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Geothermics 49 (2014) 90–98

Contents lists available at ScienceDirect

Geothermics

journa l homepage: www.e lsev ier .com/ locate /geothermics

igh temperature instruments and methods developed forupercritical geothermal reservoir characterisation andxploitation—The HiTI project

agnar Ásmundssona,b,∗, Philippe Pezardc, Bernard Sanjuand, Jan Henningese,ean-Luc Deltombef, Nigel Halladayg, Francois Lebertd, Alain Gadaliad, Romain Millotd,enoit Gibertc, Marie Violayc, Thomas Reinsche, Jean-Marc Naisse f, Cécile Massiota,h,ierre Azaïsc, David Mainpricec, Costas Karytsas i, Colin Johnstonj

ÍSOR – Iceland GeoSurvey, Akureyri, IcelandHeat Research and Development, Akureyri, IcelandGéosciences Montpellier, CNRS, Université Montpellier 2, CC 60, 34095 Montpellier Cedex 05, FranceBRGM – 3, Av. Claude Guillemin, BP6009, 45060 Orléans Cedex 2, FranceGFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, GermanyAdvanced Logic Technology, Batiment A, route de Niederpallen, L-8506 redange sur attert, LuxembourgCalidus Engineering Ltd., Research Centre, 6 Jon Davey Drive, Treleigh Industrial Estate, Redruth, Cornwall TR16 4AX, United KingdomGNS Science, Taupo, New ZealandCRES, 19th km Marathonos Av., 19009 Pikermi, GreeceOxford Applied Technology, Oxford University Begbroke Science Park, Sandy Lane, Yarnton, Oxford OX5 1PF, United Kingdom

r t i c l e i n f o

rticle history:eceived 29 February 2012eceived in revised form 16 July 2013ccepted 19 July 2013vailable online 16 August 2013

a b s t r a c t

During the early years of the Iceland Deep Drilling Project (IDDP), development of three distinctive tech-nological and scientific approaches were formalised and then carried out until 2010 within a Europeanfunded project called HiTI (high temperature instruments for supercritical geothermal reservoir charac-terisation and exploitation). These approaches were: (1) development of several downhole instrumentsallowing them to function up to 300 ◦C and 400 ◦C, (2) identification of two new Na/Li cation ratio geother-mometric relationships valid at very high temperature, (3) tracer testing with high temperature tolerant

eywords:igh temperatureupercriticalownholeeothermometerseleviewerasalt experiments

organic isomers and finally and (4) basalt rock deformation and petrophysical properties laboratoryinvestigations at high temperature and pressure conditions.

© 2013 Elsevier Ltd. All rights reserved.

. Introduction

This paper gives an overview of achievements made within aartnership formed in the so-called HiTI project (High Temperature

nstruments for supercritical geothermal reservoir characterisationnd exploitation), funded within the Sixth Framework Programmef the European Union, project number 19913.

Supercritical reservoirs are expected to exist below conven-ional high temperature geothermal fields. The supercritical fluidan in simple terms be described as having the density of water,

∗ Corresponding author. Tel.: +354 6939172.E-mail address: rka@heatrd.com (R. Ásmundsson).

375-6505/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.geothermics.2013.07.008

but flow properties like a gas. This could mean that the permeabil-ity for the supercritical phase is very high, either in dense brittleor ductile rock. The potential of supercritical geothermal fluid uti-lisation has been described in several Iceland Deep Drilling Project(IDDP) publications; see e.g. Fridleifsson and Elders (2005) and ref-erences therein. A model of the hydrothermal processes near abrittle–plastic rock transition was described by Fournier (1999).IDDP wells were subsequently planned to be cased off at 3500 mdepth, where a boiling curve to surface would reach the criticalpoint at a pressure of 22 MPa and temperature above 374 ◦C forpure water.

Future supercritical geothermal reservoirs will need to beevaluated in situ with instrumentation and methods adaptedto the supercritical environment. The HiTI project, when firstplanned in 2004, was designed to provide at least the minimum

