What is Geohermal Energy?

Transcription

1 E n e r g y R e s o u r c e s A Brigh Fuure for Geohermal Energy Geohermal energy is a clean, readily available, and renewable naural resource. Ye he near limiless poenial remains largely unapped accouning for only a very small percenage of he oal amoun of energy we use oday. Tom Smih 36 GEO ExPro Ocober 2007

2 The world s geohermal provinces and plae boundaries indicaes areas of high emperaure resource poenial (oulined in red). EGI EGI Dynamics of he earh s convecion cells and areas of rising magma where hea flows are he highes. Geohermal energy can be seen rising in he form of seam in fron of his well being drilled a he Brady s geohermal field in norhern Nevada, 70 km norheas of Reno. GEO ExPro Ocober

3 E n e r g y R e s o u r c e s Thomas Smih, Associae Edior Surface geohermal waers in he form of ho springs and hermal pools are familiar indicaors of shallow hea anomalies. For cenuries, man has used hese for bahing, cooking, medicine, and heaing (GEO ExPro Vol. 2, No 1, 2005). The firs geohermally generaed elecriciy was produced from seam in a econically acive area in souhern Tuscany, Ialy, a Larderello, in Since hose firs seps, new echnologies have been developed ha enable wider use of geohermal energy o generae elecriciy from waers as cool as 74 C. Consequenly, here is a growing awareness of he value of geohermal as a clean energy source and he sheer magniude of is poenial. Currenly 21 counries generae elecriciy from geohermal sources. Large and small privae energy companies (including Calpine, Chevron, INEL, Orma), and naional uiliies are acively searching and developing geohermal energy. In fac, Chevron, a major oil company, is he world s larges producer of geohermal elecriciy wih operaions in he U.S., Indonesia, and he Philippines. Over 25% of Iceland s elecrical needs are generaed wih geohermal energy. All hese areas lay near plae boundaries (see world geohermal provinces map below). Hea from Wihin Hea inside he earh is generaed primarily from he decay of radioacive isoopes Geohermal Energy The word geohermal comes from he Greek words geo (earh) and herme (hea). By geohermal energy we mean hea from wihin he earh. Geohermal energy is a renewable energy source because hea is coninuously produced inside he earh. The echnology for drilling and developing geohermal energy has o a large exen been borrowed from he oil and gas indusry. However, in order o achieve he large flows required from geohermal wells, producion wells are compleed differenly han oil and gas wells, ypically wih 9 inch or larger casing diameers. Tom Smih wih some hea lef over from he fiery formaion of he plane. The emperaure gradually increases from he surface down o he 4200 C in he earh s inner core. This hea moves upward o he surface and is coninually radiaed ino space. The average geohermal gradien for he earh is abou 25ºC/km. I is highes in volcanically acive areas like island arcs where he emperaure increases by abou C/km and is lowes in areas such as he ocean s renches where he emperaure increases only 5-10 C/km. Enormous hermal-convecion cells slowly circulaing in he earh s ashenoshpere (he ho, sof, easily deformable porion of he manle) have caused he plane s hin, rigid lihosphere (he uppermos manle and he overlying crus on which we live) o break ino vas econic plaes. Mos of he world s high-emperaure geohermal provinces are locaed along plae boundaries. Oher areas of higher hea flow originae from hospos above isolaed, enigmaic ashenosheric plumes, such as hose rising beneah he Big Island of Hawaii and Yellowsone Naional Park in he sae of Wyoming. Along he plae boundaries and under he world s ho spos, rising magma Temperaure is he decisive facor Geohermal energy can be uilized jus abou anywhere in he world. The resource emperaure dicaes he ype and efficiency of use. Geohermal hea pumps, which exploi low-emperaure ground hea, can be widely used o efficienly hea and condiion he workplace, schools, and homes. In higher hea flow areas, hermal waers (35 o 150 C) are piped direcly from he ground o suppor municipal heaing sysems, aquaculure, and greenhouses. The hoes sysems (generally greaer han 150 C) use geohermal seam and ho waer o generae elecric power. This source now accouns for 5% of California s elecrical generaion and 10% for norhern Nevada. Areas of highes hea flow are concenraed in he wesern saes. EGI 38 GEO ExPro Ocober 2007

