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Faculty of Earth Sciences

University of Iceland

2014



2014

Interpretation of formationtemperature and pressure of wells inthe Þeistareykir geothermal �eld

Garðar Gíslason

INTERPRETATION OF FORMATION

TEMPERATURE AND PRESSURE OF WELLS IN

THE ÞEISTAREYKIR GEOTHERMAL FIELD

Garðar Gíslason

10 credit ECTS thesis submitted in partial ful�llment of aBachelor Scientiarum degree in geophysics

Advisor

Sigríður Sif Gylfadóttir


School of Engineering and Natural Sciences


Reykjavik, February 2014

Interpretation of formation temperature and pressure of wells in the Þeistareykir geothermal�eld10 credit ECTS thesis submitted in partial ful�llment of a B.Sc. degree in geophysics

Copyright© 2014 Garðar GíslasonAll rights reserved

Faculty of Earth SciencesSchool of Engineering and Natural SciencesUniversity of IcelandÖskju, Sturlugötu 7101, ReykjavikIceland Telephone: 525 4600

Bibliographic information:Garðar Gíslason, 2014, Interpretation of formation temperature and pressure of wells in theÞeistareykir geothermal �eld, B.Sc. thesis, Faculty of Faculty of Earth Sciences, University ofIceland.

Printing: Háskólaprent, Fálkagata 2, 107 ReykjavíkReykjavik, Iceland, February 2014

Abstract

For best results of geothermal development and exploration of a well, a thorough under-standing of the natural undisturbed state is vital. Temperature and pressure logs fromseven geothermal wells, measured during various stages of the wells were interpreted toestimate formation temperature and initial pressure. The Horner plot method is ex-plained and then used to aid analytical estimation of the formation temperature. Theaccuracy and applicability of the Horner plot method can vary and hence it is importantto interpret the data with respect to di�erent parameters.

The highest bottom well temperatures were estimated in the wells ÞG-1, ÞG-5 and ÞG-9at 340°C, 341°C and 345°C respectively. All wells with the exception of ÞG-8 are viablefor utilization and they seem to be in good contact with the reservoir underneath theÞeistareykir area.

Útdráttur

Bestu niðurstöður jarðhitarannsókna frá borholum fást með því að hafa vitneskju umótru�að ástand þeirra. Mælingar á hitastigi og þrýstingi í sjö háhitaborholum eru túlk-aðar til að meta berghita og upphafsþrýsting borholanna. Mælingarnar voru gerðar ámismunandi áföngum borholanna og var Horner plot aðferð notuð til að hjálpa til viðtúlkun og mat á berghita. Nákvæmni og nothæfni Horner plot aðferðarinnar er mis-munandi og er því mikilvægt að túlka gögnin með tilliti til ýmissa breytistærða.

Hæsta hitastig neðst í borholunum var fundið í holum ÞG-1, ÞG-5 og ÞG-9 um 340°C,341°C og 345°C hver um sig. Raunhæfur möguleiki er á því að nýta allar borholurnarað frátöldum ÞG-8 til frekari orkunýtingar eins og hefur verið gert. Að auki virðastborholurnar fyrir utan ÞG-8 vera vel tengdar jarðvarmageyminum undir Þeistareykjum.

Hér með lýsi ég því y�r að ritgerð þessi er samin af mér og að hún hefur hvorki að hlutané í heild verið lögð fram áður til hærri prófgráðu.

Garðar Gíslasonkt. 311090-2599

Contents

List of Figures vii

List of Tables x

Abbreviations xi

Acknowledgments xii

1. Introduction 1

2. Methodology 3

2.1. Temperature interpretation . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2. Pressure interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3. Wells drilled in Þeistareykir 9

3.1. ÞG-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.2. ÞG-4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.3. ÞG-5B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.4. ÞG-6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.5. ÞG-7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.6. ÞG-8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.7. ÞG-9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4. Conclusion 23

References 24

Appendix 26

A. Matlab codes 26

A.1. Temperature and pressure plots . . . . . . . . . . . . . . . . . . . . . . . 26A.2. Horner method for changeable circulation time . . . . . . . . . . . . . . . 28A.3. Horner subplots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

B. Data 31

vi

List of Figures

1.1. Tectonic map of Iceland. Location of Þeistareykir (Sæmundsson 1986) . . 1

2.1. Temperature pro�les in wells during or right after drilling (Steingrímsson2013) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2. Theoretical Horner temperature buildup (Dowdle and Cobb, 1975) . . . . 5

2.3. Horner temperature graph for a well with a 2,4 hour circulation time (Dow-dle and Cobb, 1975). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.4. Horner temperature graph for a well with a 50 hour circulation time (Dow-dle and Cobb, 1975). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.5. Horner plot for formation temperature estimation using data from Table 2.1 7

2.6. Pivot point in pressure log during warm-up of well ÞG-4. . . . . . . . . . 7

3.1. Location of the wells and their tracks . . . . . . . . . . . . . . . . . . . . 9

3.2. Warm-up temperature pro�les of ÞG-1, 2002. . . . . . . . . . . . . . . . 11

3.3. Recovery temperature pro�les and EFT of ÞG-1, 2003-2004. . . . . . . . 11

3.4. Horner plot at various depths for ÞG-1 using data from Figure 3.2. . . . 11

3.5. Pressure pro�les and initial pressure estimation for ÞG-1. . . . . . . . . . 12

3.6. Warm-up temperature pro�les and EFT of ÞG-4. . . . . . . . . . . . . . 13


3.8. Horner plot at various depths for ÞG-4. . . . . . . . . . . . . . . . . . . . 13

vii

3.9. Pressure and injection rate of drilling �uid in ÞG-5. Well length is dis-played on the x-axis (Ingimarsdóttir et al., 2009). . . . . . . . . . . . . . 14

3.10. Warm-up temperature pro�les and EFT of ÞG-5B. . . . . . . . . . . . . 15

3.11. Pressure pro�les and initial pressure estimation for ÞG-5B. . . . . . . . . 15














B.1. Formation temperature estimate of all wells. . . . . . . . . . . . . . . . . 31