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nstrumentation needed to allow the scientific community andeothermal industry to evaluate a supercritical geothermal reser-oir. The primary indicators of geothermal power are of course thehermodynamic parameters, and therefore a large emphasis wasut on direct downhole measurements of temperature, pressurend flow at as high temperature as possible. This was achievedoth with improved wireline measurements and a memory tooln a slickline. A wireline is a strong armoured cable containing oner more conducting insulated wires through which constant poweran be provided and communication to surface is achieved. A slick-ine is a metal wire that can be used to lower a non-communicatingevice to great well depths and temperatures exceeding the tol-rance limits of any commercial wirelines. Both wirelines andlicklines need to tolerate high temperature and corrosive geother-al well environments. The two tools developed and field tested for

he thermodynamic evaluation were a wireline temperature sensorperating long-term without heat shielding up to 300 ◦C and a slick-ine temperature, pressure and flow instrument in a heat shieldingask tolerating 400 ◦C for a few hours. In addition to the downholeools, a fibre optic distributed temperature sensing (DTS) cable waseveloped and installed along the outside wall of a 300 m anchorasing of a new powerful production well. This latter techniquellows for an instantaneous measurement of temperature alonghe whole length of installation. Temperature changes or anoma-ies within that section can be monitored at any time, at first duringementing and drilling (Henninges and Brandt, 2007) and then lateruring flow testing and even production.

The rock properties of the deep reservoir remain of great inter-st, since the geothermal fluid flow properties are determined byhe formation characteristics. For the purpose of identifying rockroperties (e.g. fractures, formation change, resistivity and bore-ole wall rugosity), firstly a televiewer was demonstrated at 300 ◦C

n a well in Iceland along with a spectral natural gamma radia-ion tool and secondly a resistivity tool of the dual laterolog typeas designed for operation at 300 ◦C on wireline but unfortunately

ould not be field tested within the project.Knowledge on the chemical content and the resulting properties

f the fluid at both supercritical and subcritical conditions are ofigh interest regarding both field conceptualization and utilisation

or power production. Fluid samples were collected at wellheadsrom several high temperature Icelandic geothermal fields and twoew sodium(Na)/lithium(Li) ratio geothermometric relationshipswith determination of the corresponding Li isotope values) wereesigned in order to determine with more detail the temperature

n very high temperature geothermal systems. Tracer testing withrganic isomers was also carried out within the Krafla geothermaleld in Iceland where the first IDDP well is located, showing veryigh tracer linear velocities, both from the site’s main re-injectionell and the IDDP-1 well itself.

Basalt rock from cored wells from the Krafla field was testedn a Paterson pressure cell which had been modified to allow for1) fluid injection of various compositions, (2) electrical resistivity

easurements at a few mV of AC voltages ranging from 0.1 Hz to0 MHz and finally and (3) deformation pressure experiments at upo 300 MPa and 900 ◦C.

The project intermediate results were published on the HiTIebsite www.hiti-fp6.eu during the active phase and fully dis-

eminated in several reports referenced below. This is howeverhe first publication describing the overall project outcomes. Thearticipants and their roles are listed in Table 1.

The new instruments and methods have now been field testednd applied to extract scientific information at several different

igh temperature field locations in Iceland. In addition, commer-ialisation of some of the technologies has already taken place,atisfying the conventional high temperature geothermal industrynd science while marginally keeping up to the pace of deep high

rmics 49 (2014) 90–98 91

temperature drilling efforts (the IDDP-1 wellhead became hotterthan anticipated within the HiTI project).

2. Thermodynamic parameters

For the identification of thermodynamic parameters, crucial togeothermal power production, the following four instruments andmethods were completed and applied:

(1) New geothermometric relationships using Na/Li cation ratioand the corresponding lithium isotopes values were deter-mined by BRGM using liquid samples collected in 2007, 2008and 2009 in Icelandic high temperature fields.

(2) Distributed temperature sensing (DTS) fibre optic cable wasinstalled by GFZ-Potsdam in 2009 in well HE-53 in theHellisheiði geothermal field, Iceland.

(3) Temperature sensor operating to 300 ◦C on a wireline, designedby BRGM and field tested together with ÍSOR in the Kraflageothermal field, Iceland, in June 2010.

(4) Temperature, pressure, fluid flow and casing collar location toolwas designed and built by Calidus Engineering and field testedtogether with ÍSOR in the Krafla geothermal field in July 2010.

Each of these thermodynamic parameters is discussed in sepa-rate sub-sections below.

2.1. New Na/Li geothermometric relationships

One of the major applications of water geochemistry in theexploration of the potential geothermal reservoirs involves estima-tions of their deep temperature using analyses of some chemical orisotopic species performed on fluids collected from thermal springsor geothermal waters. Most of the classical chemical geother-mometers such as silica, Na/K, Na/K/Ca, K/Mg or Na/K/Ca/Mg(Nicholson, 1993) are based on empirical or semi-empirical lawsderived from known or unknown chemical equilibrium reactionsbetween water and minerals occurring in the geothermal reser-voirs. Unfortunately, the estimations of temperatures given forthe deep geothermal reservoirs using these classical tools are notalways concordant due to different processes which can occurduring the ascent of the deep water and can modify its chemi-cal composition (mineral precipitation or dissolution due to thefluid cooling, fluid mixing with shallow aquifers, etc.). Given thesediscordances, auxiliary geothermometers such as the three Na/Liratio relationships based on statistical analyses were determined(Fouillac and Michard, 1981; Kharaka and Mariner, 1989). Evenif the chemical processes which control the Na/Li ratio are stillpoorly known, these relationships give more reliable deep temper-ature estimates than the classical geothermometers in numerouscases because of the low lithium reactivity during the ascent of thedeep fluid (Fouillac and Michard, 1981). However, their use must beselective taking into account the fluid salinity and/or the nature ofthe rocks which constitute the studied reservoir (granitic, basaltic,andesitic or sedimentary rocks, for example).