4 Phoo: Halfdan Carsens E n e r g y R e s o u r c e s EGI Typical high-emperaure geohermal sysem used o generae elecriciy. Heaed a deph, groundwaer becomes less dense han he surrounding waer and rises hrough he fracures unil encounering he cap rock. This ho waer is apped by producion wells. Some of he fluid may leak hrough he cap rock o form ho springs, fumaroles, and geysers, or reach dephs shallow enough o be encounered in waer wells and mineral exploraion holes. As he waer beneah he cap rock moves laerally away from he upwelling cener and cools, is densiy increases. Cooled produced waer is also injeced back ino he sysem This cooler waer descends along he margin of he sysem where i is be reheaed, beginning anoher cycle. The resuling hydrohermal convecion cell can be very sable, lasing for ens o hundreds of housands of years or more. ascends hrough he upper manle and ino he lihosphere someimes resuling in volcanic aciviy a he surface. This rising magma creaes areas where shallow geohermal sysems can be harnessed for heir energy. High-emperaure convecive geohermal sysems, concenraed along he econic acive areas near plae boundaries, are he mos valuable and sough afer commodiies for geohermal elecric power generaion. To be viable in his regard, hese sysems require: 1) hea, which can be supplied by shallow magma bodies or deep circulaion of fluids in areas of high geohermal gradiens; 2) a supply of waer, usually ground waer; 3) adequae permeabiliy hrough inerconneced fracures; and 4) a seal or cap rock ha confines he hermal fluids. Some of he informaion for Exploraion, Zeroing in and Tesing he Prospec is from Exploraion Sraegy for High-Temperaure Hydrohermal Sysems in Basin and Range Province, Ward and ohers, AAPG Bull. January, The paper was prepared a he Universiy of Uah Research Insiue. Dr. Moore was one of he reviewers and sill considers his paper o be one of he bes o define geohermal exploraion. Surface indicaors of geohermal aciviy include fumaroles, ho springs, and geysers. Some poenial geohermal prospecs are found by acciden. A Soda Springs, Idaho, ho waer was found while drilling a waer well o supply a municipal swimming pool. Tom Smih Jeff Hulen and David Langon examine core from geohermal drilling. Locaed in he hear of Sal Lake Ciy, Uah, he EGI Geohermal Sample Laboraory conains over 520,000 kg of geohermal drill cores as well as drill cuings. These represen several hundred kilomeers of deep geohermal drilling compleed around he globe a a cos in excess of 2 billion U.S. dollars. Research on hese invaluable subsurface samples has enabled imporan scienific breakhroughs in he undersanding of volcanoes and high-emperaure magmaicgeohermal sysems. Exploraion Geohermal exploraion has no appreciably changed since we sared exploiing his resource, says Dr. Joseph Moore a geohermal researcher a he Energy and Geoscience Insiue (EGI) a he Universiy of Uah. We rely on surface indicaors of rising hea such as seam vens, ho springs, and volcanic aciviy much like he oil indusry used oil seeps o locae prospecs around he urn of he cenury. In fac, mos poenial areas have been invenoried for he locaions and descripions of surface geohermal aciviy as well as any wells ha have encounered hea anomalies. Many of he curren sysems in operaion have such surface expressions of geohermal aciviy. Evidence of hermal aciviy a he surface hus remains he firs line of exploraion, Moore says. However, many areas have lile or no surface expression. The geoscienis is he mos imporan ool in exploring for geohermal prospecs, according o Jeff Hulen. Once poenial areas of higher hea flow are idenified, he geologis obains and analyzes saellie imagery and available geologic maps, hen complees addiional, much more deailed mapping o delineae faul and fracure rends ha could be key conrols for underlying, concealed geohermal sysems. A variey of geophysical and geochemical echniques hen can be applied for more confiden characerizaion of he resource in advance of acual exploraion drilling. Curiously, in he Sae of Nevada, geohermal prospecing has much in com- 40 GEO ExPro Ocober 2007