B.2. Initial pressure estimate of all wells . . . . . . . . . . . . . . . . . . . . . 31

B.3. Temperature pro�les from ÞG-1 . . . . . . . . . . . . . . . . . . . . . . . 32


B.5. Temperature pro�les from ÞG-5B . . . . . . . . . . . . . . . . . . . . . . 34


viii




ix

List of Tables

2.1. Data for Horner plot at 2160 m TVD in well ÞG-9. . . . . . . . . . . . . 6

3.1. Summary of the wells in Þeistareykir. Coordinates are given in ISNET93coordinates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

x

Abbreviations

ÍSOR - Iceland GeoSurveyROS - Research department of the National Energy AuthorityMatlab - Matrix Laboratorym a.s.l. - meters above sea levelTVD - True Vertical DepthBPD - Boiling Point with depthEFT - Estimated Formation Temperature

tk - circulation time, hours∆t - shut-in time or time after circulation ceases until measurement, hours

xi

Acknowledgments

I would like to thank my supervisor, Sigríður Sif Gylfadóttir, for her guidance and supportand for entrusting me with this project. Her understanding and teaching proved vital tothe success of this project.

Special thanks go to my family and friends who supported me through my academicyears. I am forever grateful to my parents, Anna and Gísli for their endless love andpatience. Their parenting skills and guidance led me to the writing of this thesis.

I would also like to extend my gratitude to Landsvirkjun for granting me the permissionto use the documents and data related to the project.

xii

1. Introduction

Situated on the Mid-Atlantic Ridge, Iceland is one of the most active geothermal areasin the world. There are two main causes for the geothermal activity in Iceland. They arethe mantle plume underneath Iceland which causes increased volcanic activity and the ge-ologic rift between the Eurasian plate and the North American plate which is responsiblefor crustal movement and therefore earthquakes and increased volcanic activity.

Figure 1.1: Tectonic map of Iceland.Location of Þeistareykir (Sæmundsson 1986)

Þeistareykir is a high temperature geothermal �eld located in a �ssure swarm in NortheastIceland shown in Figure 1.1 (Sæmundsson, 1986). The area is covered in lavas, theyoungest one being around 2700 year old. For centuries it was the main sulfur mine inIceland, providing the Danish king with raw material for gun powder.

1

Extensive research on the Þeistareykir geothermal �eld has been carried out ever sincethe �rst geothermal exploration was done there in 1972-1974 (Grönvöld and Karlsdóttir1975). In 1981-1984 a major geothermal assessment was made (Layugan 1981, Gíslasonet al. 1984, Ármannson et al. 1986, Darling and Ármannson 1989) and the area was thenmonitored periodically from 1991 to 2000 (Ármannsson et al., 2000).

In 2000, Gautason et al. suggested drill sites based on available knowledge. Preparationswere immediately made and the �rst well was drilled in 2002, the second in 2003 and asof 2012, nine deep exploration wells have been drilled in the area (Ármannson, 2012).The wells are all drilled for Þeistareykir Ltd. which is now owned by Landsvirkjun.

The fundamental objective of temperature and pressure logging in a geothermal inves-tigation is to accurately determine formation temperature and reservoir pressures. Asa result of cold drilling �uid circulation, the geothermal wells and nearby formationsundergo cooling. The pressure and temperature disturbances fade away gradually afterdrilling has ceased. After several months the wells will reach thermal equilibrium withits surroundings and the pressure will reach equilibrium with the permeable feed zonesof the well.

Wells or boreholes provide essential access deep into geothermal systems that would notbe otherwise possible. They are important in both geothermal information gatheringand utilization and are becoming the main instruments of geothermal development. Lastcentury we've seen signi�cant breakthrough of increased geothermal utilization and muchimproved understanding of geothermal systems. Wells enable vast increase in geothermalenergy extraction compared to natural out-�ow (Axelsson and Steingrímsson 2012).

The goal of this project is to analyze and interpret temperature and pressure logs fromwells in Þeistareykir geothermal �eld and hence estimating the formation temperatureand initial pressure.

2

2. Methodology

2.1. Temperature interpretation

For quality interpretation of data from geothermal wells, the data must be frequentlysampled with high precision. During drilling the logs are highly disturbed by the drilling�uid circulation and are often also a�ected by in- and out-�ow at feed-zones. Temperaturelogs are nonetheless run during drilling to give information on feed zones in the boreholes.

Figure 2.1: Temperature pro�les inwells during or right after drilling(Steingrímsson 2013)

The four most common temperaturepro�les in wells during drilling areshown in Figure 2.1 (Steingrímsson,2013). The most common pro�le dur-ing injection is pro�le A where we cansee water injected into the well andlost to the three feed zones (a, b, c).The water is gradually heated whiletraveling to the bottom of the well dueto heat conducted from the formationaround the well. The slope of the tem-perature curve changes each time itpasses a feed zone since some of thewater is lost and at the last feed zonewe can see a steep slope change. Thisdoes not necessarily mean that it's thebest feed zone but rather that the bot-tom of the well is the most undis-turbed and thus much warmer thanthe rest.

Pro�le B is also logged during injec-tion but is more commonly seen inmeasurements with low injection rates. The steep steps on the temperature pro�le aredue to in�ow of warmer water mixing with the injection water. High in�ow in the wellcan cause error in interpretation since it isn't certain whether or not there is an out�owzone between b and c. If the injection rate would be increased, there would be a pressure

3

gain and in�ow would change into out�ow, �rst at the bottom feed zone and then slowlywork up to the top. This is what is happening in pro�le A.

Artesian �ow in low temperature wells is typically characterized by pro�le C. The steeptemperature changes are caused by high in�ow which mixes with the well �ow. Thecooling below feed zone c is disturbance caused by drilling and will gradually fade awayas the well heats to the formation temperature.

Pro�le D is commonly seen in wells just after drilling. Some temperature peaks can beseen and suggest permeable feed zones, but the permeability is not high enough to causea massive �ow between feed zones. The bottom of the well is very undisturbed whichcauses the high temperature gradient.

After drilling, the borehole is closed and given time to recover. After some time haspassed (∆t) the temperature recovery is measured and then the hole is closed again.This is repeated several times and the temperature build-up measurements are then usedto interpret the formation temperature around the well.

