-- Keywords: reservoir management, reservoir sustainability, energy extraction, energy cost.

Transcription

1 t al. GEOTHERMAL RESERVOIR MANAGEMENT IN ICELAND and Snom P. Consulting Enginrs, 9,108 Rykjavik, Icland 50,108 Rykjavik, Icland ABSTRACT At prsnt, thr ar about 40 gothrmal rsrvoirs undr commrcial xploitation in Icland, and many hav bn xploitd for mor than 30 yars. This xploitation of gothrmal rsourcs startd in During th following 65 yars gothrmal rsrvoir managmnt has bcom an intgral part of th xploitation. Improvmnts in production tchnology, such as th us of down-hol pumps, hav multiplid th nrgy xtraction from th rsrvoirs. Ths improvmnts in production tchnology and improvmnts in managmnt hav mad gothrmal nrgy so attractiv, both rgarding pric and rliability, that gothrmal nrgy contributs 44% of th gross nrgy consumption in th country. An incrasing dmand for gothrmal nrgy is mt with managmnt actions such as: mor ffctiv us of rsrvoir capacity, injction, improving production and by xtnsions to nw production filds. Nvr has th utilization of a gothrmal rsrvoir in Icland bn abandond bcaus of a dpltion of th rsrvoir. Th basis for propr rsrvoir valuation and succssful managmnt is th carful monitoring of th rsrvoirs and modling work prdicting thir futur rspons. -- Kywords: rsrvoir managmnt, rsrvoir sustainability, nrgy xtraction, nrgy cost. 1. INTRODUCTION Managmnt of a gothnnal rsrvoir involvs taking dcisions during th opration of th rsrvoir in ordr to chang its oprational conditions. Most frquntly, managmnt actions ar carrid out m ordr to improv th opration m on way or anothr. Somtims, managmnt dcisions ar takn bcaus. unfavorabl conditions (cold watr invasion, prssur drop) hav volvd in th rsrvoir. In othr cass, improvmnts in production tchnology (down-hol pumps, rinjction) might a chang in production stratgy aimd at prolonging th longvity of a gothrmal rsourc. of gothrmal rsrvoirs rlis on propr information on th rsrvoir. If changs in production stratgy ar imposd upon th rsrvoir, th managmnt concpt mans that th rspons of th rsrvoir is known advanc. Othrwis, it is not to about managmnt actions. Th information ndd for a succssful managmnt of gothrmal rsrvoirs is, in gnral, th following: Knowldg of th gomtry, and boundary conditions (f. x. rcharg) of a rsrvoir. (ii) Knowldg of th proprtis of a rsrvoir such as prmability, porosity, dnsity, hat capacity, and hat conductivity. Knowldg of physical conditions which ar dtrmind by tmpratur- and prssur distribution (f. x. boiling). Information about th tim dpndnt paramtrs of a rsrvoir is collctd with a propr monitoring program carrid out during xploitation. Gothrmal vary considrably in thir and th tim constants involvd in diffrnt procsss hav a considrabl Figur 1. Location of th gothnnal rsrvoirs discussd in th papr. rang. Thrfor, th monitoring frquncy is diffrnt for diffrnt within a rsrvoir and for diffrnt rsrvoirs as wll. Th changs within a rsrvoir, affcting th production from th rsrvoir, usually tak som months or yars. Thrfor, th managmnt is a slow procss and its long trm objctivs hav to b wll dfind. Rsrvoir managmnt may hav diffrnt objctivs such as: to th oprational cost of a crtain gothrmal fild, to maximiz th nrgy xtraction from a givn rsourc, to nsur scurity of nrgy to ffcts, to avoid oprational scaling, to adhr to th nrgy policy of th rspctiv country, and othrs. Quit oftn th objctivs ar a mixtur of all th itms listd abov. In such cass it is ncssary to put a prfrnc ordr to th objctivs bcaus thy may not all convrg. It can b as an xampl, that kping th oprational cost low for two or thr yars might