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Sci. Dril., 17, 51–59, 2014
www.sci-dril.net/17/51/2014/
doi:10.5194/sd-17-51-2014
©Author(s) 2014. CC Attribution 3.0 License.
Scientific Drilling
Open Access
Workshop Reports
The Japan Beyond-Brittle Project
H. Muraoka1, H. Asanuma2, N. Tsuchiya3, T. Ito4, T. Mogi5, H. Ito6, and the participants of the
ICDP/JBBP Workshop
1North Japan Research Institute for Sustainable Energy, Hirosaki University, Matsubara 2-1-3,
Aomori 030-0813, Japan
2Renewable Energy Research Center, AIST, Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan
3Graduate School of Environmental Studies, Tohoku University, Aoba 6-6-20, Miyagi 980-8579, Japan
4Institute of Fluid Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan
5Graduate School of Sciences, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo 060-0810, Japan
6Independent scientist, Japan
Correspondence to: H. Asanuma (h.asanuma@aist.go.jp)
Received: 12 August 2013 – Revised: 7 December 2013 – Accepted: 16 December 2013 – Published: 29 April 2014
1 Introduction
1.1 Outline of the workshop
The international workshop “Japan Beyond-Brittle Project,
JBBP – Scientific drilling to demonstrate the feasibility of
engineered geothermal systems in ductile zones” was held
at the Graduate School of Engineering, Tohoku University,
Sendai, Japan, during 12–16 March 2013. The workshop
was cosponsored by the ICDP (International Continental
Scientific Drilling Program) and Tohoku University GCOE
(Global Center Of Excellence) Project. A total of 102 people
attended the workshop and 98 presentations were made (75
oral, 23 poster).
1.2 Background
Although various advantages of geothermal energy have
been widely accepted, power generation using natural hy-
drothermal reservoirs has not been recognized in Japan as
an attractive investment, mainly because of a general per-
ception of high development risks and uncertain returns on
investment.
An engineered geothermal system (EGS) is considered to
be the best solution to the problems of the hydrothermal re-
sources. However, previous Japanese hot dry rock (HDR)
projects showed that water recovery from an EGS reservoir in
a fracture-rich tectonic belt in Japan is at best 50% (Tenma
et al., 2004; Kaieda et al., 2005). Another important issue
is the difficulty of designing EGS reservoirs in a tectonic-
belt setting, where local variations in tectonic stress and frac-
ture distribution are common. Furthermore, the occurrence of
felt earthquakes from the EGS reservoirs (Majer et al., 2007;
Häring et al., 2008) introduces additional environmental bur-
dens and risks.
These problems in the development of hydrothermal and
EGS reservoirs cannot be readily solved in Japan because
they are intrinsically related to the physical characteristics
and tectonic setting of the brittle rock mass. Hence,we ini-
tiated a project, the Japan Beyond-Brittle Project (JBBP), to
investigate the feasibility of developing an EGS in brittle–
ductile transition (BDT) zone. The expected advantages of
EGS in the BDT are as follows:
1. More homogeneous rock properties and stress states in
the BDT make it conceptually simpler to design and
control geothermal reservoirs.
2. A nearly full recovery of injected water can be expected
from hydraulically closed reservoirs.
3. Sustainable production can be realized by controlling
the flow rate and chemical contents of circulated liquids.
4. Possible site-independent characteristics of ductile
zones may lead to the establishment of universal de-
sign/development/control methodologies.
5. Induced/triggered earthquakes with damaging magni-
tudes will not occur in reservoirs in ductile rock masses.
Published by Copernicus Publications on behalf of the IODP and the ICDP.
52 H. Muraoka et al.: The Japan Beyond-Brittle Project
200℃
300℃
400℃
500℃
Approx.
