"#$ J. Hot Spring Sci.! 0*! ,0+ῌ,1+ ,*+*

 !"¤ ¥ ¦ §+

Toward the Quantitative Study of Hydrothermal Systems¨An Approach to Understand Hydrothermal Systems¨
Sachio EHARA+

Abstract
The quantitative understanding of hydrothermal systems is very important in orderto advance geothermal sciences including hot spring sciences. In this paper, such a discussion is described from the viewpoint of numerical modeling techniques and a casestudy 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 naturalstate numerical model, and the third is construction of the history matched numericalmodel. 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

῍ῌ"#%&'( )*+,*-./012$%3'2('/4! 5678,9:;<*=>?@ABC2D+%! EF78:;<GHIJK! LMN@OPQR*  S*
T/UVWXYZ( [! \! ]^! ,^_*`a;bc%def! ,
9gh:;<%Gi! jk/! l./UVWX! mn:;<%GH12D+! o/! pqrs:;<GHt! uvwbxyz{|*IJK%}(mn:;<%GH12D+*@A~%f( lfX! l*M8567:;</UVWX! "#*[/ 2++/! "#6C2W4/rs\/012$78a?+8i! "#*8*?B! jk/! +"#+*/B2+*
%YZ( 18! **8% ¡XW¢£B! "#&'( *+,+
 
 2+3ῌ*-3/  1..   + Laboratory of Geothermics, Department of Earth Resources Engineering,
Faculty of Engineering, Kyushu University, Motooka 1.., Nishi-ku, Fukuoka 2+3ῌ*-3/, Japan.

261






 :    ! "  ! +    # $%&'()*+,--  ./0%1 2)*,3+456, 7 2859:;<'16/0+=<>1% 54 ?@!"AB5$C:D E

F GHῌ ῌ,I#1 J K0 #LDI#1M$%N
O M24 ῌ,2$K0&P<Q72$4=<%(% =1;< ' D  RSTU!4    $%& ( ?,V;<% WRSTU!4Hydrothermal systems $=<% 2X&()Y/04 ?@ Z.&8F%;<%

[D*% \+,$%$$%$Z.](% [^(F(%2$,-_` ab<%$4./) 4(F cd,0bFD e

f&,1/=ghi#01   N j-%4klmnopcq' p($%2,r=<%;< st3uXv Z.w$54Q7<%' 2Z.$%&)*Dxy2$,z4=<%

,   

'4 ,I#1 {|P$ (:} =< ~$=<
J56|X N7/0+ $4{89Y MYb< 8:(;?Dv MY <X&(D/#<v =1;<{ <+,2$4'(%

81 $+,&=1;< ,>;<#2$v 4
9?(+, MX&(\@$=< !f\A@,Fig. + =1 Elders, +33+  (:} ,61#4 8



Je

J56 6 S,ῌB= 89Y|P$  !
, C 2  ! C$%&4 ./0 &DD*F [>0<'<%
2  ! C4#<\ 2  !EFG(D ¡"  ! CHI(D(;1v ¢;1£¤YDv& D}¥
h EFG(  !4"  ! CJ¦ §¨,K©(FK2$D(K< ª"  ! C«%< ¡ 24  !cq%< 9?( !,L>)<2$ *FY{M$%KN; (:} ¬O; !

 C/0­#0   !4  { <$Q7
K<% P  (®QiP4(F R&S ¯&S T? U? TKM°±VX&("
Elders, +33+ SDI8 "!fD%¥%¥(J¦(/0T?ῌU? Scq%< F=( !,²9=< ³

W´X|%< X&( ;w/,+D/0 24 µ ! $DC7 1$7 0 +32+   / ¶?= - ª¸*Y ±Y °Y °± , Y
262/· [, - ª¸(¹º» <(Z¼Y[ $M,½?D8 0*  ,*+*



ῐΐ

Fig. + A generalized approach to reservoir evaluation (Elders, +33+) ! Ehara et al., ,**/ "#() *+,-. /0 1 ! 2

/ $%&'- 34*+,-.56789:;<=>'2?@A B<C (D =E % FGHIB,-./JK 2
LM8N9LO6P?QRS C? TUF<K V FGWX ,-. - 34*+! 