not agr with th goal to maximiz th nrgy xtraction for twnty or thirty yars. On of th most difficult aspcts of rsrvoir managmnt is to dtrmin a tim span for th managmnt objctivs. It may b nccssary to choos, for xampl, whthr th maximum rvnu of th rsourc should b obtaind in 5 yars or 30 yars or vn in yars. Thr ar cass whr dplting a givn rsrvoir in 5 yars of yars, mor th pur financial point of viw. Such cass ar, howvr, usually unaccptabl from political or sociological point of viw. Rliabl availability of nrgy for a long tim is considrd mor valuabl than th intrnal rat of rturn of th in th of th nrgy rsourc. Th tim usually chosn for th managmnt objctiis is rlatd to th xpctd liftim of surfac quipmnt. Powr plants ar frquntly amortizd in som yars and thrfor th longvitis of th rsrvoirs matchd to that tim span. Svral rsrvoirs hav bn xploitd succssfully for a much longr tim and it is not obvious whthr gothrmal rsrvoirs should b considrd as nrgy sourc (i.. a min) or as a rnwabl nrgy sourc. 1763

2 t al watr lvl E ai 100 ai 200 yarly Figur 2. Production and drawdown in th Laugams gothrmal fild Up to 1960 th production was fr flow from wlls, but pumping with down hol pumps has bn th production mthod sinc At prsnt, thr ar mor than 30 gothrmal rsrvoirs undr commrcial xploitation in Icland. Th largst numbr of ths ar low-tmpratur rsrvoirs utilizd for hot watr production. Dirct us of gothrmal nrgy is mainly for spac hating in Icland whr mor than 85 of th houss in th country ar hatd with gothrmal nrgy. In addition six high-tmpratur rsrvoirs ar currntly utiliid for lctricity gnration, spac hating, and purposs. In som of ths rsrvoirs production has only minor changs in rsrvoir conditions whras in othr cass hav ithr dclind continuously or th prssur drop has causd a considrabl cold- and/or salin-watr rcharg. Th managmnt of ths rsrvoirs has aimd at mting an incrasing nrgy dmand as wll as mting th dcrasing potntial of rsrvoirs whr major changs in subsurfac conditions hav occurrd. Mathmatic modls ar prsntly usd in Icland to prdict futur conditions of th rsrvoirs. Lumpd paramtr modls hav bn found vry usful for prdicting th rsrvoir rsponss of singl-phas gothrmal systms (Axlsson, 1989). In th cas of two-phas rsrvoirs and m cass whr larg variations in tmpratur or prssur conditions prvail, distributd paramtrs modls ar usd t al., Modl prdictions hav always bn found to b a rliabl basis for managmnt considrations. No gothrmal rsrvoir in Icland has bn abandond bcaus of dpltion. Th prsnt dscribs svral rprsntativ managmnt actions m along with th rsrvoir conditions that hav causd ths actions. Ths actions includ: production with down-hol pumps, improving of gothrmal utilization, injction (of wast watr) into th rsrvoirs, actions to prvnt or circumvnt th ffcts of Scaling, sarch for improvd production charactristics th systm undr xploitation, and xtnsion to nw production filds. Location of th rsrvoirs discussd in th papr is givn in Figur 1. DOWN-HOLE PUMPS Introduction of down-hol pumps, as a production mthod for tmpratur gothrmal rsrvoirs, is th singl managmnt action that has causd largst gothrmal utilization in Icland. Th first down-hol pump was tstd in Rykjavik in Svr oprational difficultis wr xprincd in th arly days and th liftim of th first pumps was rlativly short. Th dsign of th pumps was during th yars to follow undr th ladrship of th formr dirctor of Th District Hating Srvics of Rykjavik. By a rliabl typ of pump had bn dvlopd and this typ of down-hol pump has bn in us in Icland vr sinc. At prsnt thr about down-hol pumps in opration in Icland and th avrag liftim of th pumps is