Various methods have been used to infer the formation temperature by extrapolatingthe logged well temperature. The most common is the Horner plot method which wasoriginally developed for pressure build-up estimations (Horner, 1951) in reservoir analysisbut was modi�ed by Dowdle and Cobb (1975) to model temperature build-up.

Although not mathematically correct, the Horner analysis may be used for reliable es-timates of formation temperature under the assumption of short circulation times (seeFigure 2.3 and 2.4). The validity of the Horner plot is based on the equation for heatconduction.

cpρ∂T

∂t= k

∂2T

∂x2(2.1)

This is also known as Fourier's heat conduction equation. It describes the change intemperature (T ) as a function of time (t) and position (x). The material properties ofheat capacity (cp), density (ρ) and thermal conductivity (k) are also important factors.Because the Horner plot method is based on the heat conduction equation it is usedbelow the casing depth where the heat conduction of the formation plays a big role inthe temperature of the �ow.

Horner plot method uses the measured temperature, at a given depth from di�erenttemperature logs, and the Horner time, τ , shown in equation 2.2.

τ =tk + ∆t

∆t(2.2)

4

The temperature recovery data is plotted logarithmically with the Horner time. Fig-ure 2.2 shows how di�erent circulation times determine the temperature recovery. Thetemperature will gather up as a straight line at in�nite time (τ = 1).

Figure 2.2: Theoretical Horner temperature buildup (Dowdle and Cobb, 1975)

Figures 2.3 and 2.4 show how longer circulation time a�ects the accuracy of the Hornerplot estimation. After an in�nite time (τ = 1), the system is assumed to have stabilizedand it is then possible to determine the formation temperature. This is done by plottingthe well temperatures as a function of τ and drawing a line of best �t (or "trend" line).This line intersects τ = 1 at the formation temperature.

The circulation time (tk) is an important parameter in the Horner plot method and shouldtherefore be determined accurately. Since drilling (and thus injection of cold �uid) reachesdi�erent depths at di�erent times, the circulation time varies with depth.

Using the Matlab codes presented in A.1 and A.2 it can be veri�ed that this is notnecessary for the wells in this thesis. The circulation time can hence be counted fromthe start of drilling at casing depth until the well is closed for warm-up. It should benoted that this result is not absolute and can vary with di�erent geology, methods ofmeasurements and other signi�cant parameters used in the borehole. Horner plot usuallyrequires several weeks of heating up and a minimum of three temperature logs for accurateprediction of the formation temperature.

5

Figure 2.3: Horner temperature graphfor a well with a 2,4 hour circulationtime (Dowdle and Cobb, 1975).

Figure 2.4: Horner temperature graphfor a well with a 50 hour circulationtime (Dowdle and Cobb, 1975).

2.1.1. Example

In the example below, information from ÞG-9 is used (chapter 3.7).

Table 2.1: Data for Horner plot at 2160 m TVD in well ÞG-9.

Date Operation Temperature [◦C]tk

[hours]∆t

[hours]Horner time,

τ2. DEC 2012 Circulation started � 0 � �15. DEC 2012 Circulation stopped � 304 0 �9. JAN 2013 Temperature logging 285 304 604 1,5011. FEB 2013 Temperature logging 307 304 1396 1,2211. JUN 2013 Temperature logging 338 304 4274 1,07

Given this information, Horner plot method can be used to �nd the estimated formationtemperature at 2160 m. The Horner plot is shown in Figure 2.5 where the estimatedEFT is 341, 3◦C.

6

Figure 2.5: Horner plot for formation temperature estimation using data from Table 2.1

2.2. Pressure interpretation

Figure 2.6: Pivot point in pressurelog during warm-up of well ÞG-4.

After shut-in the well pressure slowlyreaches an equilibrium with the pres-sure distribution in the geothermalreservoir. During this time, the pres-sure pro�les measured in the well typ-ically pivot about a �xed point calledthe pivot point. If the well has a singlefeed zone, the pivot point is located atthe depth of that feed zone, whereasif the well has several good feed zonesthe pivot point will be located betweenthese. The depth of the pivot point isessentially a weighted average of thedepth of the feed zones in the well andgives solid information on the pressureof the reservoir at that depth. Fig-ure 2.6 shows measured pressure dur-ing warm-up in ÞG-4 along with thepivot point that lies at around 1400 mdepth with pressure about 119 bars.

7

The pressure in closed geothermal wells is calculated by the hydrostatic formula

dp

dz= ρwg (2.3)

Where dp/dz is the pressure change with depth, ρw is the density of water and g is thegravitational acceleration. By integrating equation 2.4, the pressure with depth in thewell is obtained as

p(z) = p0 + g

z∫z0

ρ(z)dz (2.4)

Where p0 is the pressure at surface and ρ(z) is the density.

Two programs, BOILCURV and PREDYP (Arason et al., 2004), developed by ÍSOR useequation 2.4 to accurately calculate the BPD and the initial pressure. The BOILCURVprogram calculates the density by assuming that temperature of the water column is atboiling point, i.e. ρ(z) = ρsat(z). The PREDYP program estimates the density with agiven formation temperature with depth, i.e. ρ(z) = ρt(z).

The information the pivot point provides is used with the BOILCURV program to cal-culate the BPD (see Figure 3.7). The BPD curve is used along with the Horner plotmethod to estimate the formation temperature, which in turn is used with PREDYP todetermine the estimated initial pressure.

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3. Wells drilled in Þeistareykir

Figure 3.1: Location of the wells and their tracks

In the last quarter of the year 2000, the research department of the National EnergyAuthority (ROS) requested permission to drill a 1600-2000 m deep exploration well atÞeistareykir, Northeast Iceland. The well ÞG-1 was �nished in 2002 and was drilledvertically and is shown in Figure 3.1. Well ÞG-4 was drilled in 2007 and was directedsouth-southeast, beneath Mt. Bæjarfjall. When drilling ÞG-5 in 2007 the angle wasnot correct so ÞG-5B was drilled in 2008 west-southwest from the existing well ÞG-5to correct that angle. Wells ÞG-1, ÞG-4, ÞG-5 and ÞG-5B were drilled from the sameplatform.

