lead to immediate fluid loss but in this case a reservoir with favorable

properties could enhance the output temperatures. The well-temperature profiles for a

greater depth suggest the injection through the inner pipe section to achieve a lower

thermal resistance of the borehole itself. The chosen flow direction in the pipes proved



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no significant difference in performance by the experiments of Holmberg et al. (2016).

Apart from using an open-, or closed-loop arrangement Gombert, Lahaie and Cherkaoui

(2018) provided an overview about the risks related to such type of repurposing. Among

other properties, they identified the risk factors of fluid, gas and solid effusion towards

the surface, aquifer pollution due to the well characteristics, ground mass movements at

establishment and operation stages. Considering the design of the hydrocarbon

exploration drillings, they are inappropriate for geothermal use, therefore they should be

cemented at the full length, because most of the lack of production line. Existing oil

wells however have both a production line and a steel casing for physiochemical

protection. Vernoux and Manceau (2017) studied the reopening possibilities of already

reclaimed wells, which require a greater investment both from survey and financial side.

Conversional potential of depleted hydrocarbon wells across Hungary for viability of a

deep heat exchanger systems were assessed by Toth et al. (2018).

4.2. Viability of Enhanced Geothermal System (EGS), Deep Borehole Heat

Exchanger (DBHE), Deep-Well Circulation System (DWCS)

Geothermal heat extraction usually focused at locations with favorable

hydrogeological anomalies, but recent alternative approaches provide space for

introduction of new innovative procedures. These technologies could harvest heat

without the presence of a “well-determined” hydrothermal system.

Therefore at the very beginning of the research two options came into

consideration, as the given pre-drilled abandoned wells could be quite valuable. In

aspect of saving the drilling costs two techniques were found appropriate for reaching

the overall goal, one was the Enhanced Geothermal System (EGS) and the other one

was a low temperature Deep Borehole Heat Exchanger (DBHE). The common point

which is also refers to the limit of these methods are the temperature (heat flows) and

the adequacy of geologic properties. The rock temperatures (~60°C) justify a utilization

of a low-enthalpy direct EGS system, proper for district,- agricultural heating and the

lower limit for hot water production. After geological prospecting of the area the layer

properties found less favorable, than taught at the first place. However the EGS based

on a special kind of process called hydraulic fracturing, which is a technique involves

high-pressure water pumping into the desired layer, creating an artificial (in-situ)

fracture system to improve the conductivity of the layers. Injected water moves through



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these fracture system, gather heat and extracted in a second borehole. While both BHE

and EGS using the injection of a secondary media (usually water or other lower

temperature liquid) to mine crustal heat, EGS is mainly designed for electric energy

production. The reason lies in the technology and its specific costs. The most important

factors which determine the viability of an EGS are the fluid flow rate and its

temperature (Tester et al. 2006; Economides 2000). These are the reasons why this

mode of applications has been discarded.

BHE also implement an energy extraction with a secondary fluid, but while the

EGS has a direct contact with the soil or rock, it does not allow it. BHE could use

different closed loop configurations buried in the ground to exchange heat energy with

the media around. BHE are applicable in shallow (50-200 m) depths, called Ground

Source Heat Pumps (GSHP) used both in commercial heating or cooling (Lund and

Boyd, 2016), based on the same principles can reach deeper regions of earth (1000-3000

m). As in case of an EGS a DBHE also depends on the crustal heat flow, but its

efficiency and feasibility mainly determined by the configuration and the thermal

properties of the layers (Silwa and Kotyza, 2003; Dijkshroorn et al. 2013). The

unfavorable low conductivity geology of the target media does not allow extraction of

any liquid and the fracturing technique did not appear to be cost effective due to

relatively low temperatures. In regard of these facts the use of DBHE was considered to

be a viable option, but the inspected wells are originally oil producers. Therefore

according to the structure of oil wells and available data of well completion showed that

the wells are drilled in multiple sections and diameters. Whereas in this case the bores

include 2-3 liners and cement layers at the target depths which will greatly impair the

heat transfer ability of a potential DBHE system. The only possible option which can

both extract heat and partially utilizing the existing bores is a Deep-Well Circulation

System (DWCS), which is a subtype of Ground Coupled Heat Pumps systems.

Unfortunately to make this technology applicable, a natural or semi-natural circulation

is required to provide convective heat transport. The compacted non-porous,

impermeable layers do not allow any cost-efficient way of circulation at the initial

depth. Many thermal wells and exploratory drillings have been deepened in the area so

far, which reached Eocene and Triassic age limestone layers below the clay-marl strata.

The well-developed karst water level monitoring system working in the area provided

additional, up-to-date long-term datasets of the recent hydrogeological characteristics



16

serving a good basis for this and other innovative scientific researches within the region

(Miklós, et al., 2020). This areal information provided both the fact and a good

reference point to delineate the position of the Eocene charred aquifer layers below.

Thus the considerable option left is to overdrill the existing potential wells till it reaches

the known fractured karstic aquifer below (Pelikán, 2005).

4.3. Configurations of Ground Coupled Heat Pump (GCHP)

Most deep geothermal projects consider the operation with well doublets, for a

balanced and sustainable operation. These types of deep-collector systems could reach

at greater (1000-3000 m) depths enabling much higher delivery temperature accordingly

the geothermal gradient. It also refers to the main advantage of these deeper holes,

which reduces the number of required wellbores to deliver the same heat effect as a

regular (GCHP) system. Therefore it takes up lesser space at settlement useful in

densely populated urban area and also means a lesser environmental burden, as no

actual water consumption happens. Long-term monitoring of such systems in Germany

and China proved negligible effects on both groundwater quantity and quality (Ni et al.

2006). The deep boreholes could be used another way around, as a geologic heat

storage. Whereas there is no facility nearby that generates a vast amount of industrial

heat which could be stored in the geological environment during summer season, this

work focuses on the possibility of heat extraction (Gehlin, 2016).

Returning the extracted water to the subsurface serves two purposes, one is to

avoid drawdown and the other is to eliminate the need for treatment of the water as in

case of letting it into a surface waterbody which depends on quality terms (Westaway,

2017). Comparing an open-loop arrangement to a closed one, due to the effect of natural

circulation can considerably reduce the required amount of bores for the same energy

output. However closed-circuit systems are known to be the industry standard because

they have no special requirements, an open-cycle using the groundwater as a resource

media, can increase the efficiency and viability of a system (Zeng et al. 2017).

The known close relation between groundwater flow and thermal convection in many

hydrogeological concepts as thermogeology refer to it. Accordingly to the manner of

how water is extracted, four configurations of groundwater heat-pump systems are

distinguished (Wu et al. 2014). Single-well extraction (Fig. 6a) means that the water



17

pumped out, is not returned to underground, but discharged into a surface waterbody.

Unlike a two-well circulation system (Fig. 6a) where the exploited groundwater is

returned to the subsurface through an injection well situated separately, apart from the

pumping well. Meanwhile both the standing column well system (Fig. 1b) and single-

well circulation system (Fig. 1b) are injecting back the extracted groundwater, through

the same bore, the only difference between the two is the manner of it. Standing column

well returns the water in a state where the two sections are not hydraulically separated.

In contrary the single-well circulation system injects the groundwater in a hydraulically

separated condition. Of the mentioned open-loop heat-pump systems, the latter one

provides the greatest geothermal capacity with the lowest space requirement. According

to Rybach (2015), a standing column system render ten times more thermal supply than

a closed-circuit one, whereas a single-well circulation system produce ten times more

energy than the standing column system. In this regard the single-well circulation

system offers the best heat exchange efficiency in case of suitable thermogeological

conditions for the establishment (Zeng et al. 2017). Hereinafter the current study will

focus on the viability of a Deep Single Well Circulation System (DSWC) and Deep

Geothermal Doublet System (DGDS), operating with a three-well establishment.

Figure 6a Two-well circulation,- and single-well extraction systems [6]



18

Figure 6b Standing column,- and single well circulation systems [7]

4.4. Geothermal well systems

Hydrothermal systems extracting heat energy from thermal water situated in

deep geothermal reservoirs. Accordingly, low-enthalpy systems utilize warm/hot water

in a direct way through a heat exchangers for different purposes other than electicity

generation. As producting electircal power in a cost-efficient way requires fluid

temperature 120 °<, this is not possible in the studied pilot area. Low-enthalpy systems

can be established wherever, with the presence of elevated geothermal gradients. In this

section two kind of wells systems will be discussed in a detailed manner.

4.4.1. Deep Geothermal Doublet System (DGDS)

The source of a geothermal well system is the „clean” groundwater originated

from a hydraulically highly conductive media called aquifer, with its water table

relatively close to the surface. In case of a karstic aquifer the depth of groundwater table

should not be greater than 30 m, for an viable longterm operation (Zeng, 2017). A

doublet system as the name implies extract two spatially separated wells, one for

production and the other for injection above, below or at the level of the same reservoir.

Such systems often referred as two-well-systems or water-water-heat-pumps, more

commonly groundwater heat pumps. The use of groundwater as working fluid has many



19

advantages e.g.: minimizes energy losses in the heat exchanger. Under the right

circumstances the source fluid temperature is relatively constant, and the advective heat

transport by the groundwater flow has a vantage point compring with the conductive

heat tansfer of the ground source heat exchangers. Limitations of a fluid source system

is mainly comes from the available amount of transfer medium which directly refers to

the properties of the aquifer. These two things define mainly a long term feasibility of a

water mining act (Stober, 2013).