The geothermometric analysis by BRGM was initiated witha bibliographic study on the application of the sodium-lithiumratio and the existing Li isotope values to identify reservoir tem-peratures and characteristics previously reported (Sanjuan andMillot, 2009). Water sampling from producing high temperaturewells was conducted in October 2007, June 2008 and June 2009(Sanjuan et al., 2010 and Sanjuan, 2010). These water samples

were chemically and isotopically analysed in the BRGM labora-tories. A new statistical Na/Li relationship was first identified fordilute non marine geothermal waters from Iceland with temper-atures ranging from 200 to 325 ◦C (Fig. 1; Sanjuan et al., 2010).

92 R. Ásmundsson et al. / Geothermics 49 (2014) 90–98

Table 1Participants in the HiTI project.

Participant name Short name Country Role in project

Iceland GeoSurvey ISOR Iceland Coordination/field demonstrationGéosciences Montpellier, UM2, CNRS CNRS France Rock physics – log data modellingBureau de Recherches Géologiques et Minières BRGM France Water chemistry, downhole tool development, tracing testsCalidus Engineering Ltd. CalEng United Kingdom Downhole tool developmentAdvanced Logic Technology ALT Luxembourg Downhole tool developmentOxford Applied Technology Ltd. Oxatec United Kingdom High temperature electronicsGerman Research Centre for Geoscience GFZ Germany Distributed temperature sensingCentre for Renewably Energy Sources CRES Greece Dissemination

1,00

2,00

3,00

4,00

5,00

1,0 1,5 2,0 2,5 3,0 3,5 4,0

log

(N

a/L

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mo

lar

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103/T (°K)

250 200 150 25100 50300350

Basalt -seawater interactions(Iceland, Djibouti, seawater,

MAR, EPR)

400 0 C

Reykjanes, Svartsengiand Seltjarnarnes fields

Krafla field

Namafjall field

Hveragerdi field

Nesjavellir field

Krafla + Nesjavellir +Namafjall + Hveragerdi

(dilute non marine fluids)

Djibouti areas

Mid-Atlantic Ridge (MAR)East Pacific Rise (EPR)

Very altered MAR, EPR

a)

b) c)

F the pe ids wa

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a

l

wet

s(fianhein(ffo

lower than that of the seawater (�7Li ≈ +31 ‰) and their higherLi/Cl ratios indicate that these waters have intensively interactedwith basalts, even at 114 ◦C (Fig. 2). However, their significant Liisotopic variations at high temperatures (despite a low isotopic

ig. 1. The two new geothermometric Na/Li relationships revealed by BRGM withinxisting Na/Li relationships in the literature: (a) Fouillac and Michard, 1981, for flund (c) Fouillac and Michard, 1981, for fluids with Cl < 0.3 M).

nother statistical Na/Li relationship was also obtained betweenand 365 ◦C for seawater–basalt interaction derived hydrothermaluids from rifts that have emerged above sea level such as those of

celand (Reykjanes, Svartsengi and Seltjarnarnes geothermal fields)nd of Djibouti (Asal and Obock geothermal areas), or from numer-us oceanic ridges and rises worldwide (Fig. 1; see references inanjuan, 2010). A new publication describing the two identifiedelationships is pending (Sanjuan et al., 2013).

These relationships were in our studies determined to have anccuracy of ±25 ◦C and have the linear generic appearance:

ogNaLi

= a

T (K)− b (1)

here the constants a and b are different for each type of waternvironment, listed in Table 2. Na and Li represent the concentra-ions of each species expressed in mol/l.