Well ÞG-6 was drilled in 2008 directionally west-northwest from a platform at the foot

9

of Mt. Ketilfjall while ÞG-7 was drilled in the year 2011 from the same platform tothe northeast direction. In 2011, another well, ÞG-8 was drilled much further west thanthe other wells and was directed west-northwest. The well ÞG-9 was drilled in 2012vertically. The location of the wells can be seen in Figure 3.1 and more information isgiven in Table 3.1. All depths in the following subchapters represent true vertical depthsunless otherwise stated.

Table 3.1: Summary of the wells in Þeistareykir.Coordinates are given in ISNET93 coordinates.

Well ÞG-1 ÞG-4 ÞG-5B ÞG-6 ÞG-7 ÞG-8 ÞG-9Database ID 60401 60404 60405 60406 60407 60408 60409Year drilled 2002 2007 2008 2008 2011 2011 2012

Coordinates (x;y)592990 (E); 593015 (E); 593014 (E); 594138 (E); 594155 (E); 590992 (E); 593424 (E);599033 (N) 599044 (N) 599056 (N) 599534 (N) 599552 (N) 599447 (N) 599593 (N)

Type Vertical Directional Directional Directional Directional Directional VerticalCasing length (m) 617 832 847 841 769 1493 826Elevation (m a.s.l.) 352 350 350 377 377 340 350Length of well (m) 1953 2240 2499 2799 2509 2503 2194True vertical depth (m) 1953 1882 2389 2479 2075 2244 2194Circulation time (hours) 386 359 389 261 620 332 304

3.1. ÞG-1

As previously stated the well ÞG-1 was drilled vertically in 2002. At the time Þeistareykirwas an untapped resource and an exploration well was needed for more understandingand knowledge of the deep geothermal system. The well location was chosen based onresults from a number of surface exploration surveys, which indicated that the reservoirtemperature was above 280°C at depth (Gautason et al., 2000). In the drilling report,increased �uid circulation loss was detectable down to 1600-1700 m depth, which indicatesa feed zone below that.

From ÞG-1, nine warm-up pro�les and three injection pro�les were available (see FigureB.3). Only six warm-up pro�les can be used for the interpretation of formation tempera-ture, since the three others either don't have enough data or there hasn't passed enoughtime from when the well was closed to the measurement (shut-in time). Those six pro�lesare: the latter measurement from 16 September 2002, from 10 October 2002, 18 August2003, 22 August 2003 and the two from 7 September 2004. A feed zone is visible at 1900m depth in the injection pro�le from 4 September.

The well was opened for discharge in 22 October 2002 to 18 August 2003 and thereforethe measurements can't be used together to �nd the EFT using Horner plot method.So we will split this interpretation in to two parts; one including only the two 2002measurements and the other one using the four remaining measurements.

Figure 3.2 shows the two 2002 measurements. The temperature measurements in Figure

10

Figure 3.2: Warm-up temperaturepro�les of ÞG-1, 2002.

Figure 3.3: Recovery temperaturepro�les and EFT of ÞG-1, 2003-2004.

3.3, which shows the recovery temperature pro�les from 2003 and 2004, shows muchwarmer measurements than the former Horner estimation indicates. This is most likely

Figure 3.4: Horner plot at various depths forÞG-1 using data from Figure 3.2.

because of lack of data, sincethe time interval between thetwo measurements was notenough and also because ofthe absence of a third mea-surement.

For the latter temperaturelog, the measurement from18 August was done tooearly to be used with Hornerplot method and was there-fore not used in the Hornerpoint calculation shown inFigure 3.3. In Figure 3.3,the Horner points look moreprobable though most pointsare the same as the tem-perature measurement from 7September 2004. This can beexplained by looking at thedischarge time (tk), which is very large compared to the short shut-in time (∆t) of the 22

11

August 2003 measurement. This is the reason why the Horner points are almost identicalto the pro�les from 7 September 2004, a year later. Figure 3.4 shows four Horner graphsat various depths for the former Horner calculation.

Figure 3.5: Pressure pro�les and initialpressure estimation for ÞG-1.

These results show that for thewarm-up measurements in wellÞG-1, the Horner plot methoddoes not accurately predict theformation temperature and theresults are only as good as thedata. By looking at Figure 3.3 itcan be seen that the temperatureis very close to the BPD whichsuggests that after a longer warm-up period the formation tempera-ture would be the same as BPD.

The 2003 pressure measurementscannot be used for estimating thepivot point since they were mea-sured after the well had been dis-charging for almost a year. Fig-ure 3.5 shows the pivot point at1300 m depth where the pressureis around 102 bars. Also shownis the BPD curve and the initialpressure estimation.

3.2. ÞG-4

ÞG-4 was directionally drilled to the fracture zone southeast of the drilling platform toinvestigate permeability and locate fractures for future geothermal wells. At 1740 m(1525 m TVD) there was a total �uid circulation loss logged in the drilling report whichindicates a feed zone.

Three temperature pro�les from the warm-up data and one temperature pro�le from theinjection data are shown in Figure B.4. A visible feed zone can be seen on the injectioncurve at around 1600-1700 m depth.

In Figure 3.6 the Horner plot method (Figure 3.8) has been used to �nd the EFT. The twolatter warm-up measurements are close to the boiling point depth curve which suggeststhat the formation temperature of the well will follow the BPD closely.

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Figure 3.6: Warm-up temperaturepro�les and EFT of ÞG-4.

Figure 3.7: Pressure pro�les andinitial pressure estimation for ÞG-4.

Figure 3.8: Horner plot at various depths for ÞG-4.

The safer estimation is how-ever that the formation tem-perature follows the BPD un-til the casing depth and thencoincides with the Hornerpoints. The well ÞG-4 wasextremely high-pressured, whichis why BPD shows sucha high surface temperature.This was adjusted by esti-mating that the 11 Octobermeasurement was correct inthe top 100 m.