5 The map shows he rif ha ransecs Iceland wih he Norh American Plae o he lef and he European Plae o he righ. In Iceland, geohermal energy is coming from boh low emperaure areas, ouside he rif zone, and high emperaure areas, inside he rif zone (colored red). The high emperaure zone is also associaed wih volcanism. Norh American Plae Krafla Halfdan Carsens Reykjanes Peninsula Eurasian Plae The geohermal power plan in Krafla, Iceland, is using ho seam and has been operaing since Alogeher 33 wells have been drilled in he area o ge sufficien energy. Aemps are now being made o muliply he oupu by drilling ino superheaed waer wih emperaures in he range of C. GEO ExPro Ocober

6 Tom Smih E n e r g y R e s o u r c e s mon wih he search for gold deposis, which in essence are nohing more han fossilized geohermal sysems. Many of he echniques in oil and gas exploraion are also used o locae geohermal sysems. Of paricular imporance are chemical analyses of ho springs and fumaroles, which help o esimae subsurface reservoir emperaures, and elecrical resisiviy surfaces, which can be used o locae areas of subsurface aleraion and ho fluids. Graviy, magneic, and seismic surveys can help delineae fauls. Geologiss Jeff Hulen, formerly wih EGI, now a consulan, and Su Johnson of Orma, a major developer of geohermally generaed elecriciy. Boh have many years of experience in geohermal exploraion and research. There are many prospecs ye o be found, claim boh Jeff and Su, many wihou disinc surface indicaors. A lo of deailed geologic work goes ino finding hese blind prospecs. Zeroing In Once high prioriy prospecs have been idenified, shallow, generally less han 1000 m, hermal-gradien wells are drilled. Temperaure measuremens are he primary produc of he drilling. Oher sudies of he drill holes include hermal conduciviy measuremens on rock samples ha permi gradien measuremens o be convered o hea flow. Lihologic and aleraion logging of cuings yield informaion on rock ypes, fracuring, and hydrohermal aleraion ha can be ied o he resuls of surface mapping. This echnique is used o gain an improved undersanding of he argeed geohermal resource a deph. A concepual model is now made o inegrae he prospec-specific geologic mapping, geochemical, aleraion, and hermal-gradien informaion wih he regional daabase. The geoscienis can esimae he laeral exen, deph, heasource ypes, and emperaure of he prospec. Deailed geologic and hea flow mapping, resisiviy, self-poenial surveys, and he drilling of slim holes (commonly cored borehole drilled o 300 o 1000 m) can be done o complee he concepual model a deph. Tesing he Prospec All exploraion work oulined above sill does no guaranee success, bu mus be done o minimize he risk of drilling poorly placed producion wells. Drilling he producion es well is by far he mos expensive and riskies porion of finding and developing a geohermal field. As drilling progresses, he concepual model of he resource is refined and he risk decreases. To be successful for he purposes of generaing elecriciy, producion wells mus yield large quaniies, abou 2,000 gallons (7.6 m³) per minue (69,000 b/11,000 m³ per day), of ho waer, says Su Johnson of Orma. Once subsanial flows are esablished by es wells, injecion wells need o be drilled o coninually recharge he convecive sysem. Mos of he fields have a raio of 2 producers for every injecion well. All produced waer is reinjeced o mainain reservoir pressure. Producion and injecion mus be carefully managed so ha emperaures and pressures are mainained. Mud logger, Neil Peerson, of Horizon Well Logging, looks for evidence of hydrohermal aleraion in cuings and moniors he well for loss of circulaion, an indicaor ha he well will be a good producer. Dr. Joseph Moore shows a ypical core and fracure ha drillers mus hi o ge he flows necessary for a successful geohermal well. Dr Moore is recognized worldwide for his research on he hydrohermal aleraion, geology, and evoluion of geohermal sysems. The resuls of his invesigaions have been published in numerous inernaional scienific journals and conference proceedings. He currenly serves on he Board of Direcors of he Geohermal Resources Council and as Associae Edior for he Americas of he journal Geohermics. Enhanced Geohermal Sysems Naurally occurring geohermal sysems are limied, no so much by he hea source, bu by he amoun of fluid and he number and size of he fracures available for he fluid o circulae and absorb he hea. According o Dr. Moore here is sill a good invenory of naurally occurring geohermal sysems ye o be pu ino producion, bu here are far more poenial high-emperaure sies ha lack adequae permeabiliy hrough a conneced fracure nework. The fuure of geohermal elecri- 42 GEO ExPro Ocober 2007

