In March 1986 a research borehole of the Salton Sea 
Scientific Drilling Project reached a depth of 3,220 m in 
the Salton Sea geothermal system of Southern 
California as part of the first major research drilling 
project of the U.S. Continental Scientific Drilling Program 
(Elders and Sass, 1988). The principal goals of the 
SSSDP were to investigate the physical and chemical 
processes of a high-temperature, high salinity, 
magmatically driven, hydrothermal system at depths 
approximately twice as deep as the usual existing 
production wells in that geothermal field. 
In mining exploration and in scientific drilling in the 
oceans continuous core drilling is the norm. However 
based on the advice of a drilling consultant the idea of 
continuously coring the SSSDP borehole was 
abandoned because of the difficulties expected at the 
high temperatures anticipated (Ross and Forsgren, 
1992). Instead it was decided to follow standard oilfield 
practice and use rotary drilling but to attempt to core 
about 10% of the drilled depth: 1 million USD of the 
budget was assigned to that task. Plans called for 
setting aside 250 hours of rig time for logging and 
downhole fluid sampling, but, because of the difficulties 
experienced, these tasks actually required 487 hours.  A 
broad suite of geophysical logs was obtained from the 
upper part of the borehole, but at depths where 
formation temperatures exceeded 300°C, the inability 
to cool the borehole sufficiently severely limited the 
logging program (Paillet and Morin, 1988). The demands 
of the logging program required that the amount of 
coring be reduced and, when the borehole reached final 
depth, only 224 m (or 7%) of the borehole had been 
cored (Ross and Forsgren, 1992).
The borehole encountered temperatures of up to 360°C 
and produced metal-rich, alkali-chloride brines 
containing up to 25 wt% of total dissolved solids.  The 
rocks penetrated exhibited a progressive transition from 
unconsolidated lacustrine and deltaic sediments to 
hornfelses, with lower amphibolite facies mineralogy, 
with locally abundant ore minerals. A key finding from 
study of the cores, with implications for commercial 
development of the resource, was that at shallower 
levels permeability is controlled by secondary porosity 
formed by the incongruent dissolution of carbonate 
cement in sandstones. With increasing temperatures 
this porosity is destroyed in hornfelsed sediments 
where permeability is entirely dominated by partially 
mineralized fractures. Flow tests of the deeper part of 
the borehole yielded flow rates of up to 370,000 kg/h at 
250psig (1,724 kPa or ~17bar) wellhead pressure, 
sufficient to provide enough steam to generate at least 
12 MWe, thus demonstrating the commercial potential 
of the deeper reservoir. 
Even although the cores recovered were limited, they 
made possible pioneering studies of petrophysical 
properties, sedimentary and evaporitic facies analysis, 
resolution of the source of the dissolved salts, 
evaluation of structural relationships, identification of 
igneous-intrusive units, resolution of mineral-
paragenesis, and of vein-deposition sequences related 
to emplacement of ore-bodies.  Detailed petrography 
and isotope analyses of the cores permitted 
identification of the sources of sulphur in the ore 
mineralization.  Two distinct types of vein mineralization 
could be seen in the cores, (a) an earlier a carbonate + 
sulphide (sphalerite, galena, chalcopyrite, pyrite, 
pyrrhotite) + epidote assemblage, and (b) a later 
epidote + hematite + anhydrite + sulphide + quartz 
assemblage. The type (a) veins appear to be older and 
the deeper production is associated with type (b) 
veins.  Laboratory petrophysical measurements of 
porosity, density, and P-wave velocity improved 
calibration and interpretation of downhole and surface 
geophysics (Elders and Sass, 1988). 