Figure 3.7 shows the pres-sure pro�les from the warm-up data, the BPD curve andthe initial pressure estimatedby the PREDYP program. The pro�les indicate that the pivot point lies at around 1430m depth where the pressure is about 118 bars.

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3.3. ÞG-5B

The main purpose of ÞG-5 was to directionally drill to the west of the platform topenetrate the fracture zone that lies between ÞG-1 and ÞG-2. The design of ÞG-5 wasto drill deep underneath well ÞG-2. After drilling 1910 m in well length the drilling wasdiscontinued because the angle had decreased too much and it was clear that it would notreach enough depth and thus not its objective. Well ÞG-5B was drilled from the casingdepth of ÞG-5 to reach the aforementioned goal of ÞG-5.

According to drilling reports from Ingimarsdóttir et al. (2009) there was total circulationloss at 863 m (831 TVD) which con�rms that there is a feed zone there. The circulationloss continued until 2277 m (2178 TVD) and can be seen in Figure 3.9.

Figure 3.9: Pressure and injection rate of drilling �uid in ÞG-5. Welllength is displayed on the x-axis (Ingimarsdóttir et al., 2009).

Figure B.5 shows all the temperature pro�les of ÞG-5. Fairly visible feed zones can beseen below the casing at 800-850 m, 1200 m, 1450 m and another at 2200 m depth.

From 1550 m well length (1426 TVD) to 1950 m well length (1863 TVD) the pressure ofthe drilling �uid seems to be fairly stable with depth as shown in Figure 3.9. This canindicate a highly permeable feed zone and in�ow at that depth.

The measured warm-up temperature in Figure 3.10 drops steadily below 1000 m andreaches stability at 1500 m to 2100 m. This is around the highly permeable feed zonewhich means it's not reliable to use those measurements with the Horner plot method

14

Figure 3.10: Warm-up temperaturepro�les and EFT of ÞG-5B.

Figure 3.11: Pressure pro�les andinitial pressure estimation for ÞG-5B.


since it is built on the as-sumption that there is con-duction between the well andthe surrounding formations.Because of that highly per-meable feed zone there is con-vection taking place in thewell.

The Horner points, in Fig-ure 3.10 are much lower thanthe BPD and a selected num-ber of them are shown in Fig-ure 3.12. A recent measure-ment from 11 February 2010shows hotter temperaturesthan during warm-up. Thereason for the high temper-ature measurements on thecomparison curve is most likely caused by in�ow of hot water from the feed zones inthe bottom of the well which heats up the rest of the well after a long time. There-fore, the formation temperature above the casing shoe and at the bottom of the well isestimated to be the same as in the measurement from 11 February.

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Figure 3.11 shows the pressure pro�les during the warm-up along with the BPD curveand the estimated initial pressure. Pivot point lies around 1350 m depth at 105 bars.

3.4. ÞG-6

ÞG-6 was drilled from the same platform as ÞG-3 and data from that well was used todesign ÞG-6. It was decided to directionally drill from the platform to a fracture zoneto the west. Around 2355 m depth, the drilling �uid pressure lowered which is a goodindication for a feed zone.



In Figure B.6, the three injection pro�les and �ve warm-up pro�les measured in ÞG-6are shown. According to the injection pro�les there are two feed zones below the casing,at 800-850 m and 2350-2400 m. One additional feed zone at 1050 m can be seen on thetwo latter warm-up pro�les.

The three warm-up pro�les were plotted along with Horner points in Figure 3.13. The�rst warm-up temperature log is fairly low and is done shortly after the well was closed.This means that the measurement is done too early to accurately use the Horner plotmethod. For that reason it was not included in the Horner plot method calculations;which means we have a shortage of data. The Horner points show lower temperaturesthan in reality due to the short time span of the two measurements.

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The bottom hole tempera-ture Horner point is howeververy close to the BPD andit is therefore safe to assumethat the temperature there isclose to that Horner point.We can also assume that thetemperature above the casingend is close to or at the BPDaccording to the last temper-ature measurements.

The BPD and initial pres-sure estimation are shown inFigure 3.14, where pressurepivot point is around 1340mat 107 bars.

3.5. ÞG-7

Well ÞG-7 is located at thesame drill platform as ÞG-3 and ÞG-6. The main goal of ÞG-7 was to investigate temper-ature state, permeability and characteristics of the geothermal �uid under Mt. Ketilfjallsince surface exploration had provided evidence for upward �ow there. The well wasdrilled directionally 45° to the northeast in hope of �nding permeable channels as wellas intersecting fractures connected to the explosion craters at the foot of Mt. Ketilf-jall.According to the drilling reports, the drilling went for the most part unobstructedand at 1963 m depth there was an increased drilling �uid circulation loss which is a signof a feed zone.

Figure B.7 shows all temperature pro�les of ÞG-7 and the warm-up temperature pro�lesare shown in Figure 3.16. A clear feed zone is visible at 700-750 m depth, right belowthe casing. Other feed zones can be seen at 1500 m and 1900-2000 m.

By looking at Figure 3.16 we can see that the Horner points fall very close to the BPDcurve until the feed zone at 1500 m. The Horner points at 1600 m and 1750 m are atlower temperatures than the last measured temperature. The temperature build-up atthese depths appears to be disturbed by feed zones, which is is outside the validity rangeof the Horner plot method. They are therefore not used in the estimation of formationtemperature.

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The disturbance from thefeed zone at 1900-2000 mcould however be causingthe temperature measure-ments to be lower than inreality. It is probable thatthe formation temperature iscloser to the BPD than theEFT shows but this is thesafer estimation. Figure 3.18shows selected Horner plots.

The pressure pro�les in Fig-ure 3.17 show a pivot pointat 1320 m with pressure 105bars along with the the BPDcurve and the initial pressureestimation.

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3.6. ÞG-8

ÞG-8 was the �rst exploration well drilled in its area and it was directionally drilled westfrom the platform. The main objective was to intersect N-S fractures close by and aneruptive �ssure under the geyser Stórihver below 2200 m TVD.

Total �uid circulation loss was detected around 1580 m depth but the �uid resurfacedshortly afterwards. The same happened at 1602 m depth which indicates multiple feedzones.