As observed in Fig. 1 and in Table 2, the two new Na/Li relation-hips are very different from those known in the literature to dateFouillac and Michard, 1981; Kharaka and Mariner, 1989) and con-rm that the Na/Li ratios not only depend on the temperature butlso on other parameters such as the fluid salinity and origin, or theature of the geothermal reservoir rocks in contact with the deepot fluids. We can conclude that it is essential to well define thenvironment in which the Na/Li geothermometer is applied beforets use. Some case studies found in the literature and thermody-amic considerations as shown in Sanjuan et al. (2010) or Sanjuan

2010) suggest that the Na/Li ratio is probably controlled by dif-erent full equilibrium reactions at high temperatures involvingeldspars, quartz and clay alteration products, partially constitutedf illitic and mica minerals where lithium would be incorporated.

roject, using sampled fluids from several geothermal fields in Iceland, and the threeith Cl > 0.3 M; (b) Kharaka and Mariner, 1989, for oil and sedimentary basin fluids;

The corresponding Li isotope values (ı7Li, in ‰, Fig. 2) indicatethat the values associated with the seawater derived hydrothermalfluids mostly range from +2 to +11‰ and that the values associatedwith the Icelandic dilute non marine geothermal waters can varybetween +3 and +8‰ (Millot et al., 2013). These data are very differ-ent from those obtained by Millot et al. (2012), for New Zealand hightemperature geothermal fluids which indicated low and extremelyhomogeneous values ranging from −0.52 to +1.42‰. For the firsttype of waters, their measured Li isotope values are clearly much

Fig. 2. Li isotopic values measured in the literature (Sanjuan and Millot, 2009) andduring this study as a function of the Li/Na ratios.

R. Ásmundsson et al. / Geothermics 49 (2014) 90–98 93

Table 2The two new geothermometric Na/Li relationships developed by BRGM within the project, using sampled fluids from several geothermal fields in Iceland, to be used withEq. (1), and the three existing Na/Li relationships in the literature (r2 is the regression coefficient).

Type of water Location Temperature range a b r2

Seawater-basaltinteraction derivedhydrothermal fluids

Reykjanes, Svartsengi and Seltjarnarnes geothermal areas (thisstudy), Djibouti geothermal areas and worldwide oceanicridges

0–365 ◦C 920 1.105 0.994

Icelandic dilute nonmarine geothermalwaters

Krafla, Nesjavellir, Námafjall and Hveragerði geothermal areas(this study)

200–325 ◦C 2002 −1.322 0.967

Dilute geothermalwaters (Cl < 0.3 mol/l)

Worldwide volcanic and granitic geothermal areas (Fouillacand Michard, 1981)

0–340 ◦C 1000 -0.38 0.965

Saline geothermalwaters

(Fouillac and Michard, 1981) 0–340 ◦C 1195 0.13 0.982

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exceptionally well and even though the armour was blackened aftersulphur scaling, the insolation looked like new when inspectedafter the different runs.

1 As a ‘historic note’, it was pointed out to the authors by Paul Bixley, ContactEnergy Ltd., that a Wheatstone bridge circuit was used in the first wireline logging

(Cl ≥ 0.3 mol/l)Saline waters Worldwide oil, sedimentary basins and geotherm

(Kharaka and Mariner, 1989)

ractionation between the fluid and the rock; Millot et al., 2010)uggest that the chemical compositions or the alteration grades ofhe basalts in contact with these fluids are different (Sanjuan and

illot, 2009). For the second type of waters (Icelandic dilute nonarine geothermal waters), their Li isotopic values are less scat-

ered but most of their Li/Na ratios are higher and indicate muchore variations than for the first type of waters. The influence of

he heterogenous chemical compositions or the different alterationrades of the volcanic rocks in contact with these waters seems toe more significant than that of the temperature on the Li isotopicalues but not on the Li/Na ratios.

.2. Distributed temperature sensing

Fibre optic distributed temperature sensing (DTS) measure-ents based on Raman backscattering allows for a quasi-

ontinuous temperature measurement over the whole length offibre optic cable. Using a permanent installation of the cable

ehind casing, long term temperature monitoring during cemen-ation, testing, production or workover activities can be performedithout well intervention. To acquire a temperature profile, a laserulse is coupled into the fibre and backscattered photons of thetokes and anti-Stokes frequency band are detected. The ratio ofhe two Stokes intensities can be calibrated to temperature.

Prior to the installation in Iceland different commercially avail-ble optical fibres, suitable for the deployment under the harshonditions of a conventional geothermal well, have been testednder in situ temperature conditions up to 350 ◦C in ambient airnd inert atmosphere (Reinsch and Henninges, 2010). Among theseere fibres with polyimide as well as metal coatings. Despite its

imited temperature range of up to 300 ◦C, a polyimide-coated fibreith a hermetic carbon layer has been shown to possess the most

avourable properties at conditions expected during the productionf hot fluid in a conventional geothermal well. After selecting theost proper fibre, a fibre optic wellbore cable for DTS measure-ents was developed and manufactured as well as temperature

ested up to 280 ◦C in collaboration with nkt cables GmbH, CologneGeckeis et al., 2009). The fibre optic cable is comprised of a double-alled steel tube with an intermediate layer of bronze wire. The

uter jacket is formed by a PFA layer with an outer diameter ofmm (Reinsch et al., 2013).