Figure 7 Schematic setup of a doublet system [8]

The sturcture of a doublet system is not really differ from a regular production-injection

groundwater well system from an engineering point of view, including full-section

pipes, screens, sealed and gravel bedded intervals (Fig. 7). From the other side, a

geothermal system uses smaller diameter pipes because of the lower production rates in

comparison. The different operational flow velocities from both sides, means that the

screened depths of the two pipes will also differ. The production pipe‟s screened length

should be situated lower than the injection well‟s, and the pump responsible for the

extraction must be above the screen depth because of the increased inflow velocities.

Meanwhile the pump in the injection well should be higher up to prevent overflow

(depending on resevoir properties and groundwater level). The two separate wells

should not be thermally linked, therfore in usual terms the injection well is placed

downstream of the production one. However there could be exceptions e.g.: the distance

of wells is adequate, the injecting happening above or below the production aquifer, the

wells are reparated by low permeability layers… etc.



20

4.4.2. Deep Single Well Circulation System (DSWC)

The DSWC is a special kind of open-loop GCHP system, which combines the

initial structure of a doublet system by molding the production and injection well pair

coaxially into single well (Satman, 2011). The setup of a single-well circulation system

is shown by Figure 8. The well itself has two screened sections, one in a shallower and

the other in a greater depth. The perforated surfaces are situated both in the outer and

inner pipe. The upper one‟s purpose is to return the cooled down water underground,

back into the aquifer, while through the bottom screen happening the extraction. These

two sections as mentioned above are hydraulically separated, with grout, plugs or in

other engineered way. The system operates with a downhole pump which circulates the

working fluid (water) with a well-determined flow rate (Zeng et al. 2017). The heat

recovery is happening through the inner pipe section when the fluid reaches the

extraction (bottom) filter. The exploited water is utilized, therefore cools down and then

returned into the groundwater. The injection of the cooled water is happening through

annular space or return pipe. The point of outflow is the upper filter section situated

above the separation seal. The plugs purpose is to shut off the entering water from the

lower parts of the well and helps the water returning into the aquifer. The reinjected

cooled water is circulated through the formation and gains the in-situ temperatures of

the aquifer by convection. The water returns to the groundwater, heats up in the media

then the cycle starts again (Dinkel et al. 2018).

Figure 8 Schematic drawing of a single-well circulation system [9]



21

A well-balanced system requires an aquifer of 20-100 m thick with favorable

characteristics. The Peclet number is used to decide if an aquifer is suitable for a

sustainable extraction. The general boundary conditions are determined by Xu and

Ryback (2003) in case of an aquifer for geothermal use. Including the hydraulic

conductivity which has to be at least 10-3

m/sec, a flow velocity of 0,5 m/day, assuming

a relatively shallow water table lesser than 30 m. The temperature changes of the

aquifer both depend on the extraction-injection rate and also on the type of utilization.

In case of a heating-dominated system the ground temperature will decrease and vice-

versa (Pertzborn et al. 2011). Detailed study on the aquifers thermogeological properties

was carried out by Wu et al. (2014). The viability and operational lifetime of an open-

loop system both defined by the chosen parameters and also the specifics of the working

fluid. In case of an open-circulation system the working fluid is water. Therefore the

physical and thermal properties have effects for development and operational terms.

The density of water is a function of its physical parameters. As for the formation

density (P-T) can be ignored under normal circumstances at planning stage. Beside that

the water density is also affected by both the pressure-temperature (P-T) and also its

total dissolved solids content (TDS). The density changes as a reverse function with

temperature according to the thermal expansibility, while the amount of TDS increase

its density. Deeply situated waters are commonly highly mineralized, which could

evoke further effects like corrosion in a geothermal system operation (Bucher and

Stober, 2013). The effects of the latter properties are not considered in this work, due to

the absence of sufficient information of the reservoir fluids and the volume of the study.

No real-time samplings happened therefore only assumptions can be made according to

the operating thermal wells nearby. As it seems, DSWC could provide a higher amount

extractable heat capacity as it does not require specified functions for each wells,

therefore each bore could be used for both purposes. However the establishment of such

a system considered more profitable but also requires a more careful planning. Due to

the lack of geological information mainly for the aquifer layer, only rough estimations

could be made for the adequate thickness for this method. If the depth range does not

have the required values, in a karstic aquifer the reinjected water will reach the

production section without much resistance. Therefore the water does not have time to

gather heat from the surrounding media, which will eventually, cools down the system.

For the latter reasons, the final decision fell on a potential “geothermal well triplet”,



22

presented in Section 4.4.1. Both the groundwater and heat transfer modelling of the

sample area was carried out according to the mentioned system.

5. Characteristics of a (karstic) aquifer system

The geological characteristics like depth, extent, thickness and other

lithological discontinuities are playing important role in determining an aquifers

distribution and fluid flow features. In a fractured environment groundwater flows are

narrowed down to pathways referred as channeling which can results in significant

heterogeneities within the rock body. Karstic aquifers are a special kind of fractured

rock media, composed of carbonate rocks (mostly limestone and dolomite). The voids,

channels created by the dissolution processes called karstification results in large void

spaces in this relatively soft rock in a form of fractures, joints, cavities…etc. Therefore

the groundwater flow in such type of aquifers may be quicker and more concentrated

than in other types of aquifers. Groundwater yield vary due to the size and distribution

of the major fractures and the rate of krastification in the media. To obtain a successful

and productive wellbore with viable supply mostly depends on puncturing a well-

connected cavity system, however the rate of groundwater movement is still remain

hard to quantify (Klepikova, 2013) (Int. 2-3).

5.1. Physical properties of the aquifer

The physical properties of a fractured karstic aquifer, is hard to define both due

to its heterogeneity and lack of hydrogeological information. The distinctive

hydrogeological features originate from a combination of rock solubility and secondary

(fractured) porosity (sometimes triple porosity) (Agosta et al., 2007). Defining an

aquifer system from hydraulic aspects, porosity, permeability, compressibility,

storage coefficient, specific yield are essential characteristics to know (LaMoreaux et

al., 2009).

Porosity () is expressed as the volume of void space per the total volume of

the soil sample. The ranges of total porosity for each rock type was studied by several

authors (Tab. 1) and the volumes of all interconnected void spaces in a sample

described as effective porosity, defined by the function (LaMoreaux et al., 2009):



23

(8)

where

: porosity [%]

Vvoid: volume of voids

Vtotal: volume of soil sample

When defining porosity, especially in carbonate rocks, it is important to specify the

origin of pore spaces which refers to its genesis. Primary porosity is inherited from the

original geological alterations meaning the size, shape and arrangement of the grains,

mineral composition and cementation processes. Secondary porosity is generated by

after consolidation phenomena like dissolution, tectonics and diagenesis which

processes further affects the initial porosity of the rock.

Table 1 Effective porosity for several rock types (Borden, 2015)

Aquifer matrix Dry Bulk Density Total Porosity Effective Porosity

Clay 1,00 - 2,00 0,34 - 0,60 0,01 - 0,2

Peat 0,3 - 0,5

Glacial sediments 1,15-2,10 - 0,05 - 0,2

Sandy clay - - 0,03 - 0,2

Silt - 0,34 - 0,61 0,01 - 0,3

Loess 0,75 - 1,60 - 0,15 - 0,35

Fine sand 1,37 - 1,81 0,26 - 0,53 0,1 - 0,3

Medium sand 1,37 - 1,81 - 0,15 - 0,3

Coarse sand 1,37 - 1,81 0,31 - 0,46 0,2 - 0,35

Gravelly sand 1,37 - 1,81 - 0,2 - 0,35

Fine gravel 1,36 -2,19 0,25 - 0,38 0,2 - 0,35

Medium gravel 1,36 -2,19 - 0,15 - 0,35

Coarse gravel 1,36 -2,19 0,24 - 0,36 0,1 - 0,25

Sandstone 1,60 -2,68 0,05 - 0,30 0,1 - 0,4

Siltstone - 0,21 - 0,41 0,01 - 0,35

Slate 1,54 - 3,17 0,0 - 0,10 -

Limestone 1,74 - 2,79 0,0 - 50,0 0 01 - 0,24

Granite 2,24 - 2,46 - -

Basalt 2,00 - 2,70 0,03 - 0,35 -

Volcanic Tuff - - 0,02 - 0,35

Permeability (K) within hydrogeological means is an essential property of

rocks defined by the ease of flow through a media due to gradient potential.



24

Permeability is strongly affecting the flow velocity through a rock body, usually with

higher values in the horizontal,-and lower in the vertical direction. The connection

between grains size and permeability refers to the frictional surface area of the grains.

Therefore smaller particle size meaning larger specific surface area resulting in lower

flow rate and permeability expressed as:

(9)

where

K: coefficient of permeability [L2]

Q: flow rate through a unit media [L2/T]

A: unit cross-sectional area [L2]

: viscosity of a fluid [L2/T]

: density of a fluid [M/L3]

L: unit length of a media [L]

h: fluid head loss along the length [L]

g: gravitational acceleration [L/T2]

Darcy modified the previous equation, turned into a simplified form for a better

hydrogeological practice. This way the equation describes the fluid movement in a

specific rock body (Chery and Marsily, 2007):

(10)

where

k: hydraulic conductivity (L/T)

Q: flow rate through porous medium (L3/T)

A: unit cross-sectional area [L2]

L: unit length of a media [L]

H: fluid head loss along the length [L]

Hydraulic conductivity is a crucial parameter for controlling fluid movement

rates below the surface. In a conjoined manner with temperature parameters decides the

economic success of a project. The available hot water for a hydrothermal system varies



25

due to the pressure and temperature circumstances. Fluid flow rate has a higher

temperature than in a colder aquifer (Stober, 2013). The hydraulic conductivity is the

ability of the fluid defining its movement through the interconnected pore spaces

depending on both fluids and soil characteristics. Meanwhile permeability is the

function of soil/rock determines the ability of pass through depending on the well-

connectivity of pore space. The mentioned parameters for different rocks and

unconsolidated lithologies studied by several authors and published in range of varieties

(Fig 9).