By looking at the temperature pro�les for ÞG-8, shown in Figure B.8, visible feed zonesare at 900-1000 m, 1400-1500 m, 1700 m, 1900 m and 2100 m depth.

Only two warm-up temperature logs were available for ÞG-8 and are shown in Figure3.19. Therefore can we not really use the Horner plot method for accurate estimation offormation temperature (see Figure 3.21 for selected Horner plots).

By looking at Figure B.8 we can see that the latter warm-up temperature pro�le showsvery low temperature at the bottom of the well although the measurement was done longafter drilling. This indicates that the formation temperature in the bottom of the well isnot much higher than the measured temperature, i.e. around 110°C.

The EFT curve is thus assumed to follow the latter warm-up temperature log and reach

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it's maximum temperature around 600 m and then gradually lower until reaching thebottom of the well at 2250 m depth with temperature 100-120°C.

Figure 3.20 shows that the depth to pivot point is around 1810m at 156 bars along withthe BPD curve and the initial pressure estimation.


3.7. ÞG-9

Location of feed zones in wells ÞG-4 and ÞG-6 suggested a highly permeable layer con-nected to a NNA-SSV fracture zone. ÞG-9 was drilled vertically to investigate that pos-sibility as well as to �nd potential permeable layers connected to dikes below 1800-2000m.

Drill �uid circulation loss was low and the �rst one, 4 L/s, was logged at 1247 m depthand the highest �uid circulation loss was at 1760 m depth at 11 L/s.

Figure B.9 shows nine injection temperature pro�les along with three warm-up temper-ature pro�les. Four feed zones can be seen at 800 m, 1400 m, 1700-1800 m, 2000 mdepth.

The temperature pro�le from 11 June 2013 shows clear indication that the formationtemperature will coincide with the BPD curve. This is con�rmed by the calculated

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Horner points on Figure 3.22 and 3.24 which follow the BPD curve very closely andafter 1700 m depth we can barely make a distinction between them and the BPD curve.Therefore we can assume that the EFT coincides completely with the BPD curve.

Initial pressure estimation and a BPD curve along with warm-up pressure pro�les isshown in Figure 3.23. The pivot point is around 1180 m depth at 94 bars.



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4. Conclusion

Temperature and pressure data from seven wells in the Þeistareykir geothermal �eld wasinterpreted with the aid of Horner plot method to estimate formation temperature andinitial pressure of the reservoir. It is interesting to see how the bottom well temperaturecan be disturbed by in�ow from feed zones.

The Horner plot method did not prove equally useful in the interpretation of all wells sincenot all temperature logs met the ideal standards for the method. In wells ÞG-1, ÞG-6and ÞG-8 there was lack of data or the temperature logs were measured too shortly afterthe drilling of the well to accurately use the Horner plot method. The temperature logsdid however give information that was used in estimating the formation temperature andin ÞG-6, the bottom wel formation temperature was estimated assuming that the Hornerplot calculation was correct at that depth. Wells ÞG-5B and ÞG-7 had highly permeablefeed zones whose in�ow disturbed the measurements. A recent measurement in ÞG-5Bproved vital in the estimation of formation temperature since it gave information on theformation temperature at the bottom of the well and above the casing end. There wasno recent measurement for ÞG-7 thus it was concluded that the bottom well formationtemperature would be the same as the Horner calculation showed. It is likely that ÞG-7is warmer than the estimated formation temperature but that could not be con�rmedwith the given data. Using the Horner plot method on data from ÞG-4 and ÞG-9 gavegood information on how the formation temperature curve would be. This is attributedto ideal warm-up temperature logs, i.e. at least three measurements that were conductedat good interval to each other after enough time had passed since the well was shut in.

Results from the two vertically drilled wells, ÞG-1 and ÞG-9 show almost an identicalformation temperature (BPD curve) seen in Figure B.1. The highest bottom well tem-peratures were estimated in the wells ÞG-1, ÞG-5 and ÞG-9 at 340°C, 341°C and 345°Crespectively. ÞG-7 and ÞG-8 were the only wells that did not reach temperatures greaterthan 320°C but it is likely that the ÞG-7 is warmer than the estimates indicate becauseof temperature disturbances below 1600 m depth. ÞG-8 yielded very low temperatureestimates, reaching highest temperature around 500 m depth but declining to 110-120°Cat the bottom of the well. All the wells are viable for geothermal utilization with theexception of ÞG-8 which is situated much farther west than the other wells. The pressureestimates in Figure B.2 all display very similar curves. The pivot points of all wells exceptfor ÞG-8 were found at 1200-1400 m depth with pressure ranging from 94-118 bars.

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References

Arason Th., Björnsson G., Axelsson G., Bjarnason J.Ö., Helgason P., 2004: ICEBOX- Geothermal Reservoir Engineering Software for Windows : A User's Manual.ÍSOR-2004/014.

Axelsson G. and Steingrimsson, B., 2012: Logging, testing and monitoring geothermallogs. Presented at "Short course on Geothermal Development and GeothermalWells".

Ármannson H., Gíslason G. and Torfason H., 1986: Surface exploration of theTheistareykir high-temperature geothermal area, Iceland, with special referenceto the application of geochemical methods. Appl. Geochem. 1:47-64.

Ármannsson H., Kristmannsdóttir H., Torfason H. and Ólafsson M., 2000:Natural changes in unexploited high-temperature geothermal areas in Iceland.Proc. World Geothermal Congress 2000, 521-526.

Ármannsson H., 2012. The Theistareykir geothermal system, North East Iceland.Case history. Presented at "Short Course VII on Exploration for GeothermalResources".

Darling W.G. and Ármannsson H., 1989: Stable isotope aspects of �uid �ow in theKra�a, Námafjall and Theistareykir geothermal systems of northeast Iceland.Chem. Geol. 76: 197-213.

Dowdle W.L. and Cobb W.M., 1975: Static formation temperature from well logs - anempirical method. Journal of Petroleum Technology, 27, p. 1326-1330.

Gautason B., Ármannsson H., Árnason K., Sæmundsson K., Flóvenz Ó.G. andThórhallsson S., 2000: Thoughts on the next steps in the exploration of theÞeistareykir geothermal area (In Icelandic). Orkustofnun BG-HÁ-KÁ-KS-ÓGF-SÞ-2000-04.