380℃
Hydrothermal
Undeveloped
HT hydrothermal
Thermal convection
/ conduction and
Silica solubility
transition JBBP Reservoir
(Type 1)
JBBP Reservoir
(Type 2)
Larger productivity and sustainability by connecting
to deep HT hydrothermal system
Controlled quartz solubility for reservoir treatment
and reduction of scaling
Lower cost relative to Type-2
JBBP Reservoir (Type 1)
Higher enthalpy
Nearly full water recovery
Universal design/development methodology
Possibility to reduce risk of large induced seismicity
Possible independency to existing hydrothermal
resources for direct use
JBBP Reservoir (Type 2)
Figure 1. Two possible types of JBBP reservoirs.
2 Possible reservoir types for JBBP
It has concluded that there are two end-member reservoir
models that should be considered (Fig. 1).
1. A JBBP type 1 reservoir would be created near the top
of the BDT, where quartz solubility andfracture density
are markedly different from those in the brittle zone.
The reservoir should be connected to preexisting hy-
drothermal systems to increase productivity and provide
sustainability.
2. A JBBP type 2 reservoir would be hydraulically or ther-
mally created beyond the BDT, where preexisting frac-
tures are less permeable, and would be hydraulically
isolated from the hydrothermal system.
3 Characterization of the beyond-brittle rock mass
3.1 Current understanding of the characteristics of the
beyond-brittle rock mass
The large strain rate by fluid injection renders the rock
mass brittle because it fractures in tensile and shear modes,
creating fractures aligned with the regional stress regime.
These fractures are observed as millimeter- to centimeter-
scale quartz veins in porphyry copper deposits, which are
quartz-filled and plugged fractures, where quartz appears to
have precipitated upon adiabatic decompression and cooling
as fluids traversed from lithostatic pressure, P(l), to hydro-
static pressure, P(h), regimes. According to the experimen-
tal findings of Okamoto, Tsuchiya, and Saishu in the Tohoku
University team, quartz precipitation at temperatures exceed-
ing 400◦C seals permeability, possibly on a time scale as
short as days or weeks.
The review above suggests that heat extraction from a 400
to 450◦C granite mass by hydrostatically pressured fluid in-
jection will be challenging, especially because of fracture
plugging by quartz and probably also because of closing of
fractures by rock creep. Quartz fracture plugging can pos-
sibly be limited by high flow rates as fluid temperature de-
scends to below 400◦C upon adiabatic decompression to
P<P(h). Quartz precipitation rates may slow sufficiently at
T<400◦C to allow fluid ascent without the plugging of frac-
tures. If not, there is a serious problem that requires investi-
gation.
Rock physics experiments at high pressures and tempera-
tures are central to the achievement of sustainable geother-
mal development. Characterization of the physical proper-
ties of rocks (e.g., permeability, Pand Swave velocities, and
electrical conductivity), is a strong indicator of the correct in-
terpretation of the geophysical field data used for subsurface
exploration.
Workshop participants suggested that laboratory-based
physical investigations of the JBBP rock–fluid system should
focus on fracture generation and the lifetime of fracture net-
works in ductile rock systems. Such studies will prove in-
dispensable information for characterizing time and distance
scales for fluid flow in ductile rocks and will also provide
data that can be used to improve stimulation techniques in
connection with new concepts of EGS beyond the brittle
field.
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H. Muraoka et al.: The Japan Beyond-Brittle Project 53
3.2 Scientific challenges
Numerical simulation is an important technique that can be
applied to assist in the realization of hydrothermal fluid flow
in the beyond-brittle rock mass. Recent advances in numeri-
cal simulation techniques allow simulation of fully transient
single- and multi-phase hydrothermal fluid flow on a contin-
uum extending to magmatic conditions (Hayba and Igebrit-
sen, 1997; Coumou et al., 2008a, b; Weis et al., 2012). These
simulations successfully reproduced a wide range of the key
features of such systems (e.g., thermal structure and evolu-
tion, temporal and spatial patterns of fluid phase states, fluid
pressure distribution).