The IDDP
 The aim of the IDDP is to explore the potential of 
producing supercritical hydrothermal fluids as an energy 
source (Fridleifsson and Elders, 2005).  Superheated 
steam produced from a fluid initially in the supercritical 
state will have a higher enthalpy than steam produced 
from an initially two-phase system, but, in addition, 
large changes in physical properties of fluids occur near 
the critical point. Orders of magnitude increases in the 
ratio of buoyancy forces to viscous forces occur that can 
lead to very high and fluctuating rates of mass and 
energy transport. Similarly, because of major changes in 
the solubility of minerals above and below the critical 
state, supercritical phenomena play a major role in high 
temperature water/rock reaction and the transport of 
dissolved metals (Fournier, 1999). Hitherto, study of 
such supercritical phenomena has been restricted to 
either small-scale laboratory experiments or to 
investigations of “fossil” supercritical systems exposed 
in mines and outcrops. Furthermore mathematical 
modelling of the physics and chemistry of supercritical 
fluids is hampered by a lack of a reliable thermodynamic 
database over the range of temperatures and 
pressures of the supercritical state, particularly for 
saline fluid compositions.
Drilling to produce supercritical fluids at depths of 4-5 
km and temperatures of 500-600°C is, with one 
exception, almost unprecedented in the worldwide 
geothermal industry. The only precedent was another 
example of collaboration between government, industry 
and the scientific community that also aimed to extend 
conventional geothermal resource investigations into 
new regimes of temperature and make direct 
observations and sample the processes that couple 
magmatic and hydrothermal processes. That project, 
funded by the Agency of Industrial Science and 
Technology (AIST) of Japan, and involving scientists and 
engineers of the New Energy and Industrial Technology 
Organization (NEDO – a government agency), the Japan 
Metals and Chemicals Co., and university scientists was 
a 3.7 km deep exploratory core hole at Kakkonda, in the 
Hachimanti Geothermal Field, Iwate Prefecture, Japan 
(Muraoka, et al., 1998). This well penetrated an entire 
shallow hydrothermal convection zone, an entire 
contact metamorphic aureole, and part of a neo-granitic 
pluton (tonalite with a K-Ar age of 0.19 Ma), a cooling 
granitic intrusion that is the heat source for the 
hydrothermal system. The shallow hydrothermal system 
is developed in Holocene to Miocene volcanic and 
intrusive andesitic rocks and shows a boiling point 
controlled temperature profile down to 3100 m depth, 
where a 380°C temperature occurs. At that depth and 
temperature, a transition from brittle to ductile 
conditions was observed and the temperature gradient 
became conductive.  Temperatures reached >500°C at 
3729 m (Muraoka, et al. 1998). At the bottom of the 
borehole the permeability was very low and the 
borehole was subject to plastic deformation. For this 
reason the drill hole was completed as a production well 
in the shallow hydrothermal system. The Kakkonda 
deep borehole represented a number of firsts that 
extended the range of geothermal investigations 
worldwide. The bottom hole temperature of >500°C is 
the highest so far measured in any drilling project, 
except for the rare occasions when still-molten lavas 
have been drilled at about 100 m depth.  The borehole 
at Kakkonda reached temperatures higher than the 
boiling point depth curve and entered a subsolidus 
cooling pluton. It was also the first geothermal well to 
penetrate the brittle-ductile transition (Muraoka, et al. 
1998).  It is theorized that, in very high-temperature 
geothermal systems, the main constraint on the 
maximum depth of hydrothermal convection is the 
control on permeability exerted by the transition from 
brittle to ductile behaviour (Fournier 1999).  
History of the Drilling Plan Adopted by IDDP  
Given this background, from the outset, the IDDP, 
recognized that the success would involve solving 
difficult technical and scientific problems and welcomed 
the inclusion of basic scientific studies. The project 
invited participation from the international scientific 
community (Fridleifsson and Albertsson, 2000).  The first 
concern was to decide how best to drill into, test and 
study supercritical geothermal reservoirs in Iceland.  