7 GE v ni EgO enr S ec gi y E NR ec se oe ux rp cl ea si N E D cal power generaion may lie in he abiliy o enhance, or even creae, subsurface fracure neworks. Research ino he formaion of enhanced geohermal is jus beginning. Projecs in he U.S., Europe, and Ausralia have demonsraed ha i is possible o creae permeable fracure neworks around injecion wells hrough hydrofracuring and hen drilling ino hese fracure sysems o produce ho waer. A 2006 MIT sudy, The Fuure of Geohermal Energy claims ha once his echnology is developed, enhanced geohermal sysems could produce 10 o 20% of he Unied Sae s elecriciy. Gahering lines o and from he power plan, boh o supply he plan wih seam and ho waer, and reurn waer back o he injecion wells ha keep he sysem charged. Direc Use Direc use applicaions for low- o moderae-emperaure (35 o 150 C) waer has grown considerably since early man s use of ho springs for basic needs. A Boise, Idaho, ho waer has been used o hea buildings since 1892 and wo of he original wells are sill operaing. The sysem is he larges in he U.S. and now serves over 360 buildings in downown Boise wih a cos savings over naural gas of abou 30%. Alhough he cos of drilling wells, insalling a hea exchanger, and piping is iniially more expensive han radiional furnaces, he difference in cos can be recovered in 4 o 6 years from he energy savings. Like all geohermal sysems, use mus be carefully managed. In he 1980 s, producion of he Boise sysem was doubled resuling in a 25% decline in waer levels. A mahemaical model of he aquifer showed ha waer reinjecion would be beneficial and a 1,000 m deep injecion well was drilled o recharge he sysem. As a resul, waer levels are rising and emperaures are now being susained. The lis of direc users and uses of geohermal hea is growing dramaically - and for good reasons. For example, he ciy of Reykjavik, Iceland has he world s larges disric heaing sysem. Once very pollued, Reykjavik has become one of he cleanes Resource Base Any discussion of he geohermal resource base can be overwhelming by he sheer magniude of is poenial. In fac, he resource numbers are so large hey are difficul o comprehend. For insance, jus in he Unied Saes, magmaic geohermal sysems o a deph of 10 km in he crus have nearly 200 imes he energy equivalen of he known U.S. oil reserves and crusal hea down o 10 km has 2,500 imes ha amoun of energy. Worldwide figures are equally impressive, bu wha represens realiy? The Idaho Sae Capiol building, one of over 360 buildings heaed by geohermal waer in Boise, Idaho, is he only sae capiol building so heaed in he U.S. Tom Smih 44 GEO ExPro Ocober 2007