The fibre optic cable was installed in May 2009 in well HE-53n the Hellisheiði field along the outside wall of the 300 m deepnchor casing and attached to the casing centralizers. The cableormed a loop, with the lowest point at 270 m depth and the two

nds extending on either side of the casing back to the surface.emperature was measured during cementing in May 2009 andhen again during flow testing in July and August as well as after amonth shut-in period in August 2010. Maximum temperatures up

as 0–350 ◦C 1590 −1.299 0.910

to 230 ◦C were measured after two weeks of flow testing in August2009. A detailed description of the installation as well as measure-ment data and data accuracy can be found in Reinsch et al., 2013.Video 3 shows a short section of the installation procedure.

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.geothermics.2013.07.008.

2.3. Temperature sensor on wireline to 300 ◦C

Commercial wirelines cannot operate beyond 300 ◦C and exper-imental wirelines rated at higher temperature are currently notavailable. One of the most notable experimental wireline is theENEL/CISE HostEn cable which was assembled almost twenty yearsago, rated to 400 ◦C and performed well at 300 ◦C (Arisi et al., 1995).Due to this wireline temperature limitation, electronic memorytools are deployed on a slickline to obtain measurements at highertemperatures (up to 370 ◦C using current high-end commercialtools). The results of a memory tool measurement only becomeavailable after the tool is extracted from the well and no ‘live’ mea-surements are available. However, in certain circumstances, therecan be a requirement to measure temperature downhole in realtime using a wireline system, e.g. to actively excite or diminishthermal inflow and follow the response interactively.

The project’s wireline temperature tool was designed by BRGM,based on a previous design by R. Gable, and demonstrated success-fully in June 2010 in the Krafla geothermal field. The tool is analogue(no electronics inside) and uses the well-established technique ofrecording platinum resistance changes that are calibrated to tem-perature (Lebert, 2010).1 The tool was demonstrated in well KS-01in Sandabotnaskarð to 316 ◦C, which is the temperature limit of thewireline cable, see Fig. 3. The tool was however designed to toler-ate 400 ◦C and field testing at that temperature can be performedonce suitable wireline cables become available. The Rochester 4-conductor wireline used within the HiTI project (labelled as type4-H-314M) for tool testing in hot geothermal wells performed

setup in New Zealand back in 1968. The BRGM sensor does not use a Wheatstonebridge, which is a resistivity balancing technique, but relies on the current commer-cial availability of higher impedance voltage gauges which allows direct and accurateevaluation of the potential drop over a resistive object with a known (measured)electric current running through it.

94 R. Ásmundsson et al. / Geothermics 49 (2014) 90–98

Fig. 3. Comparison of a temperature profile taken with the new temperature probeprovided by BRGM (red profile) and a temperature profile recorded a year earlierw(t

2o

itminnflodItbawttsl

twtat

fg

3

m

sa

Fig. 4. The time evolution of temperature recorded within the wellhead on IDDP-1recorded with the Calidus Engineering pressure, temperature, spinner and CCL tool

An hybrid experimental cell for laboratory measurements ofbasalt cores subjected to high temperature and pressure while mea-suring resistivity over the cores was performed in an upgraded

ith an accurate Kuster memory tool owned by ÍSOR. The well has been heating up.For interpretation of references to colour in this figure legend, the reader is referredo the web version of this article.)

.4. Temperature, pressure, fluid flow and casing collar sensingn slickline to 400 ◦C

Supercritical water temperatures start at 374 ◦C, and thereforet was determined that at least one instrument to be provided byhe project would need to be able to accurately measure ther-

odynamic parameters within a supercritical reservoir. That tools the so-called MultiSensor, designed and built by Calidus Engi-eering. The tool is a memory tool, i.e. lowered into a well on aon-communicating slickline and records temperature, pressure,uid flow and casing collar location into the tool’s on-board mem-ry that can be accessed once the tool reaches surface again. Aemonstration was performed in July 2010 in two wells, KS-01 and

DDP-1, the latter well at that time with 360 ◦C at the wellhead. Theemperature recordings from well KS-01 compared very well withoth BRGM’s wirline temperature tool and also calibrated temper-ture instruments owned by ÍSOR. Within the IDDP-1 well, the toolas shown to be able to tolerate the ‘shock-treatment’ of the abrupt

emperature rise when the wellhead valve is opened to the exposedool, see Fig. 4. The spinner had some difficulties in the superheatedteam environment, but both pressure readings and casing collarocations were recorded with high accuracy.