YῑZ[\B]^_9:%`('^6ab]^_9:9,-. %R cdBe῔?fg%h - 34*+LM8N9LO6,-.=>'ijCR k Nl. *+,-.
=>'2 Km῍%h noB,-.?=>p ijK FGWX ,-.%R qr
BstK - 34*+,-. u\Bvrwxy'2 ?fg%hp noBz{|} *+,-.?=>p!V %ῒC~ 1 !V%KBC 2 z{|},-.K *B%KBC?  ῍KC?K
CBC

1  

! 2 z{|},-.ῌC (D ῏ῌ|( F

~ ῏ῌ -9:%xz{|},-.;< ῏ῌ" D -9:
F '  ijC~BC 'B -9:%xBC %h
22? CBC

1 %h V2% V 'J b9 ¡¢6£ Ca¤¥?¦§  Fig. + 2 K '%¨§ +%xF z{|},-. © ª*« ¬]^_9: hCK­®«¯° +
%xFB,-.¯°'a¤¥%h V

῍B±²%+%x

-9: DB  ³ *+,-. ' %h
2´% µ# FB%  *+,-. /0K ¶῎ ·¸¹%KhGº§ 
2B~< %KBC~ »  2 FB*+,-. =>K¼½fgB·¸῏ῌC
263





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., ,**-)

  !"#$%&'()*+
,-./012345678-9:;<=>?@+ A B$
CDEῌFGH IJD+@) KLFMNO"#%$PQ(' RSTU&'()V@ WRS XYZ[" '()*+ \]$ KV+ ^_+@ WRS*+ `abc&:( C V @ WRSLde$:f'ghi:;<=+j $Kkkl'@k mn$L @Ko  XYZ["&'RS p E*
(qj Fig. ,, Axelsson et al., ,**- V E* rstuvwxy zYZ@K{|+}j ~j'@vwj& WRSvw@$ m

E* rs

ivw+y XYZ["&:vwjRS+j  m&'(V$s mn$@9@(+q &:jf' 9#$m"js
XYZ$:f'["vwWD:$@drs(+ E* &K E* (WDVj+ V E* d$Vj+}y  k&( &&j V$!$V@mn$+s K:$@ l@¡s@¢ vw$£q¤F¥¦§j RS¨©@ 

#$ª«&'()(q#d j k&( LdVk+

9¬$PQ(' Vrq j k&((q j"­& V E* x6
78-$C(' K®E*¯:$@ ,-./012345(q678-j- 



°+±²: - ³´µ¶;<=>?$C(' $·@ ¸¹ - ³´_º»;<=>?+ k¼@¢$ ¸¹ - ³´#½ O$psx V¨© LF264

0*  ,*+*

   !"# $ %&' ()   * ! +,-. /0 "1#23#456 78 9:;  %*<=> ? $ @ :;  A8 "23"B

CDEF G!:; H9 !I"BCDEF  J !23"
BCDEF KL * MBCDE NOBCDEPQRS  TU VW  NO X! "Y * >! #Z5[\]^"Y G $#
Z! _` ab` a %` %`#Z&'"#Z<Kc  3
> de#Z<Kc  $#Z]^f3  Xg5[\ "# de hQ#ZS ij  TU 5[\<(>  $ )!]^k* l+ !R"Y*< I * SL MBCDE"Y X5[\!#Z]^"Y> < G!#Z!m<? n fSQ NO X!<,

"3 BCDER?*" **U!456+ o m,<? " G   gU!456p- qRI"/0 "Yrst[\!NO T
U ? $ * !G #Zuv=>(#rst[\ R? <Q U!
456 7w! TU!.Ux k*Hl+ y)!/zSL{3 |z<& <Q &# |z TU "YSQ 456<} SL/0 ~=LR "c *<K"c  $ Y  0
 KLSL"|z! 1 Y8"!<hc *" Fig. + y)!]^ 2 3 +,-.R"Y>  * > y)!R G
! * (*<4"|z<&# y)! * (5 U!456 pKL<?  G U!456 7w6Q! +,-.+,78 BCDE=> ? ?\6[  9:*<K"c * SQ k  i. 
F3

MBCDENO

X! k* 


!<"Y>  +,-.<"Y>  iK*S y)! ;456 7*< I  <=   g>? 7 $ ! @ - @&



xL*

#*RKc "c !"<I¡¢[\£C¤s¥ U!¦§¢¨[\ <©A3  8ª"<= « U!456 7w! BC U!¦§¢¨[\ STAR,TOUGH, H HYDROTHERM  <hc  " U!4567w<=D! U!¦§¢¨[\$ R ?LS * 456 7w8 } ¬}E* * y)!<?=L*­>  =o SF®¯ y)!<?*<° ?
 $ 8°"rst[\!NO ±TU ?  NOG TU

)*!²H ?< $ !  BCDE¨³6 TU!´"  G
 ± TU!I± JµEC"¶+/0 ~ TU f!