A new approach to the characterization of deep rock
masses may arise from the exchange of data and results. For
example, in the new Swiss–Icelandic combined hydrologi-
cal, geochemical and geophysical modeling of geothermal
systems (COTHERM) project led by Thomas Driesner (Fed-
eral Institute of Technology, Switzerland), the capability of
new simulation techniques to accurately predict the distribu-
tion of strongly varying fluid properties in the subsurface will
be used to better calibrate interpretations of geophysical and
geochemical signals. A similar technique could also be used
in our project.
In the Kakkonda geothermal field (Japan), a geother-
mal drill hole (WD-1a; Fig. 2) penetrates the boundary
between the hydrothermal-convection and heat-conduction
zones (Doi et al., 1998); this is a unique example of drilling
beyond the brittle rock mass. Drilling has shown that quartz
solubility has a local minimum at ∼3100m depth (380◦C,
24MPa), which is consistent with the depth of the hydrolog-
ical boundary. Quartz precipitation has possibly created an
impermeable siliceous layer at this depth. This water–rock
interaction would lead to spontaneous development of the
bottom of the hydrothermal-convection zone, which controls
fluid flow.
In geothermal fields, we need to consider a coupled chemi-
cal and mechanical model to evaluate beyond-brittle geother-
mal reservoirs.
4 Creation and control of EGS reservoirs in the
ductile zone
Although we know very little about the geometry of artifi-
cial or natural fracture systems in the BDT, we can spec-
ulate on the basis of two end-member scenarios. On one
hand, hydraulic stimulation may produce a single fracture, or
a zone of fracture deformation controlled by local stresses.
On the other hand, a more complex cloudlike fracture net-
work might be produced, where the geometry of the frac-
ture system would depend on many factors. These could in-
clude deformation mechanisms, stresses, and rock properties.
The natural systems of fluid flow can indicate the growth of
such a fracture network, as shown by the movement of fluids
or changes of pore pressure during and after stimulation. It
is also possible that hydraulic fracturing could inadvertently
trigger fault motion. Thus, it is very important to understand
the mechanical characteristics of faults or fractures.
Our workshop discussions of the creation and maintenance
of EGS reservoirs in the BDT and ductile zone were based
on current knowledge, especially from the view point of rock
mechanics. The subjects specified at the workshop as being
of prime importance are (1) mechanical and hydraulic prop-
erties of rock, (2) in situ states of stress, and (3) seismic ac-
tivity on fractures or faults.
4.1 Mechanical and hydraulic properties of rock
The creation and maintenance of such an embrittled zone em-
bedded within a nominally ductile region in the deep crust
poses significant scientific challenges. The first challenge is
to understand how deformation, in the form of either tensile
or shear fractures, can be nucleated in a matrix that will de-
form anelastically by, for example, cataclastic flow. The vis-
coelastic rheology and failure in such a transitional regime
would likely involve a combination of semi-brittle mecha-
nisms, including crystal plasticity, diffusive mass transfer,
and microcracking. To formulate modeling methodology, it
is advisable to take into account recent advances in the mod-
eling of analogous geodynamic processes, such as stress re-
laxation during interseismic phases of the earthquake cycle.
It is of considerable importance to address coupled
thermo-hydro-mechano-chemo (THMC) processes when
considering the effective extraction of energy from geother-
mal reservoirs. Under high pressure and temperature condi-
tions, chemical reactions such as mineral dissolution and pre-
cipitation are very active, and may quickly change the me-
chanical and hydraulic properties of host rocks. Therefore,
the effects of the dissolution and precipitation kinetics on the
physical properties of rocks should be examined microscop-
ically.
4.2 In situ state of stress
Knowledge of the in situ state of stress and the geometry and
hydrologic properties of potential failure surfaces (fractures,
faults, and foliation) is required in order to create an EGS
reservoir with optimal geometry, fracture density, and heat-
extraction efficiency.