The existence of this tool has recently become of more impor-ance, since the IDDP-1 well has continued to heat up, withellhead temperature now reaching 450 ◦C. The setup during tool

esting at IDDP-1 is shown in Fig. 5. Videos 2 and 3 show thepproach to the well and the instrument inside the tool tube duringesting.

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/j.eothermics.2013.07.008.

. Rock physical parameters and borehole characteristics

In any geothermal exploration, knowledge on the reservoir’s for-ation properties and rock physical parameters are essential.2 As

2 The term ‘rock physical parameters’ covers properties such as rock or formationtrength, resistivity, porosity, radiation and density with the addition of well pathnd casing features.

(red curve). The green curve shows the internal heating buildup. (For interpretationof references to colour in this figure legend, the reader is referred to the web versionof this article.)

an example, the brittle to ductile transition in deep rock formationsmay strongly influence the rock’s geothermal fluid transport prop-erties. Core fragments were obtained from well IDDP-1 in Krafla,but only at roughly 2100 m depth (Friðleifsson, 2010). Despite thelack of deep cores, laboratory testing of basaltic rock was performedwithin the HiTI project on available cores from shallower depthwithin the Krafla field, in order to obtain knowledge on the inter-action of supercritical geothermal fluids with basaltic formations.Also within the HiTI project, a televiewer and a spectral naturalgamma radiation instrument, both operating at 300 ◦C have beendeveloped. The images and radiation spectrum from that combinedtool can be used in a complete comparison study with extractedfuture deep cores.

3.1. Basalt core studies

Fig. 5. Setup during tool testing at well IDDP-1. The 400 ◦C tolerant instrument isinside the measurement tube which is supported by the crane. Dry superheatedsteam flows from the gravel mounted silencer to the left and condensations occurssome metres above the top.

eothermics 49 (2014) 90–98 95

PTcttutaTopÁbodas

3i

w2sKsaatt22ia2nth

3

t

Fig. 6. Casing thickness evaluated with the high temperature televiewer prototypein well B-14, Bjarnarflag, in 2008. The sharp spikes with apparent thickness loss

FBu

R. Ásmundsson et al. / G

aterson Press, situated at Géosciences Montpellier (Violay, 2010).he new cells allows resistivity measurements to be conducted atonfining pressure up to 200 MPa, pore pressure up to 50 MPa andemperature up to 500 ◦C (Violay et al., 2011). In addition, deforma-ion studies of both glassy and non-glassy basalt were conductednder oceanic crust conditions, revealing a possible brittle to duc-ile transition in Icelandic glassy basalt at around 100–200 ◦C butt 550 ± 100 ◦C for non-glassy basalt (Violay et al., 2010, 2012).hese studies improve our understanding of the thermal propertiesf unconventionally deep geothermal systems, currently unex-lored directly and only inferred with e.g. seismic studies, see e.g.gústsson and Flóvenz (2005). An interesting comparison of theasalt studies mentioned above can be made with previous lab-ratory experiments on granite (Takahashi et al., 2003), where arastic enhancement of permeability was observed when temper-ture in a pressurised cell exceeded the critical point of water. Aimilar study with basalt cores should be conducted.

.2. Televiewer and natural gamma radiation boreholenstruments

The first demonstration of the televiewer prototype combinedith a natural gamma spectrometer was carried out in November

008 in well K-18 at Krafla. That well is located on top of a knownupercritical region that was ‘unintentionally’ drilled into from well-39 a month earlier (Árnadóttir et al., 2009). Casing thickness mea-urements were also carried out in two wells using the televiewer,pplying the new casing thickness evaluation features made avail-ble within the HiTI project (Massiot et al., 2009, 2010). A casinghickness profile from well B-14 in Bjarnarflag was recorded withhe high temperature televiewer prototype from ALT in November008, shown in Fig. 6. During the final demonstration in December009, the high temperature televiewer successfully reached 300 ◦C

n well B-14 in Bjarnarflag, see Fig. 7. On tool recovery from the wellfter a total operation time of around 7 h, thereof for roughly 3 h in a50–300 ◦C environment, the electronics were at 43 ◦C. This inter-al electronics temperature gradient is near linear and indicateshat the 125 ◦C tolerant electronics could last up to 30 h at theseigh temperatures.