about yars (Gunnarsson, 1992). Figur 2 shows th production history of th Laugams fild locatd within th town of Rykjavik. This is th first gothrmal fild m Icland utilizd for public district hating. Th utilization startd m 1928 and th production from th fild from 1930 to 1993 is prsntd m Figur 2. Up to 1958 th production was only by fr flow from wlls but aftr 1964 th production has bn ntirly by down-hol Th rat of fr flow from th fild was som 2040 bfor pumping startd but th avrag production in th yars has bn about 160 An nhancmnt in production by a factor of 4 to 8 was, thrfor, obtaind by th introduction of down-hol pumps in th fild. It should b notd that th production rat during th 30 yars of pumping has bn rlativly constant and that th prssur (watr lvl) in th rsrvoir has also bn rlativly stabl during th pumping priod. A comparabl nhancmnt factor by th introduction of down-hol pumps was also obtaind for th Rykir fild locatd just outsid Rykjavik. Th avrag rat of fr flow bfor pumping was about 250 but with pumping th avrag production has bn about 1100 an nhancmnt by a factor of NEW GEOTHERMAL DEVELOPMENTS Th first dcision to b takn is whthr a givn rsrvoir should b dvlopd for utilization. Th pric of gothrmal nrgy as compard to th pric of othr nrgy sourcs is usually th dciding factor. On of th most snsitiv paramtrs for th pric of nrgy is th distanc btwn th fild and th consumr. Transportation of hot watr is not conomical ovr vry long distancs. Although widly sprad throughout Icland, ar still som populatd aras conomically to far away from known gothcrmal Th longst transporting hot watr from th fild to th markt is 64 km, srving a town with inhabitants (Akrans). Anothr long gothrmal piplin transports hot watr producd at to consumrs in Rykjavik. Th tmpratur dclin along this 30 km long lin is 1-2 High oil prics in th last two dcads mad viabl many hot watr distribution projcts that wr formrly conomically marginal. Furthrmor, grat mphasis was placd on fforts to discovr nw gothnnal rsourcs nar larg hot watr markts. Th rsult of this can b sn in Figur 3. In 1970 about 45% of rsidntial buildings in Icland wr hatd by gothrmal nrgy. During a priod of tn yars thr was a rapid incras in th us of gothrmal nrgy for spac hating and at prsnt mor than 85% of buildings in Icland ar hatd by gothrmal nrgy. 1764

3 130 Hatd spac yars Figur 3. Th ffct of incrasd oil pric on th utilization of gothrmal nrgy for spac hating in Icland Figur 4. Masurd and draw-down in th rsrvoir. Powr hot watr sal usd up to 1986, but nrgy tariff in us EFFICIENCY OF GEOTHERMAL UTILIZATION Th utilization of gothrmal nrgy by th consumr may b pricd in diffrnt ways. In Icland two mods of hot watr for spac hating and othr domstic uss hav bn mployd. On mthod is to charg th consumrs according to th nrgy consumd (actually th of hot watr), th othr is to bas th nrgy bill on th pak powr dmand of th (maximum flow rat). A tariff basd on th powr is in som ways simplr in us for th district srvics. Each consumr pays a pric for th whol yar and th administrativ work for th distribution is minimal. Th xprinc of th two tariff systms has shown that th nrgy consumption of consumrs paying according to th powr tariff was gratr than thos of consumrs paying according to th nrgy tariff. This is quit natural. Th consumrs paying according to th powr tariff us th powr thy hav paid for, rgardlss of whthr thy th powr or not. infficint nrgy consumption by th district hating srvics applying th powr tariff rsultd in a gratr burdn on th gothrmal rsrvoirs than actually was ncssary. Grat prssur drawdown was obsrvd in th rsrvoirs, and it was forsn that th gothrmal rsourcs would only sustain th load for a limitd way to prolong th lif of th gothrmal rsourc is to forc th consumrs to us th nrgy mor fficintly. A rlativly simpl way to obtain this objctiv to chang th hot watr from th powr mthod to th nrgy mthod. This has bn don in svral instancs in Icland. 