Figure 9 Permeability and conductivity ranges for different type of lithologies [10]

Transmissivity (T) describes the fluid flow rate through a unit width of an

aquifer under a unit hydraulic gradient, expressed as the product of conductivity and

thickness (LaMoreaux et al., 2009):

(11)

where

T: transmissivity [L2/T]

k: hydraulic conductivity [L/T]

b: thickness of a layer [L]



26

Compressibility () in a porous medium is defined as a relative volume

change due to external pressure changes. Expressed as a function of the aquifer

thickness including both formation and fluid pressure (LaMoreaux et al., 2009):

(12)

where

α: compressibility of the aquifer [P]

Δv: differential volume [L3]

Δp: differential pressure [P]

V: initial volume [L3]

The value of compressibility in case of an aquifer rock in engineering terms is generally

neglected due to the lack of published data for limestones and sandstones. The effective

compressibility of any reservoirs is the result of relaxation of grains as the surrounding

fluid pressure decreases, and the further formation compaction due to the fluids which

could not effectively maintain the pressure as the reservoir pressure declines. Both of

these phenomena leads to a porosity decrease (Hall, 2013).

Specific yield (Sy) refers to the amount of extractable water under gravitation

force, available when rocks are drained results in a deceasing water table. Meanwhile

the specific retention (Sr) described as the volume of water retained by adhesion and

capillary forces under drainage (Harter, 2005) [Water Encyclopedia - Spec Yield

Storage Equation]. Comparing unconfined aquifers to confined ones lesser head change

can be expected in the case of an unconfined aquifer considering the same yield.

Table 2 Values of specific yield for various medium after Heath (1983); Şen (2015)

Material Minimum Average Maximum

Clay 0 0,02 0,05

Sandy clay 0,03 0,07 0,12

Silt 0,03 0,18 0,19

Fine sand 0,1 0,21 0,28

Medium sand 0,15 0,26 0,32

Coarse sand 0,2 0,27 0,35

Gravelly sand 0,2 0,25 0,35

Fine gravel 0,21 0,25 0,35

Medium gravel 0,13 0,23 0,26

Coarse gravel 0,12 0,22 0,26



27

Limestone 0,06 0,18 0,38

Basalt (young) 0,04 0,08 0,25

Storage coefficient (S) phrase the amount of water released from a storage

media per unit decline in hydraulic head, per unit area of the aquifer. Expressed as a

dimensionless number falls between 0 and the given effective porosity of the reservoir

(Freeze and Cherry, 1979):

(13)

where

Vw: volume of water released from storage [L3]

Ss: specific storage

Sy: the specific yield

b: aquifer thickness [L]

Table 3 Specific storages (Ss) in different medium according to Vedat, 1998 and Şen, 2015

Material Specific storage [m-1

]

Clay 9,81 x 10-3

Silt, fine sand 9,82 x 10-4

Medium sand, fine 9,87 x 10-5

Coarse sand, medium gravel, highly fissured 1,05 x 10-5

Coarse gravel, moderately fissured rock 1,63 x 10-6

Rock without fissures 7,46 x 10-7

Rock with fissures, jointed 3,61 x 10-5

5.2. Thermal properties of an aquifer

The heat transport of an aquifer takes place in the interconnected void space of

the rock media via groundwater flow, through both the solid matrix and the void

network within. In case of a convective heat transfer the movement of heat is depending

on the fluid velocity, therefore can be divided to advective and dispersive components.

Convection can also be regulated by density differences caused by temperature

gradients. Advection is expressed as the thermal energy transport due to a linear flow of

water in the media. Whilst the dispersion meaning a spreading at a microscopic level in

three dimensions. Heterogeneities and the varying conductance of the aquifer can

significantly affect the dispersion of the thermal plume. Beside the latter processes, an



28

aquifer‟s heat conduction depends on the thermal properties of the aquifer. These

characteristics include volumetric heat capacity or specific heat (both for the solids

and the fluids), and the effective thermal conductivity (Bridger and Allen, 2005).

Thermal conductivity ( m) is expressed as the quantity of heat conducted

through a unit media in a unit of time, under a unit of thermal gradient. However the

properties of rocks can vary in a wide range, geologic media considered to be good

insulators with relatively low differences of thermal conductivity values (Lee, 2013).

Specific heat (c) is defined as the quantity of heat required for a mass to achieve a unit

change of its temperature. Refer as the heat response of an aquifer media absorbing or

losing a specific amount of energy. The temperature of a solid or a fluid phase in the

same aquifer should be on the same due to the law of thermal equilibrium. Hereby the

temperature for both phase calculated with an average value (Lee, 2013). For example

water has a relatively high (4,187 kJ/kgK) value of specific heat capacity, therefore it

requires large amount of heat to elevate its temperature and it also release an equally

large amount when the temperature drops. The heat transfer processes situated in the

aquifer and the manner of introduction to the groundwater flow model is detailed in

Section 7.2.5.

6. Description of the research area

6.1. Location of the sample area

The research field as a sample area is located in the foreground of a

mountainous region of Hungary. The micro-region is an area of low,-and mid

mountainous ridges with low (0,5-1 %) to medium (1-2,5 %) slope hills to the south-

,southeast (Huggett and Cheesman, 2002). The elevation of the area falls between 100-

220 maBsl. The investigated abandoned wells are situated in a foothill area for which a

topographic map with adequate resolution was not provided therefore the presented

elevation map of the examined sample area is hand-digitized by the author (Fig. 10).

The map of the territory with the existing and additional pseudo-wells is shown by

Figure 7. The highlighted A1,- A2 geological cross-sections are shown in Figure 11-14.

The emphasized potential wells for further well design and modeling purpose will be

inspected in Section 6 and 7 in a detailed manner.



29

Figure 10 Location of the examined wells and geologic cross sections (author‟s own work) [11]

6.2. Geological and thermal properties

The foothill/research area itself embedded in a Miocene volcanic ring of the

main mass of the mountains consists of sedimentary rocks. This mass is formed mostly

by Upper Triassic carbonates which alternates with deep-sea sediments in the

southeastern parts. In geomorphological terms it is a hilly region bordering the

mountains on the southeast in an 8-10 km long line. On 30% of the surface of the

micro-region contains hydrocarbon-indicating Oligocene marl and sand assemblages,

with Triassic carbonate intrusions. The southwestern part of the sample area is covered

by a large mass of Pliocene sandy-marl sediments formed by multiphase regression.

These sediments are interbedded with lignite formations in large thickness which

occurrence is common in the foreground of the mountains. Pleistocene slope,- and loess

materials of varying thicknesses can also be found in the southern bordering areas.

The starting point for the geological analysis was a digital database of well logs

provided by the Hungarian Office of Mining and Geology (short in Hungarian:

MBFSZ). The datasets contain the results of detailed geological and geophysical

surveys of boreholes from 1970 to 1986. It is important to point out the age of the data,

therefore one have to reconsider their punctuality before making any conclusions. Due



30

to their reliability is lower than results of measurements performed with a more up-to-

date tool or technology. Of the whole dataset, within this time period 14 wells (Fig. 10)

were found to be adequate and sufficiently informative to draw further conclusions. In

the first step the meter resolute strata with age description have been simplified for a

manageable further use. The simplification happened according to the stratigraphic

descriptions indicated in the drilling logs, also taking the additional geological

information into account. In some cases it means for each type of rock mostly by

alterations and sometimes by color. From 14 of the detailed well logs 4-4 wells were

selected based on the available data and location, shown by Figure 10. To map the

geological conditions two cross-sections were created one with SE-NW (A1) (Fig. 11)

orientation and the other with the direction of S-NE (A2) (Fig. 13). Temperatures

calculated (3) with the area‟s average geothermal gradient (0,07°C/m) and the depth

information from the well logs gave the opportunity to make cross-sectional

temperature distribution maps (Fig. 12 and 14). The visualization of the geological

cross-sections have been created by Golden Software Strater 5 using the simplified

interlaced log data (Fig. 11 and 13). The cross-sectional representation of the subsurface

thermal conditions was created by Golden Software Surfer 12 (Fig. 12 and 14).

Figure 11 A1 geological cross section (SE-NW) (author‟s own work) [12]



31

Figure 12 Temperature distribution in the A1 cross-section (SE-NW) (author‟s own work) [13]

Figure 13 A2 geological cross-section (S-NE) (author‟s own work) [14]



32

Figure 14 Temperature distribution in the A2 cross-section (S-NE) (author‟s own work) [15]

In the A1 (Fig. 11 and 12) cross-section the depth of the boreholes vary between 600-

700 m. These wells are penetrating clay-marl layers in great thickness (Fig. 11). The age

of the rocks, according to the well logs are Oligocene clay and mixed clay-marl

formations, both present in the A1 and the A2 cross-sections. One borehole in A1

punctured an Eocene imbedded carbonate block, but the main aspect is on the potential

of the previously mentioned impervious formations. On an average basis from 350 m

depth till the bed of the formation the temperatures vary between 30-60°C in the

mentioned clay-marl material, which is considered to be a low enthalpy media (Fig. 12)

(Schreiner, 2013).

