Toward the Quantitative Study of Hydrothermal Systems An Approach to Understand Hydrothermal Systems

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1 J. Hot Spring Sci. 0*,0+,1+,*+* + Toward the Quantitative Study of Hydrothermal Systems An Approach to Understand Hydrothermal Systems Sachio E + HARA Abstract The quantitative understanding of hydrothermal systems is very important in order to advance geothermal sciences including hot spring sciences. In this paper, such a discussion is described from the viewpoint of numerical modeling techniques and a case study of the geothermal system at the central part of Kuju volcano, central Kyushu, Japan. The construction process of the numerical thermal models is shown as follows : The first step is construction of a conceptual thermal model based on the geological, geochemical, geophysical and hydrological data, the second is construction of the natural state numerical model, and the third is construction of the history matched numerical model. The final history matched thermal model can estimate geothermal potential quantitatively and also predict changes in the physical state of the geothermal reservoir during exploitation. Furthermore, it will contribute to symbiotic utilization of geothermal energy, that is, symbiotic utilization between geothermal power generation and hot spring utilization. Key words : Hydrothermal systems, Conceptual model, Numerical model, Hot spring sciences, Geothermal reservoir *-3/ Laboratory of Geothermics, Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Motooka 1.., Nishi-ku, Fukuoka 2+3 *-3/, Japan. 261

2 : + Hydrothermal systems, Fig. + Elders, +33+ Elders, / - -, 262

3 0*,*+* Fig. + A generalized approach to reservoir evaluation (Elders, +33+ ) Ehara et al.,,**/ / Fig

4 Fig., Sustainable production of geothermal energy from an individual geothermal system. E* shows the level of sustainable production rate. Excessive production rate E is EE *. The value of E* is not known a priori, but it may be estimated on the basis of available data, i.e. through monitoring and modeling of the geothermal system (Axelsson et al.,,**-) E Fig.,, Axelsson et al.,,**- E E E E E * E * * * * * *

5 0*,*+* + Fig STAR, TOUGH, HYDROTHERM + +* 265

6 + Fig ,*+* / Yahara and Tokita,,*+* + 266

7 0*,*+* +33/ /** m A, B, C, -.** +3/* /*2 Mizu- tani et al., ** MW / Fig , +*, km / km Fig. - Hydrothermal system model of Kuju-iwoyama estimated from observed water and heat discharge rates. The values in parentheses contain only the water flow rate discharged from the fluid reservoir (Ehara et al., +32+ ) 267

8 MT +33* ; Mogi and Nakama, +332 / km,, km /** m,, km, Fig.., Ehara, +33,,** -.*, 1/ Ehara, +33, +33/ -** m ** MW +*** MW - Fig. /, (a), (b), (c), (d), Ehara et al.,,**/, +* - 1* 268 Fig.. Vertical cross section of the cylindrical geothermal reservoir of Kuju-iwoyama (Ehara, +33, )

9 0*,*+* Fig. / Three dimensional hydrothermal system model of Kuju-iwoyama. (a) Conceptual hydrothermal system models. Left : before the +33/ phreatic eruption. Right : after the +33/ phreatic eruption. (b) Plan view of the block layout of the numerical model. (c) N-S vertical secpresents the permeable volcanic geo- tion of the numerical model. The red rethermal reservoir. (d)temperature distribution of the N-S vertical section of the hydrothermal system model. Left : before the +33/ phreatic eruption. Right : after the +33/ phreatic eruption (Ehara et al.,,**/). 269

10 Fig. / Continued 0 Axelsson, G., Armannsson, H., Bjornsson, S., Floventz, O. G., Gudmundsson, A., Palmmason, G., Stefansson, V., Steigrimsson, B. and Tulinus, H. (,**-) : Sustainable production of geothermal energysuggested definition, +. Elders, W.A. ( +33+ ) : Geologic models of geothermal reservoirs. Geothermal Resources Council 270

11 0*,*+* / +0,. BULLETIN,,*, +3.,*-. Ehara, S. ( +33, ) : Thermal structure beneath Kuju volcano, central Kyusu, Japan. J. Volcanol. Geotherm. Res., /., +* : +33/.2 1, * : +,,0-, :,0 -/ /0. Ehara, S., Fujimitsu, Y., Nishijima, J., Fukuoka, K. and Ozawa, M. (,**/) : Changes in the thermal state in a volcanic geothermal reservoir beneath an active fumarolic field after +33/ phreatic eruption of Kuju volcano, Japan. Proc. World Geothermal Congress,**/, CD-ROM. Mizutani, Y., Hayashi, S. and Sugiura, T. ( +320) : Chemical and isotopic compositions of fumarolic gases from Kuju-iwoyama, Kyushu, Japan. Geochem. J.,,*,,1-,2/. Mogi, T. and Nakama, K. ( +332) : Three dimensional resistivity structure beneath Kuju volcano, central Kyushu, Japan. J. Volcanol. Geotherm. Res.,,,, ,*+* : Yahara, T. and Tokita, H. (,*+*) : Sustainability of the Hatchobaru geothermal field, Japan. Geothermics (to be published) :,