4 shows, as an xampl, th prssur drawdown in th Hamar rsrvoir srving th district hating at (pop. 1,500) in North Icland. Up to 1986 th hot watr was sold to ach consumr to th powr tariff. Th avrag production rat was about 41 during this priod. In 1986 th tariff was changd and th hot watr sold according to th volum of hot watr usd. Th nrgy dmand of th town dcrasd to about 27 aftr th chang of th tariff. Bcaus of th rducd production from th rsrvoir th rsrvoir prssur has rcovrd by approximatly 2 bars, as is shown Fig. 4. Figur 4 also shows th drawdown in rsrvoir up to th yar Assuming that th production from th rsrvoir will b constant in th 1986 drawdown not b rachd until th yar This indicats that th action of changing th sals tariff for th hot watr in has prolongd th liftim of th gothrmal by at last 80 yars. Th valu of this managmnt action can b stimatd by considring th pric of of hot watr for 80 yars. By using th prsnt avrag pric of gothrmal nrgy to th consumr Icland, th valu of th prolongd liftim of th Hamar rsourc is about 26 US dollars. Th cost involvd in changing th tar8 was som. US dollars. This shows that propr managmnt actions in th opration of gothrmal rsrvoirs can b vry profitabl. 5. INJECTION Injction has not yt bcom an intgral part of th opration of gothrmal rsrvoirs in Icland. This is partly bcaus disposal of wast gothrmal watr has nvr bn an nvironmntal issu in Icland in contrast to othr countris. Th concntration of dissolvd in watr th Iclandic low-tmpratur is usually low nough for th watr to b as drinking watr according to by intrnational (EC, WHO, FAO). Wast watr can thrfor b disposd of at surfac or into swrs. In th surfac of wast watr has also bn th Invstigations hav not suggstd that th wast watr will caus advrs cological ffcts. In fact, in on cas (Nsjavllir) th wast gothrmal watr improvs th living conditions of trout in th lak In anothr cas (Svartsngi), a disposal in th lava fild surrounding th powr plant has bcom an important tourist attraction. As a consqunc of ths circumstancs, injction is primarily considrd in Icland as a tool to improv th oprational conditions of th rsrvoirs rathr than an issu. Injction xprimnts hav bn carrid out in of th low-tmpratur gothrmal filds in rcnt yars (Axlsson t al., and limitd injction has bn in th high-tmpratur fild sinc 1984 and Stingrimsson, 1992). In both th high-tmpratur and cass, is into dominatd rsrvoirs such that th rduction in prssur drawdown was obsrvd in all cass. 5 shows th simulatd rspons of th limitd rinjction on th drawdown btwn 1976 and 1992 is about 22 bars and th avrag mass production sinc 1980 is about 250 Th injction rat during varid btwn or about 8-16% of th production. Th rduction in th prssur drawdown du to injction is about 4 bars as shown in Fig. 5. Injction of watr in Svartsngi causd som. cooling in a narby production wll and th was changd in Modl calculations hav shown that injction wlls in Svartsngi nd to b locatd som 3-5 away from production wlls in ordr to rduc thir thrmal influnc (Vatnaskil, 1989). Dtaild studis of th of injction in four low-tmpratur filds t 1995) rval that th bnfit from injction can b a dlicat problm whr th injction rat, tmpratur of injctd watr, and distanc btwn injction and production wlls to b slctd with grat car. In som cass only a low rat of 1765