4<&  deS ·"TU (*< I< $R (> !de9g! ? * SL"*=> TU!  NOGg)
! deS)!]^f! "Y *+ o rst[\.b3¸k*ῌl+ y)!/zTU ¹8£Cº»"*<&¼½ 456 7w6Q! "Y> rst[\   GUKLU +* K
265







  #$  %

 



  !"&'()#$*)+$ , -./'0123$45 ,64#$!+&'(7#$*)+$ " 4 ,65 + 89:;*)5+" <=>? @AB01 " 4 C DE FGHDE -.
/'01#$*)29I$&'(7#$ * !+JKLM0N>OPQ R=SK-&'( &'( T&'( )U43$ Fig. +95 %

+V **&'(5 !W XY"!WZ#!W [ $%) \&]^

)_$2 `'a 3$( H b9T2c !) _"d)_*+,e-93$ * !+fa'g]_T)

h.2ijk)6$*)

/_ * !+*+ 0l1mn) 2o +32- !$ 2pqr6$ * mn
95 st34563$Y 78+9u 49 :';# ) < t=V_ 
v2w#+2rxE>)?#$)_y@A_zB2{C|6_$| DE! JKLM0N>OPQ!E}F~^T&'(5zB!
"#$*)29I$)_\&G^93$2 * &'( H5 %zB I)_%1^95+_ 5 * &'( J,e-+XY" !WZ J,<#K )_LI+H23$ ,1+ 012M+d)(V_5 |6 NO!$ NO" ! ?PK 2z95  NO"#$ Q3#$&'( XY"!W9I$!
+_$  RS )( V_5 ?</ K 5T

932 z95&'(g]_e-)+I_$* ! 01 Oz566 TILI"#$!+I_$   01 )UJ,^95+" J, 4V1J, %zB !EW+X_ Y+J,Z_"*)2[)far6$ *!+\ I g]5 )UJ,)J,) ?9I$ )far6$^_ )U)) ?}`#$ ¡ ,*+*
* !+"1+T&'("¢ ,6ra4/_"*)! TIb_£c2dr6$ 95+_d)fa_$ e . @AB01f¤¥ T ) )_cdr "1+mn gh /_/  

| ¦D95 vV^$) ;6 §E¨_ R=SK#$©i1+fa'
Eª z95 * !+fa'g]_ e )UjvV_5 Wk+&'(2«l|6 XY"!W)) mnK 6_$ )a¬ Yahara and Tokita,,*+*

d+2r _o95 p­2q (r1+&'(dr!Eg¥&'(7_$R=SK Ls®¯st456v ¯t°t N±v45)#$v u)v²_ *65 ³vV^$T I &'(75_´_´+%µ3$)I|6 ¯st ¶0K5, g/R=SK·# + ¸ !_u+$ 95+_d)fa$dr93$ W _&'(2³v9«l|6$!+$+r¬ ¹;º #$5L_gh#$)fa_$ +V " vV^$z&'(5 <=>?dr+$w»+ 2_2 _.x9·#¯st u5y¼1½
7+ 93$266

 0*  ,*+*



ῌ

 !"#$%&'()*+, +33/ - ./01234#$5+ 6 78*9:$ ;<$=%!">?$ @A BCD5EEF E41GH/I JKL /** m MA, B, C, - N H/ICOPQ COPQ H#RSTH/UV% .** WX

Q3Y CZ[

Q G +3/* -\]^% /*2

Q_UVC`aQ Mizu-

tani et al., +320 b3 cdeῌ%L +** MW 5 6 &*'%H/fFeg

Q3

hij +32+ Y ῍I%kQlmnopqCGr Y b5stu3f_GvwGxyznm{|}op~3nopC5%GQ C+C +33/ -./01% stf_  GQ./: GjXY E5u   +C5%GQ X3!">?$I5% bH/If6 I5`aQ H/Ijeg
cdegῌ H/ U fῌHg ῌ I  ῌ3 6
 6 j `aE  {p{  ¡¢£¤3CF ¥¦§¨Q
3 ©ª«3 Fig. - hij +32+ Y f, H/I ¡C+ £ A /¬ , ­ ®¯Q ¡5+, eg_°  +*῍ C{p{2±5+ ¤3² ¥¦§¨r ©ª« ³´5% µ¢£¶·%¸¹'5 ¡f{p{ k6 , km f / kmº»Q3 6Y5 ¼½ µFig. - Hydrothermal system model of Kuju-iwoyama estimated from observed water andheat discharge rates. The values in parentheses contain only the water ﬂow rate
discharged from the ﬂuid reservoir (Ehara et al., +32+)

267





  MT  ! " +33* ; Mogi
and Nakama, +332#" $%&'()*+,-%&'. /0 1234567%8

9:/ ;<= >? / km, @# , km A  BCD >? /** m, @# , km A
EF GHI&JKL4567%89:/ CD A

B4 M" FNJ'8O&

PQPGRST#U ;UVW4 M"+ XRST#U M"+YGRZ
 [\])7%84 ^_ , `Rab)cdefgh&'i[  Gj7%89:/
Fig. ., Ehara, +33,

;<= CD F GHI&J\+ ,** -.* ^_ , `Rklm)