The A2 cross-section located southern direction from the A1, furthermore its

boreholes vary between 700-900 m depths, therefore higher temperature values have

been experienced in some of the wells (Fig. 13-14). The cap rock of the studied section

is a nearly 220 m thick Miocene rhyolite tuff, which also does not offer any potential

opportunities from aspect of the present research. In this cross-section as also in the A1,

clayey layers are present dominantly under the top layer, with imbedded sand,-

sandstone lenses (up to a certain extant). In case of A2 (Fig. 13 and 14) from the depth

of 500 m down below the temperature values vary between the temperatures of 50-70°C

(Fig. 14) (Zsemkó, 2013).



33

As the initial analysis proved, under the near surface formations, there are low enthalpy

average 60°C temperature clay,-clay marl layers in great thicknesses. Although clay

minerals due to their properties are inherently good at water retention, insulating layers.

Considering the temperature distribution map of Hungary this foothill region gives a

potential possibility for a heat extraction via EGS or BHE utilization. Due to the

relatively low temperatures problem solvation went on the applicability of a deep BHE

system.

6.3. Subsurface modeling of the target area

A status-survey regarding the wells current condition began in 2018. The

research carried out in the PULE project is intended to classify the present state of the

wells mentioned above according to different aspects. The categorization happened on a

scale 1-10 and also included the current function of the boreholes (Szűcs et. al, 2019).

The so far results of project PULSE showed that five of the examined wells are

reclaimed therefore the idea of changing the purpose of these has been rejected. The

further investigations include the remaining three boreholes, W-23, W-28 and W-35.

The survey also revealed that two of these three wells are still producing oil while one is

just abandoned. The subsurface modeling of the wells with special regard to the thermal

characteristics of the target medium was done by Schlumberger Software PetroMod

2016.2.b. The software is suitable for modeling the geologic strata according to various

attributions, using the age and phylogeny of the rocks. This way it can give a more

accurate representation of the subsurface conditions. However it is important to cite that

the result is still a modeled environment which is not necessarily cover the real state of

the layers, but it can provide a good approximation. The software offers a way to

calibrate the initial results with real-time field measurements (e.g.: temperature

measurements) data to enhance the model output.

Due to the unique geological history of the area Eocene age limestone is

immediately followed by Triassic age limestone strata, omitting the sequences of two

(Palocene and Jurassic) geological ages. Nonetheless the PetroMod calculations require

the information and events of these missing,-eroded surfaces/ages to accomplish the

modelling and calculations. The assessment of the geological events both for the

existing and missing strata were happened according to several authors work and

publications of the area (Pelikán, 2005; Völgyesi, 2002; Pethő és Vass; 2011; Haas,



34

1998). The utilized data for the program‟s calculations, served as an input for the

simulations can be found in Annex 2.

As it was previously mentioned both the surface and subsurface environment

of the area is poorly assessed. The subsequent modeling, especially heat transport

requires specific input parameters (e.g.: bulk density, heat capacity…etc.), which can be

measured in-situ, modelled, calculated or using literature data. Evidently the latter two‟s

results has a lesser accuracy, but considered to be more realistic than working with

(averaged) literature data all along. (PetroMod modeling was carried out for each of the

three wells, and necessary output parameters were used later.) The calculation results

were plotted out, visualizing each modeled parameter in relative depth thorough its

lithological strata (Fig. 15-17). Section 7.2.4 presents the results of calculated and

modeled input data used for the heat transfer model.

Figure 15 PetroMod results of well W-23 for variable physical parameters by depth (author‟s own work) [16]



35





36

The temperature results of PetroMod calculations were correlated separately

with the calculated temperature data using the area‟s average geothermal gradient (0,07

°C/m). Therefore the values of the temperature distribution cross sections‟ (Fig. 12 and

14) were compared with those calculated by PetroMod. The correlations were

visualized in Grapher 14 program (Fig. 18-20). The result of the comparison showed

that apparently there is no big difference between the two methods, but the PetroMod

considered to be more punctual as it takes into account the events of the geological

ages.

Figure 18 Correlation between PetroMod and Calculated temperature data well W-23 (author‟s own

work) [19]



37


work) [20]


work) [21]



38

The temperatures in the screened sections stated in Figure 18-20 has been calculated by

both cited methods then the averages of the results were compared. The average

temperatures thus obtained in the potential wells‟ each screened section were used in the

late heat transport modeling as the assessed media‟s initial temperature (Tab. 4).

Table 4 Average temperatures at the potential wells‟ screened depths based on PetroMod and Calculated results

(author‟s own work)

Well

ID

Screen

top

[maBsl]

Screen

botom

[maBsl]

Average

Calculated

temperature

[°C]

Average

PetroMod

temperature

[°C]

Avarage

summarized

temperature

[°C]

W-23 -432,86 -454,16 54,52 57,40 55,96

W-28 -373,67 -386,95 52,77 53,94 53,36

W-35 -471,69 -485,57 60,9 62,27 61,59

6.4. Technical parameters of the potential wells

As it was mentioned in section 4.2., for reaching the karstic aquifer layers as

well as the goal of the research, overdrilling of the existing wells is needed. As it was

also mentioned, the wells originate from the oil industry, therefore their function was oil

exploration, - or production. From geophysical measurements the ones who drilled these

wells known where the limestone layers situate below the thick clay/marl formations,

and accordingly the well books, usually the drillings are terminated in the latter ones.

Drilling these highly pressurized karstic aquifer layers would have spoiled the oil above

and have made the cementation process almost impossible. As the scope of the thesis

work is to utilize the relatively hot fluids from the Eocene limestone aquifer layer,

several pre-measurements and preparation has to be made including the overdrilling of

the existing wells. The extent of overdrilling depends on the information and trends of

the geologic sequences from the surrounding wells nearby.

In case of well W-23 according to the well books punctured the Eocene

limestone layers (Fig. 11). Therefore the thickness of the aquifer is known in this area

which also gives a good starting point for the further estimations. It lowers the extent of

required overdrilling to 7 m in case of this well including a sump of 15 m right after the

screened section for the longer viability of the established well. The limestone layers in

well W-28 and W-35 have not been reached therefore the location of these formations

can be estimated from the water-cut of the oil wells and the trends of the clay/marl



39

sequences above in the area (Fig. 11-13). In case both of these wells the required

overdrilling is more, ascertained in a length of 33,88 m. The sumps are also included in

these wells. Hence, all three potential-considered wells are stopped in the Triassic age

limestone layer as both the sump and the bottom of the filter pipe should be situated in a

more rigid and stable formation. In this case, it refers to a less charred rock formation.

The chosen screen type is the so-called Louvre slot or bridge slot screen. In this type of

screen the slot openings are pressed in the pipe, resulting in a consistent slot size. Both

advantage of this design the strength at relatively long screen length and applicability in

corrosive environment which extends the lifespan of wells. This screen type is

commonly used in deep wells with a relatively low permeability media. Due to the

relatively large apertures of the screen preferably installed with a gravel pack, for

clogging prevention (especially in sand formations). As the screened section situated in

a relatively rigid limestone layer, installation of gravel pack is not absolutely necessary

(Raymond, 2017).

As most of the well records originate from the 1980-90‟ and the latest (2013,

2018) assessments are not covered the condition of the wells, first of all a geophysical

well inspection is required before making any far-reaching proposal of the future fate of

the wells. The condition survey should include the current state of the well casing, with

any visible bents, cracks or scaling. The investigations also have to cover the condition

of the cement coat and detect any leak points from both sides. One is the initially laid

cement around the pipes and the second is the cement plugs of the originally perforated

sections. According to the well books, not all the perforated (former producer) sections

were plugged of unknown reasons. Therefore cementing these sections to avoid any

leakage is essential. Usually when a deeply situated, highly pressurized oil well is

established one or more production packer is emplaced at the critical points of the well

which existence and location are not mentioned by the well books either. If there are

still packers installed and also any exploitation pipes left in the well, first have to be

removed before the drilling starts. The following (estimated) well designs are made with

the previously mentioned facts taken into consideration. Both the tables (Tab. 5-16) and

layouts (Fig. 21-23) highlight the planned sections and parts of the overdrilled wells

marked with red color. The layouts were made in Autodesk AutoCAD software.



40

6.4.1. Technical parameters of well W-23

Elevation: 192,14 maBsl

Static water level height: +18,765 m

Static water level: 210,905 maBsl

Table 5 Drilled lithology of W-23 well, estimated lithologies marked with red color (author‟s own work)

Lithology Depth ranges [m] Depth ranges [maBsl] Layer thickness [m]

Volcanic rocks 0 125 192,14 67,14 125

Clay/marl 125 617 67,14 -424,86 492

Limestone 1 617 654,3 -424,86 -462,16 37,3

Limestone 2 654,3 706,8 -462,16 -514,66 52,5

Table 6 Drilled segments measured from ground level of the W-23 well, overdrilled segments maked with red color


Drilling Depth ranges [m] Diameter [inch]

Segment 1 0 138 12 ¼

Segment 2 138 625,5 8 ¾

Segment 3 625,5 654,3 6 1/8

Segment 4 654,3 661,3 6 1/8

Table 7 Inner pipe casings measured from ground level of the W-23 well, planned filter pipe marked with red color


Inner pipe Depth ranges [m] Diameter [inch] Material

Inner casing 1 0 136 9 5/8 Stainless steel

Inner casing 2 0 609,5 7 Stainless steel

Filter pipe* 599,5 661,3 4 1/2 Stainless steel

*planned screened section measured from ground level: 625-646,3 m

Table 8 Originally perforated sections measured from ground level and status of cementation in well W-23


Depth ranges [m] Cemented

609,5 654,3 No

550 583 Yes

509 530 Partly

497,5 501 Yes

486,5 492 Partly



41

Figure 21 Layout of well W-23 with existing (marked with black) and planned sections (marked with red)

(author‟s own work) [22]