4 Stfhsson t al masurd drawdown total production a Magnsium silicat dposits hav bn of concrn in cass whr hatd groundwatr is usd for spac hating (Svartsngi, Nsjavllir, and Ths dposit may usually b avoidd if hatd groundwatr is not allowd to mix with natural gothmal watr. Gothrmal watrs ar saturatd in silica but dpltd in magnsium. Cold groundwatr, on th othr hand, contains som magnsium Th mixing of such watrs may caus magnsium silicat prcipitation although dposits do not occur in ach of ths typs of watrs. whn th Nsjavllir powr plant was commissiond in 1990, it was found ncssary to sparat th distribution systm within Rykjavik m ordr to prvnt mixing of th hatd ground watr from Nsjavllir with gothrmal watr from th low-tmpratur filds (Laugams, and Rykir). Figur 5. Prssur and production history of th Svartsngi rsrvoir. Modl calculation for production with and without rinjction. injction will improv charactristics of production wlls, whras m othr cass, whr th flow paths btwn injction and production ar sufficintly long, injction of cold watr will rais th production capacity considrably. Thr sms to b no gnral rul rgarding th applicability of injction as a production mthod in gothrmal xploitation. Th individual natur of ach gothrmal rsrvoir calls for spcific for ach cas in ordr to dtrmin th most appropriat rinjction schm. A gnralizd on th bnfits of rinjction into high-tmpratun: utilizd for lctricity gnration has shown that rinjction causs surprisingly small gain in lctrical production whn two phas condition prvail in th rsrvoirs t al., 1995). Rinjction stratgy is, howvr, always favorabl whn th managmnt mphasis is on th thrmal swp of th rsrvoir. 7. IMPROVED PRODUCTION CHARACTERISTICS On of th district hating srvics that startd opration in th priod of high oil prics is th on at in East Icland. A tmpratur gothrmal rsourc had bn locatd blow Lak som 5.5 from A fw wlls wr yilding nough hot watr for hating of th town. Opration startd in 1979, but during th first four yars substantial cooling of th producd watr was obsrvd (Fig. 6). It bcam obvious that th hating could not continu to mt th dmand of th hating markt unlss th production charactristics wr improvd. A comprhnsiv rinvstigation of th gothrmal systm was carrid and th up-flow zon of th systm locatd. A nw wll (wll 8) was targtd to intrsct this channl at dpth gratr than th dpth of production zons in th othr wlls. 6. SCALING PREVENTION Part of th gothrmal utilization is to dsign th utilization schm in such a way that th fluid will caus minimal ffct on th gothrmal opration. In Icland it is mainly prcipitation of calcit, silica, and magnsium silicat that hav influncd th utilization as wll as th managmnt of th gothrmal rsourcs. Prcipitation of calcit is a fast procss taking plac whr th gothrmal fluid for th first tim. Calcit scaling is of concrn in filds in Icland, Svartsngi and Th scaling is rmovd rgularly by mchanical claning of th wlls whil thy ar In that way th dbris rachs th surfac and th production is only disturbd for on or two days. By using wid production casings (13 3/8 such as Svartsngi, th wlls may b for 4-5 yars bfor th scaling starts to affct th production rat and claning is rquird. Calcit scaling has thrfor a rlativly minor ffct on th oprational cost. Calcit scaling is somtims obsrvd in low-tmpratur filds as a of hot and cold watr mixing m th production wlls. Th scaling affcting th down-hol pumps, and a yarly claning of th pumps ncssary in som cass Slfoss). Silica scaling affcts th dsign of th utilization procss rathr than bing an oprational problm. This is bcaus silica dposition