The magnitude of the least horizontal principal stress
(SHmin) is best determined using small-scale hydraulic frac-
turing stress tests (minifracs). Owing to the difficulty of find-
ing reliable open-hole packers for use at high temperatures,
such tests are best carried out in geothermal wells by drilling
a short (∼20 m long) pilot hole from the bottom of cemented
casing and pressurizing the cased hole to carry out a minifrac
in the pilot hole. Ideally, minifracs would be conducted be-
low every casing shoe during the drilling of a JBBP borehole,
thus obtaining as complete a vertical stress profile as possi-
ble.
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54 H. Muraoka et al.: The Japan Beyond-Brittle Project
Figure 2. Location of the Kakkonda geothermal field in the Hachimantai volcanic field, northeastern
Japan, and schematic cross section of the Kakkonda geothermal systems (modified after Doi et al.,
1998). Well WD-1a encountered the boundary between the hydrothermal-convection and
heat-conduction zones at a depth of 3100 m (Doi et al., 1998). This figure will be published by
Saishu et al. (2013).
4. Creation and control of EGS reservoirs in the ductile zone
Although we know very little about the geometry of artificial or natural fracture systems in the
BDT, we can speculate on the basis of two end-member scenarios. On one hand, hydraulic
stimulation may produce a single fracture, or a zone of fracture deformation controlled by local
stresses. On the other hand, a more complex cloud-like fracture network might be produced, where
the geometry of the fracture system depends on many factors. These may include deformation
mechanisms, stresses, and rock properties. The natural systems of fluid flow can indicate the growth
of such a fracture network, as shown by the movement of fluids or changes of pore pressure during
and after stimulation. It is also possible that hydraulic fracturing could inadvertently trigger fault
motion. Thus, it is very important to understand the mechanical characteristics of faults or fractures.
Our workshop discussions of the creation and maintenance of EGS reservoirs in the BDT and
ductile zone were based on current knowledge, especially from the view point of rock mechanics.
The subjects specified at the workshop as of prime importance are: (1) mechanical and hydraulic
properties of rock, (2) in situ states of stress, and (3) seismic activity on fractures or faults.
Figure 2. Location of the Kakkonda geothermal field in the Hachimantai volcanic field, northeastern Japan, and schematic cross section
of the Kakkonda geothermal systems (modified after Doi et al., 1998). Well WD-1a encountered the boundary between the hydrothermal-
convection and heat-conduction zones at a depth of 3100m (Doi et al., 1998). This figure will be published by Saishu et al. (2014).
Acoustic and electrical borehole imaging tools can be used
to determine the orientations of in situ principal stresses
(through observations of breakouts and drilling-induced ten-
sile cracks) as well as the distribution, orientation, and appar-
ent apertures of preexisting natural fractures and faults. Im-
age logs so acquired should be augmented with density logs
to determine the vertical (overburden) stress, temperature–
pressure-flow meter logs to identify preexisting permeable
fractures and other fluid loss zones, and Pwave velocity logs
to allow for in situ estimation of rock strength. The latter esti-
mates are used to relate borehole breakout width to the mag-
nitude of the greatest horizontal principal stress (SHmax) by
using the magnitude of Shmin as measured during a minifrac
test.
Below the brittle regime, preexisting fractures, if present,
might have very low permeability and/or cohesive strength
due to closure by plastic creep and sealing by secondary min-
erals. In such a case, it would be necessary to use higher fluid
pressures to increase formation permeability through tensile
failure. This could be augmented by extended circulation of
cold fluids to lower the mean stress, creating a more perva-
sive, mixed-mode fracture network comprising both tensile
and shear fractures. When a cold fluid is injected into a high-
temperature rock, the fluid cools the rock locally around the
injection borehole and fractures, and the cooling induces lo-
cal shrinkage of the rock. Such shrinkage leads to a consider-
able reduction in the fluid pressures required for fracture ini-
tiation at the borehole wall, fracture extension, and the open-
ing of fracture networks.
5 Geothermal exploration and monitoring of
EGS reservoirs
5.1 Current status of technology
Temperature mapping is an essential component of geophys-
ical surveys for EGS development. The use of aeromagnetic
survey data to map depth to the Curie temperature isotherm is
the only known way to directly detect temperature at depth. A
spectral analysis method has been developed that assumes a
fractal distribution of crustal magnetization; this method has
recently been used to estimate depth to the Curie temperature
at a potential EGS site in a continental environment.