8 E n e r g y R e s o u r c e s ciies in he world by using geohermal energy as he main source of hea. The poenial o direcly use geohermal energy does no end in Boise or Reykjavik. Many oher ciies in he U.S., and around he world, can reap similar benefis. The U.S. Deparmen of Energy has invenoried 1,277 geohermal sies wihin 5 miles of 373 ciies in jus 8 wesern saes. Japan is he world s larges user of direc geohermal hea. Any counry lying near plae boundaries, like he Philippines, Mexico, Ialy, Greece, are all using and expanding he heir use of geohermal hea. Geohermal Hea Pumps For areas wihou higher hea flows, lowemperaure ground hea can be uilized wih efficien ground-source geohermal hea pumps (GHP). Their use can reduce energy consumpion by 30 o 60%. GHP's circulae waer or oher liquids hrough pipes buried in a coninuous loop. Working similar o a refrigeraor (a one-way hea pump), he GHP can exrac hea from he ground in he winer for delivery o he building or home and remove hea in he summer. Elecric power is required o move he hea bu no produce i, delivering 3 o 4 imes he energy i consumes. An ineresing example of he wide use of GHP echnology will be implemened a he 2008 Bejing Olympics. There will be 160 geohermal wells compleed in he ciy by The consrucion of a geohermal heaing projec in Olympic Garden will provide space heaing for mos of he sadiums and gymnasiums here. Is Time Has Come Could he wesern Unied Saes become he Saudi Arabia of geohermal energy? The poenial is here, Dr. Moore says. I am very encouraged of wha has happened over he pas few years in he progress companies and local governmens have made oward he developmen of his clean and valuable resource. I has been very hard for geohermal, or any oher viable alernaive resource, o compee wih convenional coal and gas in generaing elecriciy. Now, local and sae governmens are seeing he need for clean energy alernaives o help mee growing energy demands, lessen our dependence on foreign oil, and address environmenal concerns. Geohermal developmen is progressing even faser overseas. In many counries, privae companies are working closely wih naional uiliies o expand producion and develop new convenional hydrohermal and enhanced geohermal sysem resources. We can expec a significan increase in he generaion of elecriciy from geohermal sysems worldwide over he nex few years, Dr. Joseph Moore, research scienis If only a small fracion of his poenial is capured, geohermal energy could help mee he world s growing needs. a he Energy and Geoscience Insiue (EGI) a he Universiy of Uah, concludes. Mos expers agree ha he geohermal resource base is huge and we have barely scrached he surface in he uilizaion of his energy source. If only a small fracion of his poenial is capured, geohermal energy could help mee he world s growing needs. How much will depend on economics and developing echnologies o exploi his resource. Now, wih very rare excepions, only high-emperaure (>150ºC) geohermal sysems have been harnessed for he producion of elecrical energy. Geohermal companies have a large invenory of hese higher emperaure geohermal sysems awaiing developmen, in some cases pending only enacmen of appropriae ax incenives. Enhanced sysems will bring even more of his poenial ino he energy equaion, Says Dr. Moore. Geohermal energy is considered by mos a renewable resource. Magmaicallyheaed sysems are driven by ho igneous inrusions, some sill parially molen, ha gradually cool over hundreds of housands of years. Ialy s Larderello field has been in producion since 1904 and no a single geohermal field has been exhaused o dae. Carefully managed geohermal sysems can be susained indefiniely o provide baseload power, says Dr. Moore. Generaing Elecriciy The fluid in mos geohermal reservoirs is liquid waer. As he waer is brough o he surface from dephs of up o 4 km, and he pressure is reduced, he waer flashes or explosively boils. The seam is piped o urbines and elecriciy is produced. The remaining unflashed liquid is reinjeced. A some plans, his waer may be flashed again a lower pressure, or used o hea anoher fluid ha vaporizes a a lower emperaure han waer in a binary plan and generae addiional elecriciy. Binary power plans can generae elecriciy over a broader range of emperaures han flash plans. Flash and binary-cycle plans can be combined for efficien use of all he geohermal waer produced. Using binary echnology, wase waer from oil and gas wells in many areas can used o generae elecriciy. The plan a Chena Ho Springs, Alaska, operaes a 74ºC, he lowes emperaure geohermal source currenly used for commercial power producion in he world. Unlike hese liquid-dominaed sysems, wells in a few high emperaure reservoirs like The Geysers, California and Larderello Ialy produce only dry seam. Power plans are modular so ha as a geohermal sysem is developed, more generaing capaciy can be insalled as needed. Sysems can be small requiring only 1 or 2 wells o operae. Large sysems, such as wih he Seamboa Hills 100 MW geohermal complex, will supply Reno, Nevada wih is enire domesic elecriciy needs abou 70,000 homes. Flash-seam power plan. are passed hrough a hea exchanger o hea a secondary fluid ha vaporizes a a lower emperaure han waer. In a closed loop, he secondary fluid vapor spins he urbine-generaing uni, is condensed back and revaporized. 46 GEO ExPro Ocober 2007