is a much slowr procss than th prcipitation of calcit. It is thrfor possibl to slct a suitabl plac in th utilization schm whr it is convnint to rmov th silica scaling. Production wlls ar always opratd at wll-had prssur abov th scaling point of amorphous silica bcaus rmoval of scaling is asir m surfac piping than in wlls. bd hat xchangrs ar for xampl usd in th powr plant at Nsjavllir such that th prcipitation taks plac whr th dposition can b rmovd rlativly asily (Gunnarsson t al, 1992). wll 8 [- I I I, I I I I I /I a I, I d 00 HO 87 A Figur 6. Production history fild. Nw wll with improvd production charactristics takn in us i983 rsulting m stabl tmpratur dlivry. Wll 8 turnd out to b highly succssful yilding mor than hottr watr than th oldr wlls. Th production capacity of this singl wll was, furthnnor, svral tims gratr than th capacity of ach of th oldr wlls. Figur 6 shows th production history from Aftr 1984 wll 8 has fulfilld compltly th district hating dmand. Th tmpratur of th producd fluid from wll 8 has changd littl during an oprating priod of tn yars. A similar situation aros in th Krafla high-tmpratur fild in North Icland. During th construction priod of a 60 powr plant this fild a volcanic ruption startd within th caldra whr th gothrmal fild is locatd 1981; t al., 1987). rsrvoir bcam contaminatd with magmatic gass th chmical quilibrium btwn th gothrmal fluid and th rsrvoir rock t al. 1989). Complx prcipitations startd to clog som of th wlls and acidic fluid was obsrvd othrs and Gislason, 1992). As a rsult a larg part of th gothrmal rsrvoir in was dmd xploitabl. Considrabl ffort was thrfor put into finding parts within th gothrmal systm whr th rsrvoir fluid was not affctd 1766

5 Stfhson t al. by th magmatic gass. Two such subsystms and wr discovrd and hav dlivrd most of th stam usd for th 15 yars of powr gnration in Both and ar small compard to th main contaminatd rsrvoir m Th. availabl stam has thrfor only bn nough for 30 production instad of th dsignd 60 xampls from and Krafla show that a sarch for bttr production charactristics within a givn gothrmal systm can b a viabl managmnt option whn undsirabl changs tak plac in th rsrvoir undr xploitation. 8. NEW PRODUCTION FIELDS Th. dmand for gothrmal nrgy is incrasing continuously in Icland at th avrag rat of 6% annually during th last 30 yars. indicats that soonr or latr th dmand for gothrmal nrgy will xcd th capacity of th rsrvoirs undr xploitation. Most of th District Hating Srvics xploit on gothrmal rsrvoir only. Th District Hating Srvics in Rykjavik and Akuryri, howvr, utiliz four rsrvoirs ach. Up to th prsnt only ths two Utilitis hav bn facd with th dcision whn th dvlopmnt of a nw production should tak plac. A prrquisit for this managmnt option is of cours that thr ar som unxploitd gothrmal rsourcs in th nighborhood. In th trms of invstmnt, th largst managmnt dcision takn m Icland was th dcision to dvlop th Nsjavllir high-tmpratur fild as additional nrgy sourc for th Rykjavik Municipal District Hating Srvic. Th cost involvd in th dvlopmnt of th Nsjavllir Co-Gnration Powr Plant is about 125 million US dollars (Gunnarsson t al., 1992). It is obvious that a long sris of othr managmnt actions had prcdd th dcision to dvlop th fild. Th. Rykjavik Municipal District Hating Srvic startd opration in 1930 by 2.5 MW from th Laugams fild. In 1943 a nw production fild opration and th powr of th hating srvic incrasd to 45 Production with down-hol pumps startd in th Laugams fild in (s chaptr 2). In th yars a nw production fild at was addd to th distribution systm. Rdvlopmnt (by down-hol pumps) of