SMP44-01

Room:102B

Time:May 25 09:15-09:30

Characteristics of silica solubility in the geothermal fields presented by the deep drilling
dataSAISHU, Hanae1∗ ; OKAMOTO, Atsushi2 ; TSUCHIYA, Noriyoshi2
1AIST, 2 Tohoku University

Permeability is one of the important parameters for geological events and the development of geothermal systems. Water-RockInteraction has a role for spatial and temporal change of permeability in the Earth’s crust, although geophysical properties havebeen mainly focused on. Especially silica is one of the dominant components in the Earth’s crust, thus dissolution-precipitationof silica minerals is an important geochemical reaction in the crust. The permeable-impermeable boundary is regarded to consistof the brittle-ductile transition (BPT) at around 300-450 degree C (Scholz, 2002). The depth of the boundary is typically at 10 to
20 km depth (Ingebritsen and Manning, 2010), which is too deep and too strict to be observed by drilling. Therefore, the structureof the Earth’s crust has been studied by indirect geophysical measurements and experiments, not by geological observation andgeochemical analysis.
In the geothermal field, however, some deep drilling wells are expected to reach the permeable-impermeable boundary ataround 3 km depth because of high temperature gradient. The deep drilling wells in Italy, Iceland, and U.S.A. recorded higher
pressure than hydrostatic pressure at the bottom of these wells (Fournier, 1991). The deep drilling well WD-1a at the Kakkonda
geothermal field, Japan, is the only well in the world to penetrate the boundary between the hydrothermal-convection zone andthe heat-conduction zone (permeable-impermeable boundary) at 3.1 km depth, in 24 MPa and 380 degree C (Doi et al., 1998),
which is in the supercritical conditions of water. The Kakkonda granite at >2.9 km depth is the heat source of the hydrothermal
system of the Kakkonda geothermal field. Saishu et al. (2014) calculated quartz solubility along the well WD-1a, and revealed
that the local minimum of quartz solubility consists of the permeable-impermeable boundary at 3.1 km, indicating the possibility
that a large amount of quartz precipitate induce fracture sealing, blocking the downflow to the impermeable zone, and control thedepth of the permeable-impermeable boundary.
In this study, quartz solubility is calculated to reveal the relationship between the permeability and dissolution-precipitationof silica minerals in the 4 deep drilling wells recorded overpressure at the bottom: (1)San Pompeo 2, Italy, (2)the well NJ-11,
Iceland, (3) Wilson No. 1, U.S.A., and (4) San Vito 1, Italy. In the geothermal fields, including the Kakkonda geothermal field,
the condition at the permeable-impermeable boundary is in or near the supercritical conditions of water, and quartz solubility
decreases and increases drastically in hydrostatic and lithostatic pressure, respectively. If fracturing occurs at the boundary,
downward fluid from the shallower part would dissolve large amount of silica and enhance quartz precipitation due to decreaseof quartz solubility in deeper part. In addition, upward fluid of high quartz solubility from the over pressure zone would also
trigger precipitation of quartz because of pressure decrease. Thus, in the geothermal field, the permeable-impermeable boundary
would be controlled by precipitation of silica minerals.
References) Doi, N., Kato, O., Ikeuchi, K., Komatsu, R., Miyazaki, S., Akaku, K., and Uchida, T., 1998, Geothermics, 27,663-690.; Fournier, R.O., 1991. Geophys. Res. Lett., 18, 955-958.; Ingebritsen, S.E. and Manning, C.E., 2010, Geofluids, 10,193-205.; Saishu, H., Okamoto, A., Tsuchiya, N., 2014, Terra Nova, 26, 253-259.; Scholz, C.H., 2002, 2nd edn. Cambridge
University Press, Cambridge.
Keywords: silica solubility, precipitation, deep drilling, geothermal field, permeable-impermeable boundary

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SMP44-02

Room:102B

Time:May 25 09:30-09:45

Electromagnetic imaging of fluids under the brittle crustOGAWA, Yasuo1∗
1Volcanic Fluid Research Center, Tokyo Institute of Technology