42







Volcanic rocks 0 245 231,93 -13,07 245

Clay/marl 245 600 -13,07 -368,07 355

Limestone 1 600,00 623,88 -368,07 -391,95 23,88

Limestone 2 623,88 676,38 -391,95 -444,45 52,5




Segment 1 0 16 17 ½

Segment 2 16 39 12 ½

Segment 3 39 600 8 ½

Segment 4 600 633,88 5




Inner casing 1 0 14,6 13 3/8 Stainless steel

Inner casing 2 0 592,5 5 1/2 Stainless steel

Filter pipe* 582,5 633,88 4 1/2 Stainless steel





563 600 No

544 556 Yes

521 534,5 No



43





44







Volcanic rocks 0 200 239,31 39,31 200

Clay/marl 200 706 39,31 -466,69 506

Limestone 1 706 729,88 -466,69 -490,57 23,88

Limestone 2 729,88 782,38 -490,57 -543,07 52,5




Segment 1 0 38 17 ½

Segment 2 38 360 12 ¼

Segment 3 360 706 7 5/8

Segment 4 706 739,88 5







Filter pipe* 655 739,88 4 1/2 Stainless steel





457 641 No



45





46

7. Groundwater modeling of the research area

A model intends to represent the processes take place in a given area. It

describes a system‟s relationships between its components providing a simplified

version of the real environment and phenomena takes place within via approximate

simulations (Bear and Cheng, 2010). In the present work the area introduced in Section

5 is modeled from the aspect of thermal energy extraction possibilities in “one

production, two injection wells setup in open-loop” circumstances.

7.1. Basic concept of modeling

For modeling flow phenomena and processes in the sample area Groundwater

Modelling System (GMS) 10.4.9 was used. The software made by Aquaveo LLC is

suitable for solving various environmental problems by creating 3D models. GMS can

replicate both groundwater and surface water phenomena with speed and simplicity by a

visualized interface. One of the software‟s advantage is that both hydrodynamic and

transport modeling tools are included. This way it is able to carry out underground

water simulations on a graphical level. The program consists of a number of code

embedded modules using finite element numerical methodology. From the supported

GMS codes the present work displays the utilization of two, the MODFLOW and the

MT3DMS/RT3D to carry out the overall heat transport modelling (Int. 4).

The analysis of groundwater flow conditions in the sample area was

implemented in MODFLOW-2005 version. The MODFLOW itself is a cell-centered,-

saturated flow model which can perform simulations in two ways, in steady state or in a

transient manner with a variety of user defined boundary conditions and inputs. Since

the aim of the recent work is the question of sustainability, the model itself has been

created in a transient way, with a one year long steady state period for stabilization,

revealing the time-dependence of the investigated transport processes. In the

MODFLOW “purely fluid” modeling part the well (WEL1) optional packages were

used, for simulating a pumping-injection well triplet. MODPATH is a particle tracking

code, displaying the results of MODFLOW on the level of water droplets movement

through the media forward or backward in time. The MT3DMS and SEAWAT codes

are both used to simulate heat transport of a certain area in conjunction with

MODFLOW. MT3DMS is capable to analyze chemical reactions and the processes of



47

advection, dispersion in groundwater systems. However the code can be implemented to

carry out heat transfer processes along the results of MODFLOW (Zheng and Wang,

1999) (Int. 4) (Int. 5).

7.2. GMS model buildup and parameters

The visualization of subsurface conditions was made from the side of

conceptual model approach using the simplified strata of well bores. The conceptual

model could draw a more realistic picture of the subsurface strata than the grid approach

and also gives the opportunity to handle complex (e.g.: space variant) characteristics in

a manageable manner. Due to the spatial distribution of the boreholes, additional

pseudo-wells (Pw) (Tab. 17) were created based on available information from the

target area and information of the surrounding bores. This step was necessary to

enhance the punctuality of interpolation throughout the sample area. From the chosen

wells, only three (W-23, W-10, W-13) are reaching the Eocene limestone layers.

Triassic age limestone layers are punctured by wells situated in the direction of S-SW,

relatively far from the sample area. Where the limestone aquifer layers are not present,

their depth and presence was estimated using trends of thickness form the existing

location information.

Table 17 Boreholes used at model build-up with potential production well marked blue,-and injection wells marked

with red color (author‟s own work)

Hole_ID Elevation top Depth Depth

[maBsl] [m] [maBsl]

D_23 192,14 706,8 -514,66

D_28 231,93 750,5 -444,45

D_35 239,31 759,5 -543,07

D_25 252,1 692,5 -791,93

D_26 236,13 748,5 -444,28

D_27 246,23 718,52 -440,25

D_29 231,86 731,88 -485,65

D_31 242,85 746,38 -514,52

D_32 232,22 763,37 -553,53

K_13 157,47 739,5 -564,16

K_10 172,00 772,5 -766,42



48

Table 18 Pseudo-wells used at model build-up for spatial distribution enhancement of the modeled area


Hole_ID Elevation top Depth Depth

[maBsl] [m] [maBsl]

Pw_1 170,00 723,92 -553,92

Pw_2 220,00 779,75 -559,75

Pw_3 220,00 745,29 -525,29

Pw_4 215,00 827,45 -612,45

Pw_5 220,00 701,59 -481,59

Pw_6 210,00 724,59 -514,59

Pw_7 200,00 740,92 -540,92

Pw_8 195,00 902,50 -707,50

The modeling was carried out as a combination of real time data respected to the

geological information with additional assumptions for lithological and hydrogeological

parameters resulting a four layered model with a planned well-triplet system (Fig. 24).

Figure 24 3D elevation subsurface model with marked selected potential wells (author‟s own work) [25]

7.2.1. Boundary conditions

The model area has been visualized as a 2700 x 2100 m grid, with 10 x 10 m

cells, consists of four layers, refined around the wells for a detailed outlook. The



49

available information was enough to determine the approximate head but was not

adequate to correlate hydraulic heads with each of the wells. Neither data is available,

no measurements happened to determine the static water level in the sample area,

therefore an average water level of +18,8 m is determined for the whole area according

to information from existing karstic thermal wells situated in the southwestern direction

from the sample area. To achieve continuous replenishment in the karstic aquifer layer

the boundaries of the model are set from active to fixed as specified heads (cell value =

-1) on the sides of the grid, setting up the initial boundary condition. Privileged

groundwater flow direction is unknown in neither the aquifer nor in the upper layers.

Therefore the only available information is that the aquifer is a pressurized, confined

one with a head above surface level (~210 maBsl) (Fig. 25).

Figure 25 2D view of the initial heads in the aquifer layer (author‟s own work) [26]

7.2.2. Lithological units and hydrogeological parameters

The geological information originating from the well books provided a strong

basis for determining the lithological units and locations in the subsurface area. These

strata were simplified and extended according to the experienced and expected trends

for a more viable modeling. As the well book data was “over-detailed” to be handled

correctly within a model, the occurred lenses and geological transitions were simplified,

merged together. Most of the analyzed wells are not reaching the depth of the target

media, the karstic limestone layers, explained in Section 4.4.2. Therefore, to reach the

target depth estimations were made according to the layer thickness trends of the



50

surrounding wells data. It was both essential from the aspect of required overdrilling

length in the potential wells and for a more punctual interpolation of the subsurface

geology. However plenty of bores are situated in the research area, most of them are

densely located. Therefore additional pseudo-wells (Pw) were created from the data of

the existing boreholes for a more accurate spatial extension of the subsurface geology.

As hydrogeological parameters were not included in the previous surveys, therefore

ranges of several literature data from the authors Freeze and Cherry, 1979; Heath and

Ralph, (1983); Vedat, (1998) Borden, 2015; Şen, (2015) was used for defining the

layers‟ properties (Tab. 1, Tab. 2, Fig. 9).

Table 19 Applied hydrogeological input parameters (author‟s own work based on Freeze and Cherry, 1979; Heath

and Ralph, (1983); Vedat, (1998) Borden, 2015; Şen, (2015))

Lithology Horiz.

cond.

Vert.

cond.

Hor./Vert.

anisotopy

Spec.

storage

Spec.

yield

Porosity

[m/d] [m/d] (-) (-) (-) (-)

Volcanic rock 8,64*10-1

8,64*10-2

1 5,00*10-5

0,2 0,11

Clay-marl 8,64*10-3

8,64*10-4

1 9,00*10-3

0,03 0,04

Limestone

(Eocene)

8,64 8,64*10-1

1 4,00*10-4

0,18 0,2

Limestone

(Triassic)

8,64*10-2

8,64*10-3

1 1,00*10-4

0,16 0,15

7.2.3. Groundwater exploitation

For investigating heat recovery via groundwater exploitation, a well-triplet was

established with two injections and,-one production well setup (Fig. 24 and 26).

Screening of the wells has been established in Eocene age limestone aquifer visualized

by layouts in Section 6.4. The exact locations of screened sections are cited in Table 20.

Based on existing and operating thermal karst water wells in the area and literature

information the potential limestone aquifer is karstified. On one hand, datasets from

operating thermal wells nearby (e.g.: W-10, W-13) confirm it, on the other hand the

geological history of the area also refers to the Eocene age limestone as a “well-

charred” rock (Pelikán, 2005). Knowing the distance of the wells, and other hydraulic

properties of the upper limestone body two options arose. One was to inspect the system

response by extracting and re-injecting in the same layer, and the other is to re-inject

one layer below. However the properties of the Triassic age limestone layers situated



51

below are poorly assessed. Therefore, according to the information about the two

distinct aged rock body the decision was made and the most feasible idea was to

implement a circulation system within the same Eocene age limestone layer (Fig. 26).