Two international workshops in 2002 funded by the 
International Continental Scientific Drilling Program 
(ICDP) were held. The workshop, held in March 2002 
brought together more than 50 international experts to 
develop the optimum strategy of meeting the difficult 
technical challenges of drilling and sampling wells to 
depths of up to 5 km with temperatures of >450°C.   
The workshop led to a clearer definition of the range of 
conditions likely to be encountered and developed the 
guidelines for planning drilling and sampling.  
The drilling workshop considered different options for 
obtaining cores in a high-temperature geothermal 
production well. Fruitful discussions were held among 
panel participants with a diversity of experience in 
drilling in countries and different environments, and with 
representatives of organizations that have developed 
different approaches to drilling and coring.  The 
workshop considered four options for obtaining cores in 
two different sizes of production wells, A and B.  Well 
profile A has a 9 5/8 inch production casing to 3500 m, 
whereas Well profile B has a 9 5/8 inch casing to 2400 
m with a 7 inch production casing to 3500 m.  The upper 
part of Profile B (to 2400 m) is the design currently used 
in standard production wells used in the geothermal 
fields of the Reykjanes Peninsula in Iceland. The two 
different options for coring considered were continuous 
wireline coring and spot coring.  It was estimated that 
drilling, coring and reaming to a nominal depth of 5000 
m would take about 250 days. The preferred plan 
suggested was to use a wireline coring system 
attached to the platform of a conventional rotary rig to 
continuously core the well in two stages, the first down 
to 3.5 km depth, followed by reaming, and then 
continuous coring to the    supercritical zone expected 
beyond that depth. 
The second workshop held in October 2002 brought 
together approximately seventy international experts 
on geothermal systems. It continued the discussion on 
technical issues of drilling into and sampling supercritical 
geothermal reservoirs. In addition, panels were held on 
studies of geology, fluid chemistry and reservoir 
properties. There was a strong agreement that there 
was a crucial need for drill cores. Studies of fossil 
supercritical geothermal systems exposed in mines and 
outcrops, that formed in similar environments to those 
expected in Iceland, indicate that supercritical fluids 
have pervaded every cubic centimetre of their basaltic 
host rocks. It is not known if this occurs by diffusion 
from spaced-out fractures, or by microfracturing and 
fluid advection on a sub-millimetre scale.  Core samples 
from deep wells will give first-hand look at the process.  
Similarly, the thermodynamics of supercritical solutes is 
poorly known, as the transition from sub- to 
supercritical conditions at the magma-hydrothermal 
interface is accompanied by substantial changes in the 
physical-chemical characteristics of the reservoir fluids.  
The paired samples of fluids and rocks from the IDDP 
well will be extremely valuable for testing and improving 
numerical models of reservoir properties, of fluid-rock 
reactions, and of controls on the compositions of both 
fluids and minerals under supercritical conditions.  This 
requires obtaining as much core as possible in and near 
the supercritical transition. Study of the coupling of the 
chemical and mineral alteration, fracture propagation, 
pressure solution, and fluid flow will be based on 
analysis of data on mineral chemistry, isotopes, 
geothermometry, and fracture geometry.  The costs of 
the many man years of laboratory studies and 
interpretation are a major contribution by the scientific 
team to the IDDP. Similarly the science program is 
contributing more than 3.6 million USD to the coring 
program.  More than half the IDDP science projects 
proposed by the science team would be impossible or 
severely compromised without drill core. Coring is a part 
of all major scientific drilling projects today. The 
philosophy is that utilization of core will increase as 
science progresses in the future and cores constitute a 
robust archival record.  