Gíslason G., Johnsen G., Ármannsson H.,Gíslason G., Johnsen G., Ármannsson H.,Torfason H. and Árnason K., 1984: Þeistareykir � Surface exploration of thehigh-temperature geothermal area (In Icelandic). OS-84089/JHD-16.

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Grönvold K. and Karlsdóttir R., 1975: Þeistareykir. An interim report on the surfaceexploration of the geothermal area (In Icelandic). Orkustofnun JHD-7501, 37pp.

Horner D.R., 1951: Pressure build-up in wells. Third World Petroleum CongressProceedings, E.J. Brill, Leiden.

Ingimarsdóttir A., Gautason B., Sigurgeirsson M.Á., Jónsson R.B., Pétursson F.,Sveinbjörnsson S. jr., Sveinbjörnsson S. sr., Tryggvason, H.H., Jónsson P.,Haraldsson K., 2009:Þeistareykir - Well ÞG-5B: Drilling from borehole ÞG-5 at813 m to 2499 m depth: Drilling history (In Icelandic). ÍSOR-2009/055

Layugan D.B., 1981: Geoelectrical soundings and its application in the Theistareykirhigh temperature area. United Nations University. Geothermal TrainingProgramme Report 1981-5, 101 pp.

Steingrimsson B., 2013: Geothermal well logging: Temperature and pressurelogs. Presented at "Short Course V on Conceptual Modeling of GeothermalSystems".

Sæmundsson K., 1986: Subaerial volcanism in the Western North Atlantic. In Vogt,PR. and Tucholke, B.E. (eds.) The Geology of North America, M. The WesternNorth Atlantic Region. 69-86. Boulder, Colorado: Geological Society of America.

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A. Matlab codes

A.1. Temperature and pressure plots

1 % Program to plot temperature profiles along with a2 % BPD curve and a EFT using Horner plot method.3 % Pressure profiles also use this code but it's modified.4

5 % Data loading:6 %−−− 9. JAN 2013 15:00 −−−%7 date1 = [2013 1 9 15 00 00];8 load T_2013_0109.dat9 T1 = T_2013_0109(:,2);

10 z1 = T_2013_0109(:,1);11 %−−− 11. FEB 2013 14:47 −−−%12 date2 = [2013 2 11 14 47 00];13 load T_2013_0211.dat14 T2 = T_2013_0211(:,2);15 z2 = T_2013_0211(:,1);16 %−−− 11. JUN 2013 12:26 −−−%17 date3 = [2013 6 11 12 26 00];18 load T_2013_0611.dat19 T3 = T_2013_0611(:,2);20 z3 = T_2013_0611(:,1);21 %−−− boilcurv output −−−%22 load OUTPUT.dat23 BT = OUTPUT(:,3);24 Bz = OUTPUT(:,1);25 pos = find(Bz==0);26 %−−− EFT estimation −−−%27 load T4_plot.dat28 Fx = T4_plot(:,2);29 Fy = T4_plot(:,1);30 %−−− Shut−in time − Well closed 15. DEC 2012 10:45 −−−31 date_s = [2012 12 15 10 45 00];32 dt1 = etime(date1,date_s)/60^2;33 dt2 = etime(date2,date_s)/60^2;34 dt3 = etime(date3,date_s)/60^2;35 %−−− Circulation time − From 2. DES 2012 19:00 −−−36 date_c = [2012 12 2 19 00 00];37 tk = etime(date_s,date_c)/60^2;38 %−−−Horner time−−−

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39 ta = [(tk + dt1)./dt1 (tk + dt2)./dt2 (tk + dt3)./dt3];40 ta1 = [1 ta];41 %−−−Chosen depth for horner calculation−−−42 % Finds the closest location of depth (z) in data to43 % the chosen depth (depth1) and adds the temperature44 % to a cell.45 % I1_d is the location of the value Y1.46 depth1 = (8:16)*135;47 T = cell(size(depth1));48 for i = 1:length(depth1)49 [Y1 I1_d] = min(abs(z1 − depth1(i)));50 [Y2 I2_d] = min(abs(z2 − depth1(i)));51 [Y3 I3_d] = min(abs(z3 − depth1(i)));52 T{i} = [T1(I1_d) T2(I2_d) T3(I3_d)];53 end54 %−−− Plot −−−55 figure;56 hold on57 set(gca,'XAxisLocation','top','YAxisLocation','left','ydir','reverse', ...

'XTick',0:100:600, 'YTick', 0:500:2500, 'xlim', [0 400], 'ylim', ...[0 2300]);

58 plot(T1,z1, 'b−', 'LineWidth',1, 'LineSmoothing','on');59 plot(T2,z2, 'r−', 'LineWidth',1, 'LineSmoothing','on');60 plot(T3,z3, 'g−', 'LineWidth',1, 'LineSmoothing','on');61 % Chosen depth to plot markers at. Aesthetics62 depth2 = (1:16)*135;63 % Same for loop as before.64 for j = 1:length(depth2)65 [Y1 I1_d] = min(abs(z1 − depth2(j)));66 [Y2 I2_d] = min(abs(z2 − depth2(j)));67 [Y3 I3_d] = min(abs(z3 − depth2(j)));68 % Each point is plotted.69 a = plot(T1(I1_d),z1(I1_d),'−bp', 'MarkerSize', 6, ...

'MarkerFaceColor', 'b', 'LineSmoothing','on');70 b = plot(T2(I2_d),z2(I2_d),'−ro', 'MarkerSize', 4, ...

'MarkerFaceColor', 'r', 'LineSmoothing','on');71 c = plot(T3(I3_d),z3(I3_d),'−gs', 'MarkerSize', 4, ...

'MarkerFaceColor', 'g', 'LineSmoothing','on');72 end73 %−−− Horner calculation −−−74 % p1 plots up the horner data points75 % f plots a "trend" line for the points76 % r find where the "trend" line intercects horner time = 1.77 f=cell(size(depth1));78 r=ones(size(depth1));79 for k = 1:length(f)80 p1 = polyfit(ta,T{k},1);81 f{k} = polyval(p1,ta1);82 r(k) = f{k}(1);83 end84 %−−− Horner points −−−%85 d = plot(r,depth1,'kd', 'MarkerSize', 4, 'MarkerFaceColor', 'k', ...