Gravity surveys can provide information about the density
distributions in subsurface rocks such as the massive granitic
bodies that form EGS reservoirs. Modern gravimeters, such
as the new superconducting gravimeter, can detect weak sig-
nals caused by fluid flow in deep reservoirs.
Because hydrothermal systems normally produce diagnos-
tic resistivity anomalies, subsurface images produced from
3-D magnetotelluric data have shown reasonable agreement
with borehole resistivity logs.
Sci. Dril., 17, 51–59, 2014 www.sci-dril.net/17/51/2014/
H. Muraoka et al.: The Japan Beyond-Brittle Project 55
Monitoring of background seismic activity during the pre-
development phase of an EGS project is necessary in order
to understand preexisting seismicity at the site. Hasegawa
(2009), Imanishi et al. (2011a, b); Ma et al. (2012) and
Schwartz and Rokosky (2007) showed various characteris-
tics of seismic events which occurred beyond the BDT.
5.2 Technological challenges for detecting suitable
targets and monitoring developed reservoirs
Recently developed 3-D repeated aeromagnetic survey tech-
niques might be useful for estimation of deep magnetic struc-
tures related to hot dry rocks.
Recent advances in gravity survey technology, especially
the development of new superconducting gravimeters, allow
for the detection of small changes in the gravity field that are
caused by mass movements or the redistribution of fluids in
geothermal reservoirs.
Resolution at depth is inherently low in surface magne-
totelluric survey data. To increase resolution, high-resolution
surveys (such as cross-hole or borehole–surface electromag-
netic surveys) should be conducted close to the reservoir.
5.3 Current understanding of induced seismicity associ-
ated with fluid injection and production at an EGS
The hypocenters of large induced seismic events are usu-
ally within the central region of the seismic cloud or near
its boundary. The source radii are comparable to or smaller
than those of the hypocentral cloud (Asanuma et al., 2005,
2011; Mukuhira et al., 2013), suggesting that the size of the
rupture is restricted by the dimensions of stimulated zones
in an EGS. Pore pressures determined at the time of occur-
rence of large induced events has shown that fractures were
not critically stressed then (Asanuma et al., 2012; Mukuhira
et al., 2013; Terakawa et al., 2012). The observation that
many large induced events have occurred after shut-in and
bleeding-offindicates that reservoir pressure or stress state is
redistributed as a result of the cessation of fluid flow.
5.4 Expectations of induced seismicity for JBBP
The reservoir required to extract thermal energy in the order
of 10MW would be of dimensions in the order of several
hundred meters; seismic events of moment magnitude 4–5
can be caused. However, if the reservoir is connected to an
existing fracture system above the BDT (JBBP type 1 reser-
voir), the risk of large induced seismic events increases.
5.5 Challenges for prevention of large induced seismicity
in and around the JBBP reservoirs
Dynamic THMC modeling of the reservoir and surrounding
zones of the JBBP reservoir presents a challenge. Investiga-
tion of methods of reservoir creation such that large induced
events are suppressed should be undertaken by integrated ge-
omechanical modeling that deals with all of these factors.
6 Engineering development
6.1 Current status of related technologies
JBBP reservoirs will be created at depths of 3–5km where
formation temperatures are 350–500◦C and formation pres-
sures are 30–50MPa. Experience gained during the drilling
of well WD-1a at the Kakkonda geothermal field (Muraoka
et al., 1998) suggests that fluids of high salinity and HCl con-
tent may be present. Functioning in conditions such as these
is beyond the capability of most currently available off-the-
shelf technology.
6.1.1 Current status of drilling and well completion
technology
1. The top drive system (TDS), which can continuously
cool a borehole, enables penetration of rock masses at
temperatures exceeding 500◦C at Kakkonda (Saito et
al., 1998).