th Rykir incrasd th powr from th fild by a factor four. Ths diffrnt managmnt actions had incrasd th powr of th distribution systm to mor than 600 usd for spac hating and as domstic hot watr for 145,000 popl (Zoga, 1987). of dvlopmnt of a nw production fild is not a simpl Fasibility calculations can b carrid out in accurat way, but th assumptions bhind th fasibility study may b subjct to chang. In th cas of th dvlopmnt of Nsjavllir, th scurity of nrgy dlivry was th major componnt in th timing of th Bfor Nsjavllir cam in, thr gothrmal rsrvoirs, Laugams, and Rykir, supplid th hot watr. In all thr rsrvoirs signs of had bn obsrvd, mainly as invasion of cold watr affcting som production wlls. Although th cooling was not srious it raisd som qustions rgarding th futur volution By dvloping Nsjavllir this uncrtainty was rmovd and th ovrall of nrgy dlivry could b incrasd considrably. Th financial viability of th Nsjavllir Powr Plant has nvr bn qustiond. Th long trm hot watr pric is stimatd at which is lowr than th pr-nsjavllir consumr pric of In addition th powr plant is dsignd to gnrat 80 MW of lctrical nrgy m th futur. Th production cost of lctricity from Nsjavllir is stimatd to b which is much lowr than stimatd for nw hydro-powr stations in Icland (Gunnarsson t al., 1992). Th timing was thrfor mainly a comparison btwn th scurity of nrgy dlivry and th long trm invstmnt cost. Th. tchnical and conomical conditions for dvloping th Thlamork rsrvoir for th Akuryri District Hating Srvic hav rcntly bn dscribd by Bjomsson t al. (1994). Th Thlamork rsrvoir is a rlativly small gothrmal rsourc locatd 11 north of Akuryri. Its connction to th distribution systm would add som 4 MW to th systm. Th nrgy pric for this small gothrmal rsourc is stimatd which is much lowr than th nrgy pric of any othr nrgy sourc availabl in Akuryri. 9. ECONOMICS Whn gothrmal nrgy is usd as thrmal nrgy, it is th chapst nrgy sourc availabl in Icland. Typical production prics (pric at ar m th rang and consumr prics ar usually m th rang This is th main rason for th succssful dvlopmnt of gothrmal nrgy in Icland. Gothrmal nrgy has th largst shar (44%) of th nrgy consumption m Icland. No othr country covrs that larg part of its nrgy dmand by gothrmal nrgy. is mainly (95%) gnratd from hydropowr in Icland and gothrmal nrgy contributs only 5% to th lctricity gnration. Th cost of lctricity gnration in nw powr plants is stimatd to b similar for hydro and for gothrmal powr plants. Th production prics for both hydro and gothrmal ar stimatd to b in th rang In 1993 hydropowr contributd 16.4% of th primary nrgy consumption m Icland, gothrmal nrgy and importd fossil fuls 40.5%. Th total nrgy consumption in th yar 1993 was which ranks among th highst in th world. Th gothrmal potntial of Icland is grat compard to th prsnt lvl of nrgy utilization in th country. It is, howvr, th utilization mthods and th managmnt of th gothrmal rsourcs that hav mad gothrmal nrgy th chapst thrmal nrgy in Icland. 10. CONCLUSIONS Th xprinc of commrcial opration of gothrmal rsourcs m Icland dats back to During this priod, th production mthods and th undrstanding of gothrmal systms hav improvd considrably. Carful monitoring of th physical and chmical conditions m th rsrvoir is th basis for a propr rsrvoir valuation at ach tim. Nw production tchniqus such as th us of down hol pumps and th co-gnration of hat and lctricity hav bn introducd and such managmnt actions hav incrasd th usabl nrgy of th rsourcs by a factor of mor than four. Dvlopmnt of gothrmal rsourcs m Icland has at all tims bn dirctd by th pric of gothrmal nrgy in comparison