Figure 26 2D elevation map of the aquifer layer with the wells at the screened depth (author‟ own work) [27]

In order to assess the long-term sustainability of the exploitation system, five,

continuous operation periods were set with the pumping rates in Table 20. As the

aquifer layer is a highly porous,-and permeable one, the assumption were made that the

system withstands the same amount of re-injected water distributed between two wells

as it was withdrawn by one. The operation periods were specified as a one year long

steady state phase to stabilize the system, then operating (transient) throughout 5,-10,-

25,-50 years long, examining the head changes at different distances from the wells.

Table 20 Aquifer parameters of extraction and injection wells (author‟s own work)

Well_ID Aquifer thickness

[m]

Q

[m3/d]

Screened depth range

[maBsl]

Screened

length [m]

W_23 37,3 1000 -432,86 -454,16 21,3

W_28 23,8 1000 -373,07 -386,95 13,88

W_35 23,8 -2000 -471,69 -485,57 13,88

7.2.4. Thermal characteristics

The most crucial parameters at a hydrothermal system establishment are the

thermal conductivity ( ) and the specific heat capacity (c) (Section 2) of both the



52

geology and fluid phase within. The former one is referred as the ability of the material

to transport the energy meanwhile the latter is the ability to store it. A material‟s heat

capacity is an important element of investigating time dependence of a transient system.

Hard rocks have a thermal conductivity between 2-6 [W/mK] meanwhile highly porous

aquifers conductance varies in lower ranges. Water has lower thermal conductivity

values (0,6 [W/mK] at 293,15 K (20 °C)), but higher (4,187 [kJ/kgK]) specific heat

capacity than rocks (0,75-0,85 [kJ/kgK]) (Stober, 2013). Therefore water poorly

conducts heat but has an increased storing capacity. The thermal conductivity of water

also increases with the temperature to a maximum of 140-150 °C depending on the

pressure. The density of rocks and water varies due to the surrounding pressure and

temperature circumstances. The P-T dependence of rock density is normally neglected

at hydrothermal planning. Meanwhile the density of water is decreasing accordingly to

the rising temperature (Stober, 2013).

The heat transport model is considering a constant groundwater temperature of

54,6 °C and and a 25°C injected water (Tab. 21). The heat transport model simulates 1,-

5,-10,-25,-50 years of operation with a constant rate of production and injection.

As it was mentioned above the heat flow modeling was carried out in MT3DMS code

using the following packages: chemical reaction, advection, dispersion, Source/Sink

Mixing simulating the heat as a solute in the system by the similarities of the transport

equations. The chemical reaction package applies a linear sorption isotherm, where the

thermal coefficient (Kd) expressed as (Giambastiani, et al. 2013):

(14)

where

cs: specific heat capacity of the solid [J/kg/K]

cf: specific heat capacity of the fluid [J/kg/K]

ρf: density of the fluid [kg/m3]

The thermal retardation (R) is given (Giambastiani, et al. 2013):

(15)

where



53

: effective porosity of the fractured medium [-]

: bulk density of the fractured medium [kg/m3]

: thermal distribution coefficient [m3/kg]

The thermal diffusivity (DT) is defined as (Giambastiani, et al. 2013):

(16)

where

λb: bulk thermal conductivity [W/m/K]

λs: solid phase thermal conductivity [W/m/K]

λf: fluid phase thermal conductivity [W/m/K]

To get a model which is more realistic to the current geology calculated data were used

from the PetroMod software for the temperature, heat capacity, thermal conductivity of

the rock body, effective porosity and bulk density. The rest of the used values were

chosen from the ranges of Table 1-3 and Figure 9 for each property.

Table 21 GMS modeling heat transport input parameters (author‟s own work)

Parameter Value

Exploited water temperature [C°] 61,59

Reinjected water temperature [C°] 25

Initial avarage temperature in the reinjected points [C°] 54,66

Effective thermal cond. of the fractured media [W/m/K] 2,63

Thermal distribution coefficient [m3/kg] 5,17*10

-4

Thermal diffusivity [m2/s] 1,25*10

-6

Longitudinal dispersivity [m] 0,5

Horizontal transverse dispersivity [m] 0,1

Vertical transverse dispersivity [m] 0,1

Dry bulk density [kg/m3] 2079,26

Specific heat capacity of the solid [J/kg/C°]] 2133,84

Specific heat capacity of water [J/kg/K] 4138

Retardation factor 3,94



54

7.2.5. Observation wells arrangements

The monitoring points were set up systematically in equidistant points between

the three wells. The specific distances were measured, distributed denser around the

wells then becoming more distant as moving away towards another (Fig. 27). Table 22

presents the exact positions of observations points showed in Figure 27. Furthermore,

helps to understand and place the results of Section 8 in space (Fig 29-31) (Fig. 35-37).

Figure 27 Cropped 2D view of observation well locations in the aquifer layer (author's own work) [28]

Table 22 Observation well distances (author‟s own work)

Distances 5 m 20 m 50 m 310 m 312 m 415 m

Ob

serv

ati

on

wel

l

nu

mb

er

Ow_35_1 Ow_35_3 Ow_35_5 Ow_35_7

Ow_35_2 Ow_35_4 Ow_35_6

Ow_28_1 Ow_28_3 Ow_28_5 Ow_28_7

Ow_28_2 Ow_28_4 Ow_28_6

Ow_23_1 Ow_23_3 Ow_23_5 Ow_23_7

Ow_23_2 Ow_23_4 Ow_23_6



55

8. Results of transport modeling

8.1. Groundwater modeling

The groundwater modeling was carried out as described previously in Section

7. The results of head changes are plotted for each well in the observation locations

specified in Section 7.2.5. The results of groundwater modeling (Fig. 29-31) shown that

the heads stabilize after 1 year of operation and remain constant with the same rate of

exploiting and re-injecting throughout 50 years period (Fig. 28). Therefore it can be

stated that circulating the same amount of water in such pressurized aquifer layer under

relatively constant conditions results in a long-term sustainable system.

Figure 28 2D view of the stabilized heads in the aquifer layer at the end of 50 years operation period (author‟s own

work) [29]

The cone of depression showed in Figure 28 in case of injection well W-28 is

greater than in injection well W-23, which is caused probably due to the shorter (∆L=-

14,5 m) screen lenth. The effect of wells on each other is clearly visible on the shape of

the equipotential circles around the wells (Fig. 25), which changes in time and space are

quantified in the following plots. The head changes at multiple distances were assessed

and showed by Figure 29-31.



56

Figure 29 Head changes in the vicinity of injection well W-23 (author‟s own work) [30]

Figure 30 Head changes in the vicinity of injection well W-28 (author‟s own work) [31]



57

Figure 31 Head changes in the vicinity of production well W-35 (author‟s own work) [32]

As it was mentioned previously the heads remain constant at specific distances through

the whole operation period. The initial heads in the confined aquifer were set to a level

of 210 maBsl, the amount of water extracted and re-injected is the same. As it is shown

in the injection wells (Fig. 29-30) the highest changes in the head are expected at 5 m

distance from the wells. In case of well W-28 the head increase at the closest

monitoring (213,6 maBsl) point is 1,5 m higher than in case of well W-23 (212,1

maBsl). This difference may be caused by the longer screened section of well W-23.

This difference in head decrease with increasing distance from the wells, as the effect of

injection is inversely proportional to the distance. At 50 m the difference is reduced to

0,5 m. At 415 m from well W-23, the head sets back to the initial one, meanwhile at 312

m from the well W-28 the effect of production in well W-35 results in a decrease of 0,5

m compared to the initial head values. As for the injection wells, the greatest up-coning

(7,4 m) can be expected at the closest (5 m) observation point from the extraction well

(Fig. 31). The head values are increasing towards the injection wells and at the furthest

observation point (310 m) from the extraction point the heads (209,5 maBsl) are almost

return back to the starting head value. In case of injection well W-23 (Fig. 29) its effect

reduces to zero at the distance of 415 m from the well. According to Figure 30 the

production well loses its effect after approximately 310 m from the well itself (in both

directions towards the injection wells). From the results of the groundwater modeling

the system is hydrodynamically stable and sustainable.



58

To assess particle movement from the both sides of production and injection,

MODPATH tracking package was implemented. The access times are equal to the

length of operational periods visualizing the paths of water in the aquifer layer

separately for exploitation and re-injection wells (but running simultaneously). The

tracks of particles for each time-steps (1,-5,-10,-25,-50 years) are showed in Annex 3.

Not all these cited arrival times are carrying great importance for the subsequent heat

transport modeling. Keeping sustainability in mind, the less a system cools back by re-

injection of the cold water the more sustainable it is. Therefore, knowing when the

injected (in this case cold) particles reach the production well could give a good

estimation for a long-term sustainability from the side of heat transport phenomena. The

time of reach, called breakthrough time showed in Figure 32-33. According to the

groundwater model‟s calculations, with 2000 m3/day discharge and recharge rate, the

injected (25°C temperature) water reaches the extraction well in 1600 days time.

Figure 32 2D view of 1600 days breakthrough of the

injection wells (author‟s own work) [33]

Figure 33 2D view of 1600 day breakthrough of the

production well (author‟s own work) [34]

8.2. Heat transport modeling

The heat transport modeling was carried out as described in Section 7 using the

calculated results of PetroMod through the previously modeled groundwater movement.