.3. Dual laterolog resistivity borehole instrument

A downhole resistivity instrument (a dual laterolog ratedo 300 ◦C) was developed by Calidus Engineering but the latest

ig. 7. Televiewer parameters recorded with the ABI prototype in 2009. Temperature inoth travel time and amplitude show 7′′ liner perforation (casing holes) and a liner joint.sing stiff in-line centralizers.

correspond to casing joints at roughly 11 or 12 metre intervals. The profile indicatesa semi-continuous thickness reduction with depth, with the cutoff at 735 m beinglinked to a communication error.

prototype could not be prepared for field testing within the project,primarily due to budget constraints. The tool remains with CalidusEngineering and can be made available for future deep and hotwells.

4. Reservoir and fluid circulation characteristics (tracingtests)

For utilisation of a supercritical geothermal system, the sub-surface fluid flow properties at depth need to be revealed. Tracertesting remains the most powerful method to determine fluid flowpaths, using foreign elements mixed with re-injection waters andmonitoring their return from production wells and other flowingwells within the geothermal area under study. By combining geo-logical and geophysical studies, such as transient electromagnetic,magnetotelluric and seismic methods with the reservoir properties

revealed by tracer testing, an overall conceptual model is derived.

Selection of tracer relies on a combination of properties: a goodtracer must be inexpensive, absent from the original fluid, con-servative (chemically non reactant, nor adsorptive), easily soluble,

the far left column is recording 300 ◦C over the measured section (2211–2224 m).Note the near-perfect centralization at tool inclination 34◦ from vertical, achieved

96 R. Ásmundsson et al. / Geothermics 49 (2014) 90–98

– NDS

dAbkicaitSbAagtp

aswIvsta(

-

partnership on improving geothermal techniques was recentlyformed between several countries and one of the working groupsformed recently published a White Paper listing the current con-

Fig. 8. Monitoring of the 1,5

etectable at very low concentration and environmentally safe.dditionally in geothermal fields, tracers have to be thermally sta-le and their behaviour with respect to the phase change must benown: preferably they have to concentrate either in the liquid orn the vapour phase (see e.g. Rose et al., 2009). The supercriticalonditions strengthen the last selection criteria. The tracers testedt Krafla within the HiTI project are naphtalene disulfonate (NDS)somers, selected since they are tolerant at high temperature (upo 340–350 ◦C; Rose et al., 2001 and a geothermal application inanjuan et al., 2006), concentrate in the liquid phase, non-toxic, cane discriminated and accurately detected (by HPLC) until 0.25 �g/l.s the Krafla geothermal field has been studied and harnessed forbout 40 years, the tracing operation could rely on the relativelyood knowledge of its structure that includes schematically fromhe top to the bottom a liquid dominated system, a vapour one andossibly, locally supercritical conditions.

The tracers were injected into both the main injection wellt Krafla (K-26) and the IDDP-1 well during drilling. The resultshowed very high linear fluid velocities: 14 m/h from the K-26ell, into the upper part of the field and, at least, 320 m/h from the

DDP-1 well, presumably near a cooling magma body. From thoseelocities, very high flow rates could be inferred in the geothermalystem (see Fig. 8). This result was so unexpected that at the firstracing test in K-26, the tracing material could not be recovereds the tracer was dispersed before the beginning of its samplingGadalia et al., 2010).

Moreover:

the recovery rate was exceptionally low: 0.5% in the first case(tracer injection from K26) and even less in the second one (tracerinjection from IDDP),

content from selected wells.

- there were significant anomalies that could be interpreted asthermal impacts: they were correlated to the thermal instabil-ity of the isomers and to the location of very high temperature atthe bottom holes,

- the dispersion of the tracers was extreme and the tracers reacheda majority of productive wells, allowing to infer that all the faultsystems were involved.

All those results are consistent with abnormally high tem-perature conditions and particularly mobile fluid, both of thesupercritical characteristics. Work is now in progress to evaluatefurther the implication of the findings and to repeat the testingwith additional tracer types.

5. Future instrumentation

The instrumentation and methodological improvements madewithin the HiTI project only open the door to the supercriticalregime, and will do not provide a full solution for the geothermalscientists and industry. Several new downhole tools have been sug-gested, applicable also for existing conventional high temperaturefields and future enhanced geothermal systems. An international

3

sensus on the immediate improvements required by both the

3 International Partnership of Geothermal Technology (IPGT), consisting of theUnited States of America, Iceland, Australia, Switzerland and New Zealand.

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cientific community and well owners (Ásmundsson et al., 2011).he main results are that downhole tools for supercritical thermo-ynamic measurements will need to tolerate up to 600 ◦C, whilepen hole formation tools such as seismic, including acoustic andesistivity need to be upgraded to 300 ◦C. For the currently oper-ting geothermal industry, a tool capable of determining casinghickness with high accuracy and other detailed casing evaluationsn fully heated production wells needs to be developed and madevailable, allowing casing evaluation to be carried out in steam orater phase without cooling down the well.