7%8n3[ PQPGo+M"Z#U)G 1/῍ n E3PQPHGj7%8\()pncdq/ r p7%8M"s#U)t+^B4um)vwxym)pn3! Ehara, +33,
;z +33/ {$|}~ ^kl/0*^B

-** m \G^!Ru/ z nnx " /  +332

;<=#"

Ux]3$+ G^!z  +** MW %& ' +*** MW %
& ;n+G^Zi) R/pnn }R/0(/pn\(!
 ;p\ }(ym)i3 - `7%85m)pn Fig. /, (a),(b), (c), (d), Ehara et al., ,**/

;<= G^!)!0 *tR

/ +M"(4GR}CD  C,T#U }CD (m)- ^RZ#U)n*7%8\¡¢y\])pn"M/ 7%8.£3H£^_ , `*kl c +*῍ x¤¥PQPG]3o +p7%8\x/¦\+()
R nb§ 4¨©)0b)ª«= 0G¬1«=n­!0xi* R]*pn3®q/ m3¯° ª«=R!0*3*A ª7%8\+ 4M"
ST#UnRM"Z#U0*R ª±2/ - 7%8\+ M"Z#U)+ 1*῍ %&\([ ²[+³´$µ-¶·¸-0n3!0}
44m)pnR"M#U ¨;" p R}3klm)G¹º3!0*)\+3*Mn»µ/0*)V¼\+ {$}CD ^ῌB4n!0 Gj7%85½5q/R

Fig. .

268

Vertical cross section of the cylindrical geothermal reservoir of Kuju-iwoyama (Ehara,+33,)

 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 thenumerical model. (c) N-S vertical section of the numerical model. The red represents the permeable volcanic geothermal reservoir. (d)Temperature distribution of the N-S vertical section of thehydrothermal system model. Left : before the +33/ phreatic eruption. Right :after the +33/ phreatic eruption (Ehara
et al., ,**/).

269





Fig. / Continued !"#$%&'()*+,-. /012&34567)* 8*9:;<567=+,-. /0
>?567&@AB :CD;<5EFGH IJ&K!9)*HLM
N,OPH

0
:CQ!E,/R*FGH :IQH!QM 345/067&ST
LUH+,: "#$VWXYZ[\ -]^S_`abcd:efgh!
2

Ei

Ej6

/6 k:;<= /0lm$%:nB )9 8*  "#$% op\qVrstWXYZ[\:u!"#$%:nv
GH+,ST1:wx=! 8= 8-y"#$%  :CDE,/FGHz{|2}H+,&L~ )94/  5
&,B ,,KGH,7:gh! E
:.M :CDEFGH IJ&TLUH,OPH|῍:U!. :Q!!|  ¡¢ I£@¤¥¦:§GHKLUH A! ¨©ªῌ«M ¬:­®¯~ ° ±wx:¯~ |῍²³´µ¶.! @¤¥GHKLUH +·῍& ¸¹H,/R*
FGH 567,yº»¹LM ,,KGH+,:b-B¼½=!

Axelsson, G., Armannsson, H., Bjornsson, S., Floventz, O. G., Gudmundsson, A., Palmmason, G.,
Stefansson, V., Steigrimsson, B. and Tulinus, H. (,**-) : Sustainable production of geothermal energy

Suggested deﬁnition , +.

Elders, W.A. (+33+) : Geologic models of geothermal reservoirs. Geothermal Resources Council270

 0*  ,*+*



BULLETIN, ,*, +3.ῌ,*-.
Ehara, S. (+33,) : Thermal structure beneath Kuju volcano, central Kyusu, Japan. J. Volcanol.
Geotherm. Res., /., +*1ῌ++0.

 !" +332 : +33/ #$%&'(& ) * +, -./0 .2 1,ῌ2-.
 !" 1234 5678 96:! +33* : ;<=(>? @AB;<$%CD
' E FG0HI +, ,0-ῌ,2+.
 !" J KL MNOP +32+ : $%CD'QR Sῌ(T ῌ&'UVST )RQRWX  Y &'UV Z[ &' ,0 -/ῌ/0.
Ehara, S., Fujimitsu, Y., Nishijima, J., Fukuoka, K. and Ozawa, M. (,**/) : Changes in thethermal 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 offumarolic gases from Kuju-iwoyama, Kyushu, Japan. Geochem. J., ,*, ,1-ῌ,2/.
Mogi, T. and Nakama, K. (+332) : Three dimensional resistivity structure beneath Kuju volcano,central Kyushu, Japan. J. Volcanol. Geotherm. Res., ,,, 33ῌ+++.

FG0H \] -.^_`abcdefH ,*+* : \] -.^_`a +ῌ0,.
Yahara, T. and Tokita, H. (,*+*) : Sustainability of the Hatchobaru geothermal ﬁeld, Japan.
Geothermics (to be published).

ghij +32- : kl

< mnopqrstuv

w

xy F

G0HI / ,-ῌ-2.

271