This paper presents a 3D inversion result of the magnetotelluric soundings over the caldera regions in the central part of NE
Japan arc. The 181 MT stations in total were located around the Naruko volcano with ˜3km grid covering the area of 40km (EW)x 80km (NS). We have used the full tensor components of impedance tensors at representative eight periods (0.4˜1300s). The
inversion code of WSINV3DMT was used. The initial model had a uniform earth of 100 ohmm with surrounding oceans (0.25ohmm). The final model gave rms of 2.5 with error floor of 10%. Significant features of the model are the thick resistive upper
crust in the caldera regions and sub-vertical conductors arising from the lower crust to the geothermal manifestations. The topof such sub-vertical conductor coincides with the cutoff depth of the shallow seismicity. In particular, the sub-vertical conductor
at Naruko volcano has a deep root in the mid-to-lower crust underneath the Mukaimachi Caldera, which is located 20km westof the Naruko volcano. The supply of the high salinity fluid may be originated sideways and may imply the path of the fluids,
which is presumably blocked by the impermeable consolidated volcanic rocks directly above the lower crustal conductor. The
resistivity of the mid-to-lower crustal conductors is significantly low at the Naruko volcano, compared with those at Sanzugawa
caldera to the north. The difference may be due to the salinity as well as the porosity of the fluid, because seismic tomography
result does not show such differences. A helium isotope anomaly at Naruko volcano may support that the flux from the upper
mantle is large.
Keywords: fluids, brittle crust, resistivity, electromagnetic induction, magnetotelluric method

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SMP44-P01

Room:Convention Hall

Time:May 25 18:15-19:30

Permeable-Impermeable or Elastic-Plastic Transition of Granite
NUMAKURA, Tatsuya1∗ ; WATANABE, Noriaki1 ; SAKAGUCHI, Kiyotoshi1 ; TSUCHIYA, Noriyoshi1
1Graduate School of Environmental Studies, Tohoku University

Recently, geothermal energy is attractive in the fields of developing sustainable energy. Conventionally, geothermal reservoirs
have some troubles which are loss of injection water or induced earthquakes because of brittle dynamics zone. On the other
hand, creating geothermal reservoir at the temperature-pressure zone of supercritical fluid can resolve these troubles and bringhigh power-producing potential. However, evaluating flow characteristics such as permeability is very difficult because of ductile
dynamics zone from the high temperature-pressure zone.
In this presentation, we reported the temperature-pressure condition which shows dynamically semi-brittle or ductile zone
and the flow characteristics in each condition for Inada granite. Firstly, we developed experimental system which can carry out
hydraulic test at the selectable environment. Secondly, we set experimental condition to temperature was 350, 380, 400, and450 ℃, effective confining pressure was 5, 10, 20, 30, 40, 50, 60, 70, 80, and 90 MPa. Pore fluid pressure was controlled 1 or
2 MPa in constant pressure. As a result, we revealed that the rapid decreasing permeability clarified stress dependency of thebrittle-ductile (or elastic-plastic) transition was different on each temperature condition. In addition, decreasing permeability rate
at the brittle and ductile zone provided a way to predict permeability at various combinations of temperature and pressure.

Keywords: geothermal reservoir, granite, elastic-plastic transition, predicting permeability

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SMP44-P02

Room:Convention HallTime:May 25 18:15-19:30

Depth estimation and evaluation of geothermal resource by melt inclusion analysis
SUZUKI, Taku1∗ ; TSUCHIYA, Noriyoshi1 ; YAMADA, Ryoichi2
1Graduate School of Environmental Sciences, Tohoku University, 2 Graduate School of Environmental Studies, Tohoku University, 3 Tohoku Univ.
Geothermal power generation in Japan has abundant resources. However it requires long periods of time for survey: theamount and distribution of geothermal resource have been estimated by the volumetric method, and by the reservoir layer evaluation method for narrower region. In this study, melt inclusions in volcanic products were utilized for a faster and simple method
for evaluation of geothermal resource. As melt inclusions remain the composition of the magma prior to eruption, they provide
thermal-chemical conditions of the magma chamber at depth. To evaluate the depth, distribution and amount of geothermal
resource, melt inclusions in quartz was analyzed for Shirasawa caldera,Japan.
Shirasawa caldera is located in west of Sendai city and erupted in late Pliocene. Shirasawa layer is composed of mud andsand deposited on the Old Sendai Lake in center of caldera. The high-temperature region (Low-velocity region) in the Shirasawa
caldera at 2˜5 km has been confirmed by seismic reflection survey. [1]Eight samples were collected from the center of the caldera to the south, and 4 samples were collected from north end in
Shirasawa caldera. These are classified into tuff sandstone, tuff breccia and pumice tuff.
Quartz crystals were picked up from the samples, and the crystals were encased in resin flake. The compositions of melts
included in quartz crystals were analyzed by electron micro probe analyzer (EPMA) for 10 elements (Si, Ti, Al, Fe, Mn, Mg,Ca, Na, K, P). Three samples from north end of the caldera were not analyzed because the crystals were trace amount and small.
The depth of crystallization was estimated from percentage of quartz(Qtz)-albite(Ab)-orthoclase(Or) by CIPW norm calculation(Fig.1). As the eutectic line changes position by pressure and the melt in the quartz crystal is located on the eutectic point of theAb-Qtz phase diagram, the crystallization pressure was decided from the plot and eutectic line on Qtz-Ab-Or diagram [2].
Eight samples from the center and south part of caldera were classified into low-alkali tholeiitic rhyolite, and 1 sample fromcaldera north(09) was high- alkali tholeiitic rhyolite. The norm percentage of 44 melt inclusions is Qtz: 30˜43%, Ab: 35˜52%,
Or: 9˜26%, and forms a straight line on Qtz-Ab-Or diagram, with the exception of some samples. Crystallization pressure is0.1˜320 MPa, and most of the samples concentrate on 30˜50 MPa. Certain samples (2305) were plotted in 5˜320MPa. Assuming
the density of ryholite magma as 2.0 g/cm3 , the crystallization pressure was estimated to be 16˜1.5 km. It is suggested thatmagma had assent from about 16 km depth, become a gravitational equilibrium at 1.5˜2.5 km, and subsequently erupted. In
the other samples, crystallization depth is concentrated at 1.5˜2.5 km. The depth of magma decided by melt inclusion in quartz
is consistent with the high-temperature region observed by seismic reflection. Considering the sample location, Or rich sample
considered to be derived from another magma reservoir.
Based on these data, the depth, distribution and amount of geothermal resource associated with the magma chamber will be
discussed.
[1] Sato et al. (2002) Earth Planets Space,54,1039-1043.
[2] Blundy and Cashman (2001) Contributions to Mineralogy and Petrology,54,631-650.