.1. High temperature electronics

Most commercially available high temperature electronics areimited to 225 ◦C operation with some excursions to 300 ◦C. Theselectronics are based on Silicon-on-Insulator technology (SOI). SOIas initially developed for radiation hardening silicon devices andore recently, for low power applications. This works by limiting

he actual devices to a thin layer of silicon separated from the bulky a buried oxide layer thus isolating the active devices from major

eakage pathways. SOI has also been exploited for high tempera-ure devices; again the advantage afforded by confining the activelectron part of the device to only a thin isolated layer cuts off theajor leakage paths that would otherwise limit the temperature

t which devices could operate. High temperature SOI devices areow widely available with tens of thousands of certified operating

ifetimes at high temperatures (225 ◦C).However, silicon, even SOI, has an intrinsic upper operating tem-

erature derived from the thermal activation of carriers within theaterial. To overcome this, wider band gap (WBG) materials such

s silicon carbide (SiC) and diamond must be utilised. However, theechnology for producing devices from these materials is much less

ature than that for silicon or SOI.The development of SiC has been driven primarily by solid state

ighting applications where SiC is used as a substrate material forrowth of hetero-epitaxial GaN which can be fabricated into a blueight emitting diode. A secondary driver for SiC development haseen for power electronic devices where SiC offers a distinct advan-age over silicon by being able to withstand higher voltages andperate at higher current loadings enabling smaller more efficientower electronics to be developed.

A few groups have demonstrated limited integrated devicesabricated from SiC. Such devices tend to be simple operationalmplifiers or very simple logic circuits. The main driver has beenor space applications particularly coming from NASA requirementsor electronics for a possible Venus Lander mission. However, inhat instance the lifetimes required are measured in hours, 10 h at

ost and thus are unlikely to furnish technologies suitable for theurrent geothermal requirements. There are exceptions to this andaytheon Systems Limited are developing relatively complex SiCased circuitry that should operate to 500 ◦C; however, such devel-pments are at an early stage and it may be several years before theevices needed by complex instruments capable of 500 ◦C opera-ion are realised.

Even if devices capable of 500 ◦C operation were currently com-ercially available, the question of how to package these devices

emains, on the whole, unanswered. The primary options wouldeem to be with hybrid thick film ceramic technology but this is aset unproven for 500 ◦C operation.

.2. Sensors

Sensors, including transducers, have now been developed inany fields of science and industry to high temperature toler-

nces. The future challenge will be to fit these new sensors into

rmics 49 (2014) 90–98 97

a functional downhole system architecture. Currently, advancedacoustic downhole instruments are available, tolerating up to260 ◦C (not mentioning the 300 ◦C televiewer discussed above),which is sufficient for the oil and gas industry and many geother-mal applications. Pressure and temperature can now be measuredin unshielded (barefoot) tools up to 300 ◦C and likely that a targetof 500 ◦C or even 600 ◦C could be reached in the near future on heatshielded electronics tools. Mechanical sensors, such as spinnersand calliper arms, suffer serious problems at higher temperaturesand the short-term approach will most likely be on increasedapplication of acoustic transducers and even optical devices.

5.3. Housing, sealing and wireline

The downhole tools need to be housed with corrosive resistantmetals, most often steel or titanium alloys or nickel-chromiumbased superalloys, chosen according to the estimated reservoircharacteristics. The thermoplastic elements which are used in sen-sor windows or electrode insulation are often of the PEEK type(polyether ether ketone). Seals are frequently fluoroelastomers orsoft metals, such as silver or gold. High temperature oil and grease isused where required, e.g. as transducer coupling material, in cable-heads and pressure capillary tubes. Future tool designs will rely onthe available material and further material improvements whenthey become available.

Memory tools (tools that store data in memory downhole) andother sensing elements can be lowered into wells on strong and cor-rosive resistant slicklines to any of the reservoirs considered here.However, following up on the Italian HostEn cable development ini-tiative mentioned in section 2.3 above, wireline cables need to beupgraded to the supercritical conditions, possibly also implemen-ting fibre optic cables. The fibre optic cables can be used as sensingelements for temperature and pressure along its length and also forfast tool communication.

Acknowledgements

The authors would like to express their gratitude to the manycollaborators who contributed to the work described here. Inparticular, we would like to thank the geothermal field ownersLandsvirkjun, Orkuveita Reykjavíkur and HS Orka. We are thankfulfor the funding received within the 6th Framework Programme ofthe European Union, project number 19913.

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