2. In the IDDP (Iceland Deep Drilling Project), the drilling
was delayed because of thermal cracking, hole collapse,
and magma quenching to glass near/inside the magma
body. Magma has also been intersected during drilling
of injection well KS-13 in Hawaii (Teplow et al., 2009).
3. Well design may need to be revised to deal with su-
percritical fluids – in particular, to avoid steam explo-
sions in the casing annulus by HCl, H2S, CO2, and
possibly HF. Recent corrosion and scaling experiments
by IDDP-1 and the Salton Sea Project indicate that
INCONELralloy 625, titanium grade 7 and Beta-C
titanium casing performed well (Ragnarsdóttir, 2013;
Love et al., 1988).
4. Collection and recovery of spot-core samples from a
high-temperature environment can be problematic (e.g.,
Lutz et al., 2012). An alternative approach is to use a
hybrid drilling rig that can switch from rotary drilling
to continuous wireline coring (e.g., Furry et al., 1996).
Successful coring was achieved at high temperatures in
IDDP wells by using a corer provided by Alister Skin-
ner.
5. The cement commonly used to set casing can with-
stand temperatures up to about 400◦C. For the WD-1a
well, casing was cemented at a formation temperature of
around 360◦C (Saito et al., 1998). Halliburton is devel-
oping a high-temperature cement (ThermaLock™) that
can be used at formation temperatures up to 538◦C.
6. A bentonite- and water-based, low-solids, low-density
mud was used with a high-temperature dispersant (G-
500S) in the Kakkonda WD-1a well. Telnite Co., Ltd.
www.sci-dril.net/17/51/2014/ Sci. Dril., 17, 51–59, 2014
56 H. Muraoka et al.: The Japan Beyond-Brittle Project
(Tokyo, Japan) now provides a Hypergel/G-500S high-
temperature mud system. M-I SWACO (part of the
Schlumberger group) can also provide high-temperature
water-based muds usable at temperatures above 260◦C.
7. A research and development project funded by the US
Department of Energy to develop a high-temperature
(300◦C) directional drilling system is being undertaken
by Baker Hughes.
6.1.2 Expectations for drilling and well completion for
JBBP
1. The borehole will need to be effectively cooled to be-
low 160 ◦C during normal TDS drilling operations. Cas-
ing and cementing under extreme high temperature sub-
surface conditions should be possible if cooling of the
borehole during TDS drilling is sufficient.
2. There will be risk of the buckling or breaking of cas-
ing pipe and the destruction of the cement sheath in re-
sponse to the thermal stress induced by injection during
the circulation of cool liquid into the high-temperature
borehole. Corrosion of the casing pipe and wellhead
may also occur.
3. Drilling of a highly deviated borehole into the BDT to
create subvertical reservoirs of sufficient thermal capac-
ity will be difficult.
6.1.3 Current status of stimulation and injection
technology
1. Many EGS reservoirs have been created by full-hole
pressurization, either by pressurizing the entire open-
hole section (Häring et al., 2008), or by isolating the
open-hole section using casing packers.
2. The maximum operating temperature of the Halliburton
RTTSrtool and inflatable packers is 180–190◦C. An
alternative for zonal isolation during reservoir stimula-
tion is the use of chemical diverters, which have been
successfully deployed by the Newberry EGS project in
Oregon, USA (e.g., Petty et al., 2013).
3. Multistage hydraulic fracturing equipment has been de-
ployed in recent shale gas developments in North Amer-
ica, although in a relatively low-temperature environ-
ment. The stability and behavior of fracturing fluids and
proppants under high-temperature conditions are poorly
understood.
6.1.4 Expectations for stimulation and injection for JBBP
1. Multilevel stimulation to investigate changes in the re-
sponse of the BDT rock mass with increasing depth
should be undertaken within JBBP. A multilevel frac-
ture system would be expected to increase the extent of
fracturing for heat exchange. However, no packers that
can be used under conditions expected in the BDT are
currently commercially available.