The temperature distributions are plotted for each well in the same observation locations

introduced on Fig. 27. The operational circumstances were the same in both cases, but

the results of heat transport (temperature distribution) shows a more varied/less stable

outcome in time and space (Annex 4) than in case of the former groundwater model. As

it was mentioned, the amount of circulated water is the same, but with attached heat

parameters. As the system was considered to be hydrodynamically stable, a constant



59

natural replenishment of 54,7 °C hot water was assumed (Tab. 20) through the 50 years

operation period. The heat transport of the subsurface aquifer media was performed

with and exploited 61,6 °C through well W-35, and a cooled down re-injected 25 °C

water through well W-23 and W-28 (Tab 21). All three screened sections in the wells

are situated in the same aquifer layer but the exploitation takes place at a greater depth

than the injection as the presumed limestone layers are tilted in a southeastern direction

(Fig. 26). As the injection wells are situated approximately at the same depth but with a

different screen length the temperature of 54,7 °C were determined as an average

temperature in the rock body around them (Tab. 21). The groundwater model showed

that the first injected particles reach the production point in 1600 days. This may lead to

the perquisite conclusion that with a constant rate of operation, the system would cool

down in a relatively short time and the long-term feasibility cannot be achievable.

Figure 34 2D view of 10800 days heat breakthrough of the injection wells (author‟s own work) [35]

As mentioned above the heat transport phenomena were assessed though 50 years,

which results are showed in Annex 4. From the aspect of feasibility, the most significant

information is provided by the reach out times of cold water plumes. As Figure 32

shows the injected particles reach the point of exploitation in less than 5 years, the

effect of cold water only appears in 10800 days (~29,6 years). It means that however the

first particles appear earlier, the temperature decrease in the exploited water expected

after 30 years of operation. The life expectancy of a geothermal system highly depends

on the manner of utilization and environmental circumstances. The average lifespan of



60

heat pump systems is usually over 20,-somewhere between 25-50 years (Int. 6). The

observation locations were the same for both modeling, logging different parameters

(Tab. 21). In Figure 35-37 the different subsurface temperature distributions are plotted

showing the time and space effect of the injected cold water.

Figure 35 Temperature distribution in the vicinity of injection well W-23 (author‟s own work) [36]

Figure 36 Temperature distribution in the vicinity of injection well W-28 (author‟s own work) [37]



61

Figure 37 Temperature distribution in the vicinity of production well W-35 (author‟s own work) [38]

The injection wells (Fig. 35 and 36) shows that the plume of injected cold water

simultaneously advances with time. The rate of diffusion is different in the mentioned

two wells, faster in well W-28 and slower in well W-23. The probable reason for this is

the length of screened sections, as it was previously mentioned in Section 8.1. It shows

that in well W-23 the effect of longer screened length results in a more distributed

temperature in the vertical direction around the well, which causes a less focused,

slower cool down effect. This phenomena also affects the horizontal plume extent and

of well W-23 and the breakthrough time in case of both models (Fig. 32 and 34). One of

the observation points namely Ow_28_3 (Fig. 35) shows a sight areal cool down, which

reason that the plume of re-injected water is dragged towards the production well lesser

reaching this point. The plot of the extraction well W-35 (Fig. 37) is much more

variable, representing how the plumes reach the field of exploitation and how it cools

down by time. At the point of breakthrough (Fig. 35) the value of extractable water

temperature according to Ow_35_1 and Ow_35_2 (Fig. 37), drops down to ~50 °C then

it slowly cools down to the initially re-injected cold water temperature of 25 °C till the

end of operational period of 50 years (Annex 4). Therefore the findings prove that the

assumed system with such reservoir characteristics and a constant rate of circulation

leads to a relatively stable operation through a 30 years period.



62

9. Conclusion

The basic premise of the current work was the revitalization idea of former oil

producer wells in a mountainous region. Based on both international trends and also on

the assessed country‟s potential resources, the possibility of geothermal utilization was

chosen. Well books from 1980‟ provided the initial information about mentioned wells

in the area. From these well logs, according to their details 14 datasets were chosen for

further inspections. The geological information allowed mapping the subsurface regions

to understand the hydrogeological, hydrodinamical environment. For this assessment

geological cross-sections were created, compared with several authors‟ work and

publications to verify the reliability of first ascertainments. The first findings showed

that below a volcanic cap rock (dominantly tuff) several hundred meters thick clay,-

marl assemblages are situated. The primary goal was to explore the potential of the

latter strata. To reach it the next step was to investigate the temperature conditions of

these layers. The perquisite investigations implemented through calculating with the

area‟s average geothermal gradient for a rough preliminary estimation. The results

showed that the temperatures could reach up to 40-50°C in compacted clay,-marl layers.

The next phase was finding the most adequate technique to exploit and utilize this heat.

According to the temperature distribution and system practices, several techniques came

into consideration.

The first and most evident due to the properties of clay and relatively low

enthalpy medium, the possibility of a borehole heat exchanger was considered. After the

assessment of the wells‟ structures, it showed that according to the well book data has

telescopic structure with 2-3 casing sections, with cementing around them. The

thickness of cement coating varies due to each section‟s diameters but trying to achieve

a heat transfer between two media through the these double layers could lead to

significant heat loss. Due to these facts the implementation of Deep Borehole Heat

Exchanger system discarded. Enhanced Geothermal Systems are operating with a

natural or semi-natural fractured environment and working fluid. According to the low

temperatures, and the high initial costs of creating a fracture system in thick,-compacted

sediments did not found to be financially worthwhile. A pre-assessment dated back to

2018 showed that three of the examined 14 wells are not reclaimed yet. Studies and

works about this region proved that the Oligocene age clay,-marl strata is followed by



63

Eocene age well charred limestone layers then Triassic age limestone assemblages. The

latter one is poorly assessed, but both the properties and location of Eocene age

limestone strata is known by several drillings and functioning thermal wells in the area.

Therefore it was chosen for the aquifer medium of a preplanned well circulation system.

Well circulation systems have two major types. One is the Single Well

Circulation System, requires the same well to circulate (exploit and re-inject) water in

the aquifer. In addition to its many benefits, a significant areal extent of fractured or

porous media is needed for a long-term sustainability. According to the subsurface

information, the average thickness of the Eocene age limestone aquifer is an

approximate 23,8 m, which considered to be too thin for a proper establishment and

eventually the re-injected water cools down its environment in a relatively short time.

The other type of circulation systems is called Dual Well Circulation System, due to its

name, using separate wells for extraction and re-injection. Due to the spatial distribution

of the available non-reclaimed wells and known features of the aquifer media the latter

system implementation was chosen. Except of well W-23 the potential considered wells

are terminated in the clay,-marl strata, therefore overdrilling of the existing wells is

required to reach the limestone layers below. To visualize the initial and overdrilled

sections, modified and completed well plans were created in AutoCAD software.

After the perquisite statements, the next step was building the environment of

modeled area. For this purpose, the existing well log data were extended by the layer

trends among the area and additional pseudo wells were fabricated from the information

of nearby wells. The latter was required because of the adequacy of spatial distribution

of the wells. These lithology sequences were simplified to four horizons, for a better

implementation of the subsequent groundwater modeling. The well books did not deal

with the hydrological and geothermal characteristics of the rock bodies. Therefore,

endeavoring for the most accurate representation of the subsurface conditions

preliminary calculations were made by PetroMod software and its results were

correlated with literature and estimated data of the area. This way every input parameter

was available to carry out both groundwater and heat transport modeling by GMS

groundwater modeling tool. The modeling‟s goal was to assess the long-term feasibility

of a well-triplet system with one production and two injection well setup in the same

aquifer layer. The examined (transient) operation period lasted for 50 years, starting



64

with a one year steady-state condition for the sake of stabilization, observed in 1,-5,-10,-

25,-50 years time-steps. The operation considered a constant rate of 2000 m3/day

extracted and injected (1000-1000 m3/day) water amount. The MODFLOW

groundwater modeling results showed that under the cited circumstances, the model set

to a stable and hydrodynamically sustainable state until the end of the operation.

However the system considered to by hydrodinamically feasible, the MODPATH

particle tracking results showed that the first re-injected water droplets reach the

exploitation point in a relatively short 1600 days (~4,4 years) time. Knowing this

breakthrough time led to the conclusion that the system may not be long-term

sustainable and the extracted, approximately 60°C temperature water will be cooled

down way too early by the 25°C temperature re-injected water. The implemented

MT3DMS heat transport model refuted this claim. The results showed that however the

re-injected particles reach the extraction point less than 5 years time, but the cool down

effect appears after 30 years of constant operation. The average life expectancy of a

geothermal system is over 20 years, for example a heat pump systems‟ lifespan vary

between 25-50 years, depending on the operational and environmental circumstances.

10. Future plans

The present study investigated the feasibility of a three-welled circulation

system without taking into account the effects of the actual operational variability. For

example, utilizing the present system connected to the utility network via a heat

exchanger, several, multistep losses and operational difficulties could occur. As every

system and utilization practice has its own specificity, the operation parameters must

also be adjusted accordingly. In case of realistic simulation, implementing the modeled

system for a referred heating purpose in a conjoined manner, every fluctuation of the

network would be specified and taking into account. From the modeling side, it means

an adaptation to the heating needs (increased during winter) including different

production periods caused by seasonal variability. Introducing such periodicity into the

operational conditions could have either a positive or negative feedback on the system

and its environment. The previous example allows a more realistic sustainability study

for a karst environment based heating system to be tested.



65

11. Summary

The basic premise of the current work was the revitalization idea of former oil

producer wells in a mountainous region. Based on both international trends and also on

the assessed country‟s potential resources, the possibility of geothermal utilization was

chosen. The first part of this recent work was a preliminary assessment of the sample

area for future studies and potentials to be explored. Knowledge of geological strata and

thermodynamic relationships is essential for setting up heat transport models. Therefore

it meant the starting point and preparations of this endeavor. The layer sequences were

simplified in terms of later model-level (simpler) manageability.