There are also practical reasons for coring. Nowadays, 
the usual practice in Iceland is to drill with downhole 
motors, for their high rate of penetration, and this 
produces very fine-grained drill cuttings. Irrespective of 
the cutting size, unravelling the nature and chronology 
of fracture failure and vein in-filling and detection of time 
serial fracture events and determination of constitutive 
rock properties is impossible without drill cores. Similarly 
understanding the nature and formation of permeability 
requires study of cores.  Measurements of mechanical 
and thermal properties of core as a function of 
temperature are necessary to quantify processes 
related to brittle-ductile behaviour. The permeability and 
thermal diffusivity of fractured and intact, fresh and 
altered, basalt comprise essential baseline information 
for fluid circulation models. Furthermore, as indicated 
above lost circulation during drilling, which is common in 
highly permeable zones in Iceland, would prevent 
recovery of drill cuttings. Similarly the use of borehole 
televiewers and most other logging tools is impossible 
because of high temperatures. Furthermore, recognizing 
when the supercritical conditions are reached during 
drilling will best be done by studying mineral 
assemblages and fluid inclusions in cores as they are 
recovered. 
However, the estimated costs of continuous coring 
followed by reaming to the diameter of the well to 
production size escalated considerably at that time. 
Because continuous coring is slower and therefore more 
expensive than conventional drilling, a compromise was 
proposed and adopted to reserve continuous coring for 
the supercritical zone below 3.5 km depth and to take 
only spot cores in the upper part of the well.
These recommendations were reviewed and refined in 
the two year long feasibility study funded by Deep 
Vision (Fridleifsson et al., 2003). Meanwhile the science 
team has obtained awards of 3 million USD from the US 
National Science Foundation, and 1.5 million USD from 
the ICDP to obtain drill cores and data for scientific 
studies as part of the IDDP. Given the extremely 
competitive nature of funding for science projects and 
the limited budgets available, these are very large 
awards.  This is an indication of the global significance 
of the IDDP project.  In 2003 an Icelandic energy 
company, a member of the IDDP consortium, offered 
one of its planned exploratory wells on the Reykjanes 
peninsula, in southern Iceland, to the IDDP for 
deepening. This well reached 3.1 km in February 2005, 
and research on the downhole samples began. 
Unfortunately the well became plugged during a flow 
test and was abandoned in February 2006 after 
attempts to recondition it failed. This led to the IDDP 
deciding to move the site for the first deep borehole to 
Krafla, near the northern end of the central rift zone of 
Iceland, within a volcanic caldera that has had recent 
volcanic activity. The Krafla geothermal system has 
higher temperature gradients than at Reykjanes. The 
drill site chosen is near an existing well that 
encountered 340°C at only 2.5 km depth. Drilling is 
expected to occur in 2008.
An informational workshop concerning the IDDP was 
held in Reykjavik in March 2007 to review the current 
situation, to reconsider goals and plans in view of new 
circumstances, and to make recommendations. It 
became apparent that there was increased interest in 
the IDDP on the part of the Icelandic energy industry.  It 
now seems likely that in the next few years deep (> 3.5 
km) exploratory boreholes will be drilled in each of three 
major geothermal fields in Iceland. Because deep drilling 
plans are furthest along at Krafla, it was decided to 
focus on that field first.   
However a prime concern is the recent even larger 
escalation of cost estimates for drilling, coring, fluid 
sampling and testing of a deep geothermal well.  As 
indicated above, continuous coring is much more 
preferable in terms of characterizing the nature of the 
resource. However Icelandic drilling engineers had 
serious reservations about the performance of a 
wireline coring system at high-temperatures and raised 
questions about the difficulty of cooling the bottomhole 
assembly in a narrow diameter corehole. Another 
concern is the cost of changing drilling rigs necessary for 
the different tasks of rotary and core drilling and the 
difficulty and cost of reaming the corehole to production 
size. Continuous coring is inherently more expensive 
than rotary drilling and there is limited experience of 
continuous coring at very high temperatures.  It is likely 
that these issues can only be addressed by actual 
experience in continuous coring at high temperature. 