6.1.5 Current status of logging and sampling technology
1. Most of the logging tools used in the oil industry can
be operated at temperatures up to 175◦C. Some high-
temperature (HT) pressure-temperature-spinner (PTS)
tools can be used in memory mode at 400◦C on a
slickline, whereas temperature, televiewer, and spec-
tral gamma tools can be operated at up to 300◦C on
a wireline. The maximum operating temperature for HT
logging-while-drilling (LWD) tools is 230◦C.
2. The maximum operating temperature for a standard
seven-core wireline cable is 315◦C; a slickline cable
can be used at temperatures exceeding 400◦C.
3. The ICDP (International Continental Scientific Drilling
Program) provides online monitoring of gas (OLGA)
during drilling to extract and analyze gases (N2, O2,
CO2, CH4, Ar, He, H2, C1–C4,222Rn) from the circulat-
ing drilling mud. This technique has been successfully
used in several ICDP projects (San Andreas Fault Ob-
servatory at Depth; Unzen, Japan) and IODP (Interna-
tional Ocean Discovery Program) riser drilling expedi-
tions (e.g., Expedition 319) (e.g., Erzinger et al., 2006).
4. A downhole sampler was developed by Lysne et
al. (1997) and by NEDO (New Energy and Industrial
Technology Development Organization) (Sato et al.,
2002). Thermochem Energy Consulting & Chemical
Testing has continued with the development of a two-
phase high-temperature (to 400◦C) downhole sampler.
5. Metal-coated optic fiber designed for industrial and
telecommunications use is thermostable up to 600◦C,
although cable length is limited to several tens of me-
ters.
6. Most commercially available high-temperature elec-
tronic apparati, fabricated with silicon-on-insulator
(SOI) technology, are limited to a maximum operating
temperature of 225◦C (a few to 300◦C).
7. Wider band gap (WBG) materials such as silicon car-
bide (SiC) must be utilized for the fabrication of HT
sensors and circuits (Azevedo et al., 2007).
6.1.6 Expectations for logging and sampling for JBBP
1. It will be possible to collect information about the BDT
rock mass by LWD, although there are risks of thermal
damage to downhole tools.
Sci. Dril., 17, 51–59, 2014 www.sci-dril.net/17/51/2014/
H. Muraoka et al.: The Japan Beyond-Brittle Project 57
2. Most of the existing logging tools can be run during or
immediately after periods of circulation, at which times
the borehole temperature will not have returned to its
initial state.
3. Although operating times will be restricted, PTS tools
can be used in memory mode during the stimulation and
circulation phases. For thermal power monitoring, pres-
sure and temperature tools will need to access the well
for the full range of expected temperatures and pres-
sures.
6.1.7 Current status of technology for EGS reservoir
maintenance
1. Reservoir productivity is maintained by injection, pres-
sure build-up, and stimulation.
2. Scaling is avoided via the acid treatment of injection
fluids.
3. Scaling within heat exchange apparatus has been ob-
served at Soultz (Scheiber et al., 2012).
6.1.8 Expectations for maintenance of the JBBP
reservoir
1. The solubility of quartz will change drastically in the
BDT, causing channeling and shortcut flow paths as a
result of the solution and precipitation of quartz within
the JBBP reservoir.
6.2 Technology developments required for JBBP
1. Drilling technologies: The risks and costs of drilling and
well completion should be reduced as much as possi-
ble. Methods of well completion and materials used for
casing and cementing that will allow long-term produc-
tion and injection for JBBP reservoirs should be inves-
tigated. Coring equipment and operations should be de-
veloped that overcome the inadequate downhole cooling
during coring operations.
2. Monitoring technology: fiber optic distributed sens-
ing (e.g., distributed temperature sensing (DTS) +dis-
tributed acoustic sensing (DAS)) should be used to mon-
itor and evaluate the stimulation treatment. Technolo-
gies to delineate the structure of the fracture system
and the distribution of permeability should be devel-
oped. New survey methods (e.g., surface-borehole com-
bination) and inversion theories (e.g., focused inversion)
will be useful in identifying drilling targets and monitor-
ing JBBP reservoirs. Highly sophisticated tracer meth-
ods (e.g., smart tracer) in combination with 3-D/4-D in-
version theory have promise for reservoir monitoring.