to othr availabl nrgy sourcs. This has rsultd m gothrmal nrgy now bing th chapst sourc for hat nrgy in th country. Propr managmnt of th gothrmal rsourcs maintains th low nrgy prics and scurs th continuity of th producd nrgy. Effctiv us of th gothrmal rsourcs hav contributd to low consumr prics and xtndd th longvity of th rsrvoirs in svral locations. In Icland, injction is primarily considrd as a production mthod that can improv th long trm xploitation of gothrmal rsrvoirs. Svral injction xprimnts hav rvald that in ach rsrvoir nds to b balancd in a carful way in ordr to obtain bnficial ffct on th oprational conditions in th diffrnt rsrvoirs. Th sarch for improvd production charactristics within a rsrvoir undr xploitation has in many cass provd to b a viabl option whn undsirabl changs tak plac in th rsrvoir. 1767

6 t al. dmand for gothrmal nrgy rsults in that nw production filds hav to b dvlopd soonr or latr. Th timing of nw dvlopmnt is not a simpl dcision. Tchnical and conomical considrations hav to b compard to political and sociological prfrncs. Th timing of a nw gothrmal on a comparison btwn th scurity of nrgy dlivry and th long trm minimum invstmnt cost. Gothrmal nrgy is th chapst thrmal nrgy sourc availabl m Icland, and it contributs 44% of th gross nrgy consumption m th Typical wll-had prics in th rang and th consumr prics ar usually in th rang ACKNOWLEDGEMENTS W thank th District Hating Srvics of Rykjavik, Dalvik, and for prmission to publish th data prsntd in this papr. W also thank Bjomsson, and Michal L. Sory for critically rviwing th papr. REFERENCES H., and Gislason, G. (1992). Th occurrnc of acid fluids in th fild, Krafla, Icland. In. Watr-Rock Intraction, and Mast, (ds), Rottrdam, pp hannsson, H., J., and Jffry, A.W.A. (1989). Gas changs in th Krafla gothrmal systm, Icland. Chmical 76, pp. hrmannsson, H., h., and Stingrimsson, B.S. (1987). Exploration and dvlopmnt of th Krafla gothrmal ara. 37, Axlsson, G. (1989). Simulation of prssur rspons data from gothrmal rsrvoirs by lumpd paramtr modls. Fourtnth Workshop on Gothrmal Rsrvoir Enginring, Jan Stanford Univrsity, pp Bjomsson, G. and Stingrimsson, B. (1992). yars of tmpratur and prssur monitoring m th Svartsngi hightmpratur gothrmal fild m SW-Icland. Gothrmal Rsourcs Council, Transactions, Vol. 16, pp Bjomsson, G., Axlsson, G., and 6.G. (1994). Fasibility study for Thlamork low-tmpratur. systm in N-Icland. Papr prsntd at th Nintnth Workshop on Gothrmal Rsrvoir Enginring, Stanford, Jan. Bodvarsson, G.S., B omsson, S., Gunnarsson, h., Gunnlaugsson, E., V., and Stingrimsson, B. (1990). Th Nsjavllir gothrmal fild, 1. Fild charactristics and dvlopmnt of a thr-dimnsional numrical modl. J. Gothrm. and Tch., Vol. 2, pp Gunnarsson, A. (1992). Hot watr pumps (in Iclandic). Hitavituhandbdk, Chaptr 5, Samband Gunnarsson, h., Stingrimsson, B.S., Gunnlaugsson, E., J., and Maack, R. (1992). Nsjavllir gothrmal Gnration Powr Plant. Gothrmics, Vol. 21, No. 4, pp. 6., Arason, P., and Stfhsson, V. (1995). stratgy for gothrmal systms. World Gothrmal Congrss Flornc, Italy, May V. (1981). Th gothrmal fild, Northast Icland. In: Gothrmal Systms; Principls and Cas Historis. L. Rybacch and L.J.P. Mufflr (Editors), Ch. 10. Nw York, N.Y., pp (1989). Svartsngi. Modl simulations. (in Iclandic) Orkustofnun Rport Zoga, J. (1987). Powr- and nrgy dmand for th Rykjavik Municipal District Hating Srvic (in Iclandic). Papr prsntd at Confrnc on Gothrmal Dvlopmnt at Nsjavllir. Rykjavik 28 January p. G., G., H., and G. (1995). Injction xprimnts in tmpratur gothrmal aras in Icland. World Gothrmal Congrss Flornc, May