The ultimate target of the study was to build a hydrodynamic model and assess

the temperature potentials of a region with such geological and hydrogeological setup in

terms of future utilization. The first results showed that beneath a volcanic cap rock

thick clay,-marl assemblages are situated which temperatures can reach up to 40-50°C.

Due to the construction of the potential wells the establishment of a deep borehole heat

exchanger system has been discarded due to the significant heat loss through the piping.

The temperature are also considered too low, for the initial costs of an enhanced

geothermal system as neither fracture system nor free fluids are available at that zone.

However single well circulation system is both space-saving and feasible technique the

aquifer layer extent not considered to be adequate for a long-term operation. Examining

all the possibilities the establishment of a dual well circulation system in a form of a

well-triplet was decided. From both side of well planning and modeling some

information were available in a form or well books which was correlated and

supplemented with literature and perquisite calculations. As most of the wells are not

reaching the potential considered aquifer layers, overdrilling is required on the existing

wells. The original and planned sections were visualized by layouts.

Groundwater modeling of the area showed that exploiting and re-injecting the

same amount of 2000 m3 of water on a daily basis through 50 years time, results in a

hydrodynamically stable and sustainable system within the aquifer layer. The heat

transport modeling revealed that with the same amount of extracted, (approximately)

60°C,-and re-injected 25°C water the system is long-term feasible up to 30 years time

without resting periods. Even so it is sustainable for a relatively long time

implementation of resting phases is highly recommended since through the operating



66

wells are distant and not directly affects each other‟s performance however it may cause

irreversible changes in their environment or other wells nearby. The current work did

not cover the assessment of set aside periods as it is strongly depends on the purpose of

geothermal energy use. The lifetime of the system probably elongates if depletion by

overexploitation is avoided and the time is given for the natural replenishment processes

to work. The present study intended to point out the identification of the knowledge

gaps where scientific approaches and modern technical solutions could be implemented.

The results of similar endeavors like the current work makes adaptation easier for

incoming unusual conditions and helps to achieve sustainable water management goals

in the future (Báldi, et al. 2020).



67

Acknowledgement

I am thankful for all the help provided by the Department of Hydrogeology. I would

like to thank Dr. Péter Szűcs for the opportunity to be a part of this new endeavor. I

would also like to thank the tireless help of my institutional supervisor Rita Miklós, and

to Dr. Balázs Kovács for sharing his unique expertise in groundwater modeling. Special

thanks to Dr. Viktor Mádai for the idea and advices of PetroMod software use which

advanced the solution to the problem. I am grateful to the MBFSZ for the well books

that are a cornerstone of the present research study.

The research was carried out in the framework of the GINOP-2.3.2-15-2016-00010

„Development of enhanced engineering methods with the aim at utilization of

subterranean energy resources‟ project of the Research Institute of Applied Earth

Sciences of the University of Miskolc in the framework of the Széchenyi 2020 Plan,

funded by the European Union, co-financed by the European Structural and Investment

Funds.



68

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List of figures

[1] Different heat pumps setups in case of a shallow heat source

(https://www.energy.gov/energysaver/heat-and-cool/heat-pump-

systems/geothermal-heat-pumps)

[2] Operational flow chart of a heat pump (Rees, 2016)

[3] T-S diagram of a heat pump (Ian Staffell et al. 2012)

[4] U-pipe, coaxial type of collector pipes (Scorpo, 2013)

[5] U-pipe, coaxial type of collector pipes 2 (Silwa, 2014)

[6] Two-well circulation,- and single-well extraction systems (Wu et al. 2014)

[7] Standing column,- and single well circulation systems (Wu et al. 2014)

[8] Schematic setup of a doublet system (Stober, 2013)

[9] Schematic drawing of a single-well circulation system (Pap, 2020)

[10] Permeability and conductivity ranges for different type of lithologies

(Freeze and Cherry, 1979)

[11] Location of the examined wells and geologic cross sections (author‟s own work)

[12] A1 geological cross section (SE-NW) (author‟s own work)

[13] Temperature distribution in the A1 cross-section (SE-NW) (author‟s own work)

[14] A2 geological cross-section (S-NE) (author‟s own work)

[15] Temperature distribution in the A2 cross-section (S-NE) (author‟s own work)



79

[16] PetroMod results of well W-23 for variable physical parameters by depth






[19] Correlation between PetroMod and Calculated temperature data well W-23






[22] Layout of well W-23 with the existing (marked with black) and planned sections

(marked with red) (author‟s own work)





[25] 3D elevation subsurface model with marked selected potential wells (author‟s

own work)

[26] 2D view of the initial heads in the aquifer layer (author‟s own work)

[27] 2D elevation map of the aquifer layer with the wells at the screened depth


[28] Cropped 2D view of observation well locations in the aquifer layer (author's own

work)

[29] 2D view of the stabilized heads in the aquifer layer at the end of 50 years

operation period

[30] Head changes in the vicinity of injection well W-23 (author‟s own work)

[31] Head changes in the vicinity of injection well W-28 (author‟s own work)

[32] Head changes in the vicinity of production well W-35 (author‟s own work)

[33] 2D view of 1600 days breakthrough of the injection wells (author‟s own work)

[34] 2D view of 1600 days breakthrough of the production wells (author‟s own work)



80

[35] 2D view of 10800 days heat breakthrough of the injection wells (author‟s own

work)

[36] Temperature distribution in the vicinity of production well W-23 (author‟s own

work)


work)


work)



81

Annex

Annex 1 Well book data for each borehole (marked with black) and pseudo-wells with

the estimated coninuation of the sequences marked with red color

Materials Volcanic rock Clay,

marl

Eocene

age

limestone

Triassic

age

limestone

Hole_ID

Top

[maBsl]

Bottom

[maBsl]

Bottom

[maBsl]

Bottom

[maBsl]

Bottom

[maBsl]

W_23 192,14 67,14 -424,86 -462,16 -514,66

W_28 231,93 -13,07 -368,07 -391,95 -444,45

W_35 239,31 39,31 -466,69 -490,57 -543,07

W_24 240,57 -6,43 -720,43 -739,43 -791,93

W_25 252,10 76,10 -367,90 -391,78 -444,28

W_26 236,13 -68,87 -363,87 -387,75 -440,25

W_27 246,23 65,23 -409,27 -433,15 -485,65

W_29 231,86 151,86 -438,14 -462,02 -514,52

W_31 242,85 34,85 -477,15 -501,03 -553,53

W_32 232,22 -13,78 -487,78 -511,66 -564,16

W_34 209,96 -10,04 -690,04 -713,92 -766,42

W_13 157,47 -7,53 -587,93 -600,53 -653,03

W_10 172,00 -25,50 -498,00 -524,50 -577,00

Pw_1 170,00 -4,17 -476,67 -501,42 -553,92

Pw_2 220,00 38,75 -487,70 -507,25 -559,75

Pw_3 220,00 17,50 -451,80 -472,79 -525,29

Pw_4 215,00 12,05 -535,78 -559,95 -612,45

Pw_5 220,00 69,50 -398,50 -429,09 -481,59

Pw_6 210,00 109,50 -431,50 -462,09 -514,59

Pw_7 200,00 -1,03 -469,27 -488,42 -540,92

Pw_8 195,00 -10,08 -632,75 -655,00 -707,50



82

Annex 2 Input data for the PetroMod model calculations

W-23 well PetroMod modeling input parameters (author‟s own work)

Top

[m]

Bottom

[m]

Lithology

[-]

Age

[myrs]

Paleo

Water

Depth

[m]

Erosio

n [m]

Heat flow

[mW/m2]

0 44,41 Tuff Miocene 5,3 0

44,41 44,41 Eroison 11,6 0 -80,59

44,41 125 Tuff 12,6 0 80,59

125 617 Clay, marl Oligocene 23,03 0

617 654,3 Limestone Eocene 33,9 100 38

654,3 654,3 Eroison Paleocene 56 0 -1500 32

654,3 654,3 Hiatus Jurassic 174,1 6500 50

654,3 706,8 Limestone Triassic 201,3 1000 1500 33

201,83 1000


Top

[m]

Bottom

[m]

Lithology

[-]

Age

[myrs]

Paleo

Water

Depth

[m]

Erosio

n [m]

Heat flow

[mW/m2]


87,01 87,01 Erosion 11,6 0 -158

87,01 245 Tuff 12,6 0 158




623,88 623,88 Hiatus Jurassic 174,1 6500 50


201,83 1000


Top

[m]

Bottom

[m]

Lithology

[-]

Age

[myrs]

Paleo

Water

Depth

[m]

Erosio

n [m]

Heat flow

[mW/m2]


87,01 87,01 Erosion 11,6 0 -158

87,01 245 Tuff 12,6 0 158




623,88 623,88 Hiatus Jurassic 174,1 6500 50


201,83 1000



83

Annex 3 Groundwater modeling results, MODPATH particle tracks for 1,-5,-10,-25,-50

years times of operation

2D view of 1 year pathlines of the injection wells


2D view of 1 year pathlines of the production well


2D view of 5 years pathlines of the injection wells


2D view of 5 years pathlines of the production well








84









Annex 4 Heat transport modeling results, subsurface temperature distribution in 1,-5,-

10,-25,-50 years times of operation

2D view of 1 year heat radation of the injection wells


2D view of 5 years heat radation of the injection wells




85

2D view of 10 years of heat radation of the injection

wells (author‟s own work)

2D view of 25 years of heat radation of the injection

wells (author‟s own work)

2D view of 50 years of heat radation of the injection wells (author‟s own work)

Documents

Geothermal energy utilization by revitalization of