Keywords: melt inclusion, Shirasawa caldera

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SMP44-P02

Room:Convention Hall

Time:May 25 18:15-19:30

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SMP44-P03

Room:Convention Hall

Time:May 25 18:15-19:30

Porosity structure and permeability reduction by mineral dissolution in a fracture
TANAKA, Hiroto1∗ ; OKAMOTO, Atsushi1 ; WATANABE, Noriaki1 ; TSUCHIYA, Noriyoshi1
1Graduate school of environmental studies, Tohoku University

Fractures act as dominant fluid pathways within the crust. Fluid usually control transport of energy and heat. Silica solubility
generally increases with increasing pore pressure and temperature, and thus dissolution and precipitation of silica would provide
significant effects on fracture permeability. Even by dissolution of minerals within a single fracture, dissolution at free (noncontact) areas increases the aperture, whereas dissolution at the contact areas decreases the aperture, therefore it is not clear how
fracture permeability evolves by mineral dissolution under confining pressure.
In this study, we conducted the hydrothermal flow-through experiments at 350 ℃, 20˜34MPa under confining pressure 10˜15MPa
to understand the evolution of porosity structure of a fracture and permeability change in granite by mineral dissolution. For thispurpose, we developed a novel reactor, which has a inner tube in the vessel. Two types of granite core (Aji granite, φ10mm) wereused, first one (85mm length) contained a slit with thickness of 0.5mm as parallel prates flow path. The other one contained thetensile fracture with no shear displacement. During the experiments under constant flow rate, we monitored the fluid pressures,and periodically sampled the solutions. After the experiments, we analyzed the porosity structure by X-ray CT (resolution was10µm/pixel).
In slit-core experiment, the concentrations of Si was 100˜120 mg/kg, whereas the concentration of Al, Na, K were 7, 5 and 8
mg/kg respectively, indicating that the ration of dissolved volume of quartz, plagioclase and K-feldspar are 10:2:1. The X-ray
CT also revealed that preferential dissolution of quartz, and that the average aperture increases especially near the inlet.
In tensile fracture experiment, fracture permeability decreased continuously from 10−13 to 10−15 (m2 ) during the experimentof 90h. An increase in flow rate did not enhance the reduction whereas that in confining pressure accelerated the permeability
reduction. The X-ray CT images revealed the complex structure of porosity: quartz dissolution made the local increase in theaperture, but the overall aperture decreased by dissolution of quartz and feldspar at contact areas, which is responsible the permeability reduction.

Keywords: permeability, fracture, hydrothermal experiment, dissolution

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SMP44-P04

Room:Convention Hall

Time:May 25 18:15-19:30

Natural Analogue of Supercritical Geothermal Reservoir
TSUCHIYA, Noriyoshi1∗ ; YAMADA, Ryoichi1 ; UNO, Masaoki1
1Graduate School of Environmental Studies, Tohoku University

To understand the geological properties of a supercritical geothermal reservoir, we investigated a granite?porphyry system asa natural analog. Quartz veins, hydrothermal breccia veins, and glassy veins are present in Neogene granitoids, Tohoku Japan.
The glassy veins formed at 500-550 C under lithostatic pressures, and then pressures dropped drastically. The solubility ofsilica also dropped, and the quartz veins formed under hydrostatic pressures. Connections between the lithostatic and hydrostaticpressure regimes were key to the formation of the hydrothermal breccia veins, and the granite?porphyry system provides useful
information for understanding supercritical geothermal reservoirs.

Keywords: Supercritical fluid, Geothermal reservoir, Granite-Porphyry system

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