3. Reservoir creation and maintenance technology: the use
of proppants and appropriate fracturing/stimulation liq-
uids should be investigated. Technologies that enable
the avoidance or control of channeling and shortcut flow
paths is of critical importance for sustainable produc-
tion.
4. Zonal isolation technology: technology to isolate open-
hole sections at formation temperatures of up to 450◦C
should be developed to allow for the creation of multi-
level thermally productive reservoirs.
5. Logging and borehole testing technologies: high-
temperature logging tools that can operate at 450◦C
are needed to understand the properties of the beyond-
brittle rock mass and fracture system. A technique
to measure or estimate in situ stress at beyond-brittle
depths is of critical importance in the JBBP.
7 Contribution of JBBP to Earth sciences
The JBBP will directly contribute to a broad range of earth
science disciplines. We expect that the information so de-
rived from core analyses and bore hole tests can be effec-
tively used to improve our understanding of phenomena such
as dehydration/degassing of magmas, global hydrogeology
in the Earth’s crust, and the processes by which hydrother-
mal convection/conduction zones can be created. Laboratory
tests on fracturing in rock specimens at high temperatures
and pressures as well as the testing and monitoring of deep
boreholes can provide information on the dynamic response
and stress state of the rock mass beyond the BDT, which will
lead to improved scientific understanding and interpretation
of the mechanics of earthquakes at depth. New exploration
technologies applied to identify the BDT will contribute to
our ability to determine the thermal and structural charac-
teristics of phenomena such as volcanoes and seismogenic
zones in the Earth’s crust.
8 Roadmap and implementation plan
Two-and-a-half years of lead time might be required before
submission of a full drilling proposal to ICDP (Fig. 3). This
lead time will be used to further our scientific understanding
of the beyond-brittle rock mass, develop the new technolo-
gies, undertake surveys of possible sites, and develop a drill
program and contingency plans.
A number of current geothermal projects target supercrit-
ical fluids in shallow, still-hot, molten igneous intrusions
in young volcanic rocks along plate boundaries and at hot
spots. These include established geothermal fields in Ice-
land, New Zealand, the Philippines, Indonesia, Italy, and the
United States. International collaboration, particularly with
ICDP high-temperature geothermal projects worldwide, is of
www.sci-dril.net/17/51/2014/ Sci. Dril., 17, 51–59, 2014
58 H. Muraoka et al.: The Japan Beyond-Brittle Project
Jan.
2013 Jan.
2014 Jan.
2015 Jan.
2016 Jan.
2017
ICDP-JBBP WS ICDP Full Proposal
Jan.
2018
Basic scientific studies
Technology development
Site survey
Drill planning
Proposal
preparation
Drilling
Drill
preparation
Monitoring
preparation
Pilothole drill
Tests
Jan.
2019
Mainhole drill
Experiments, monitoring
Figure 3. Roadmap for development of the full JBBP proposal and subsequent drilling
A number of current geothermal projects target supercritical fluids in shallow, still-hot, molten
igneous intrusions in young volcanic rocks along plate boundaries and at hot spots. These include
established geothermal fields in Iceland, New Zealand, the Philippines, Indonesia, Italy, and the
United States. International collaboration, particularly with ICDP high-temperature geothermal
projects worldwide, is of critical importance for success of the JBBP by sharing of scientific
knowledge and technology.
Figure 3. Roadmap for the development of the full JBBP proposal and subsequent drilling.
critical importance for success of the JBBP, as it will allow
for the sharing of scientific knowledge and technology.
Acknowledgements. The core team of the JBBP would like to
acknowledge ICDP and Tohoku University for their support in
holding the workshop.
Edited by: T. Wiersberg
Reviewed by: one anonymous referee
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