'LineSmoothing','on');86 %−−− Boiling Point with Depth curve −−−%

27

87 f = plot(BT(pos:length(BT)),Bz(pos:length(Bz)),'k', 'LineWidth',1.5, ...'LineSmoothing','on');

88 %−−− EFT curve −−−%89 e = plot(Fx,Fy,'m−−', 'LineWidth',1.5, 'LineSmoothing','on');90 legend([a,b,c,d,e,f],'9. JAN 2013', '11. FEB 2013', '11. JUN 2013', ...

'Horner points', 'EFT', 'BPC');91 % EFT = Estimated Formation Temperature92 % BPD = Boiling Point with Depth93 xlabel('Temperature');94 ylabel('Depth');95 grid on96 hold off97 % Aesthetics98 set(0,'DefaultAxesFontSize', 12)99 set(gcf, 'Color', 'w', 'Position', [1, 1, 400, 435]);

100 export_fig T_plot.png −painters −r300

A.2. Horner method for changeable circulation time

1 % This program will calculate horner points for certain2 % depths that have various circulation time.3

4 % The program uses the same code as the temperature5 % plot code with the 'Circulation time', 'Horner time'6 % and 'Chosen depth for horner calculation' replaced7 % with this code:8 %−−− Circulation time − From 2. DES 2012 19:00 −−−9 depth1 = (8:16)*135;

10 A = ones(2,length(depth1));11 % Cell that includes a vector with each depth12 % reached and the time vector.13 % This information is included in the drilling reports by ISOR.14 date_c = {[1300 2012 12 3 24 00 00] [1500 2012 12 4 24 00 00] [1700 ...

2012 12 5 24 00 00] [1800 2012 12 6 24 00 00] [2000 2012 12 7 24 ...00 00] [2200 2012 12 8 24 00 00]};

15 % Puts the information from date_c into a more16 % organized vector form.17 for k = 1:618 for j = 1:length(A)19 A(k,j) = datec{k}(j);20 end21 end22 %−−−Chosen depth for horner calculation−−−23 % Finds the closest location of depth (z) in data to24 % the chosen depth (depth1) and adds the temperature25 % to a cell.26 % I1_d is the location of the value Y1.27 T = cell(size(depth1));28 tk = ones(size(depth1));

28

29 ta = cell(size(depth1));30 ta1 = cell(size(depth1));31 for i = 1:length(depth1)32 [Y1 I1_d] = min(abs(z1 − depth1(i)));33 [Y2 I2_d] = min(abs(z2 − depth1(i)));34 [Y3 I3_d] = min(abs(z3 − depth1(i)));35 T{i} = [T1(I1_d) T2(I2_d) T3(I3_d)];36 end37 % Starts with the first depth in the 'depth1' vector and38 % finds the corresponding depth in the 'A' vector.39 for i = 1:length(depth1)40 [X1 X1_d] = min(abs(A(:,1)−depth1(i)));41 if A(X1_d,1) − depth1(i) < 042 X1_d = X1_d + 1;43 end44 % Then it calculates the circulation time between the date45 % in A and the given shut−in time (date_s).46 tk(i) = etime(date_s, A(X1_d,2:7))/60^2;47 % If the 'depth1' variable is larger than the 'A' variable48 % it will put the horner time into the cell 'ta'.49 % If it's smaller then it will take the next circulation time50 % and put it into the cell 'ta'.51 if depth1(i) ≥ A(X1_d,1)52 ta{i} = [(tk(i+1)+dt1)/dt1 (tk(i+1)+dt2)/dt2 (tk(i+1)+dt3)/dt3];53 ta1{i} = [1 (tk(i+1)+dt1)/dt1 (tk(i+1)+dt2)/dt2 ...

(tk(i+1)+dt3)/dt3];54 else55 ta{i} = [(tk(i)+dt1)/dt1 (tk(i)+dt2)/dt2 (tk(i)+dt3)/dt3];56 ta1{i} = [1 (tk(i)+dt1)/dt1 (tk(i)+dt2)/dt2 (tk(i)+dt3)/dt3];57 end58 end

A.3. Horner subplots

1 % Program that creates a horner subplot for2 % all specified depths (depth3).3 % The program uses all the same inputs as4 % the previous program.5 depth3 = (5:8)*270;6 for i = 1:length(depth3)7 [Y1 I1_d] = min(abs(z1 − depth3(i)));8

9 [Y2 I2_d] = min(abs(z2 − depth3(i)));10

11 [Y3 I3_d] = min(abs(z3 − depth3(i)));12

13 T{i} = [T1(I1_d) T2(I2_d) T3(I3_d)];14 end15 % −−−Plot−−−

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16 for k = 1:length(depth3)17 subplot(2,2,k);18 hold on19 plot(ta,T{k}, 'ko', 'MarkerSize', 5, 'MarkerFaceColor', 'r', ...

'LineSmoothing','on');20 p1 = polyfit(ta,T{k},1);21 f1 = polyval(p1,ta1);22 plot(ta1,f1,'−k', 'LineWidth',1, 'LineSmoothing','on');23 xlim([0.75 1.6]);24 text(0.78,f1(1),[num2str(round(10*f1(1))/10) 'C' '\rightarrow'], ...

'Color', 'r', 'fontsize', 7, 'BackgroundColor', 'white')25 title(['Temperature at ' num2str(depth3(k)) ' m.b.s.l.'])26 xlabel('$(t_k+\Delta t)/\Delta t$', 'interpreter', 'latex')27 ylabel('Temperature [C]')28 grid on29 hold off30 end31 export_fig horner_plot.png −painters −r300

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B. Data

Figure B.1: Formation temperatureestimate of all wells.

Figure B.2: Initial pressureestimate of all wells

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Figure B.3: Temperature pro�les from ÞG-1

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Figure B.5: Temperature pro�les from ÞG-5B

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