Given the  present circumstances, unless in the 
immediate future a technically and fiscally viable option 
for continuous coring can be found by 2008, the 
principal investigators reluctantly concluded early in 
June 2007 that the first deep IDDP well, should be 
rotary drilled to target depth (4.5 km) while taking a 
reasonable number of spot cores, depending on the 
budgetary constraints.  Depending on the temperature 
gradient the supercritical conditions may be 
encountered at less than 4.5 km depth.  In the future, 
based on the knowledge that will be gained from study 
of these deep wells, the IDDP should seek funds for 
continuous coring through the supercritical zone in a 
geothermal system in Iceland.  
The economic potential for developing deep high-
temperature geothermal resources in Europe has not 
yet been thoroughly assessed. However the potential 
for making a major contribution of sustainable energy in 
the region may be very large, not only in Iceland and 
Italy, but also in the Azores, Canaries, Turkey, Greece, 
Guadeloupe, Russia, and elsewhere (Elders, 2007).  Our 
limited knowledge of the nature of such geothermal 
systems is a handicap in exploring for and developing 
these deep unconventional geothermal resources 
(DUGR’s). Given the present state of our knowledge, it 
is therefore desirable to carry out detailed scientific 
investigations of different classes of high-temperature 
systems and the study of drill cores is an essential part 
of such studies.  Hence the plea to the ENGINE drilling 
community for advice on improving the technology and 
economics of both spot core drilling, and of continuous 
wireline coring at high temperatures.
References 
Elders, W.A., 2007, “Development of Deep 
Unconventional Geothermal Resources (DUGR’s) in 
Iceland and their Potential Application Elsewhere in 
Europe.”  Workshop on Exploring High Temperature 
Reservoirs; New Challenges for Geothermal Energy of 
ENGINE(Enhanced Geothermal  innovative Network for 
Europe). 1-4 April 2007, Volterra, Italy. 
Elders, W. A. and Sass, J. H., 1988.  "The Salton Sea 
Scientific Drilling Project." J. Geophysical Research, 
93 :12953-12968. 
Fournier R.O. 1999. Hydrothermal processes related to 
movement of fluid from plastic into brittle rock in the 
magmatic-epithermal environment.  Economic Geology 
94: 1193-1211.
Paillet, F.L., and Morin, R.H. 1988, “Analysis of 
geophysical well logs obtained from the State 2-14 
borehole, Salton Sea Geothermal Area, California.” J. 
Geophysical Research, 93:, 12981-12994.
Fridleifsson, G. O., Ármannsson, H., Árnason, K., 
Bjarnason, I.Th., and Gíslason, G. 2003 Part I: 
Geosciences and Site Selection, 104 p. In: Iceland Deep 
Drilling Project, Feasibility Report, ed. G.O. Fridleifsson. 
Orkustofnun Report OS-2003-007.
Fridleifsson, G.O., Albertsson, A., 2000. “Deep 
geothermal drilling at Reykjanes Ridge: opportunity for 
an international collaboration. In: Proceedings of the 
World Geothermal Congress, Japan, pp.3701-3706.  
Fridleifsson, G.O. and Elders, W.A, 2005, "The Iceland 
Deep Drilling Project: a Search for Deep Unconventional 
Geothermal Resources." Geothermics, 34: 269-285. 
Muraoka H., Uchida T., Sasada M., Mashiko Y., Akaku K., 
Sasaki M., Yasukawa K., Miyazaki S., Doi N., Saito S., 
Sato K., Tanaka S. 1998, “Deep geothermal resources 
survey program: igneous, metamorphic and 
hydrothermal processes in a well encountering 500 oC 
at 3729 m depth, Kakkonda, Japan”. Geothermics 27: 
507-534.
Ross, H.P. and Forsgren C.K. (editors) 1992 “Salton Sea 
Scientific Drilling Project: A summary of drilling and 
engineering activities and scientific results”. Final report 
DOE/CE-12429-H1. prepared for the U.S. Department of 
Energy, contract No. DE-AC07-90ID12429, pp185.

Grensasvegur 9
Reykjavik,
Iceland