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Under the auspice of:Division of Earth Sciences

Chapter 3.1

Development of Hot Dry Rock TechnologyHelmut Tenzer

Geothermic Research, Municipality WorksD-72574 Bad Urach

Introduction

Hot-Dry-Rock (HDR) for long time hasbeen a synonym for heat, extracted fromdeep hot crystalline rock (> 5000 m). 25years ago. During the first oil crisis this ideawas presented for the first time by atomicphysicists and technicians from the LosAlamos Scientific Laboratory (New Mexico)at energy discussions. The aim of this con-cept was to use the inexhaustible and at anyplace available energy for electricity produc-tion by so-called „Man-Made-Geothermal-Systems (MAGES)“. This concept rarelyagrees with present day systems for the useof geothermal energy and had been termedas “far future technology” even by sustain-ing geothermal pragmatists .

The situation has changed: first HDRprojects as well as deep and very deep scien-tific drillholes of past years have shown, thatcrystalline rock in the deep underground isnot dry. Even in great depth fluids and openflowpaths in the shape of joints and faultswere detected. Therefore the undergroundsystem is not exclusively “Man-Made”, butnatural conditions, especially discontinuity-es, have to be taken into consideration. In1978 the International Energy Agency (IEA)commissioned the Kernforschungsanstalt Jü-lich GmbH to act as operating agent forconducting a feasibility study on the pos-

sible use of geothermal energy from so-cal-led Man - Made - Geothermal -Energy –Systems (MAGES) on the basis of availableexperiences. Objective of the study was afirst systematic analysis of all single com-ponents of such a complex system, the iden-tification of the problems, suggest possiblesolutions, and to evaluate the economics forusing HDR geothermal energy. The database was increased by experiences of severalHDR- Projects during the last 25 years. Thetechnology was adapted to the conditionsunderground.

Geology and natural conditions becom-ming more important and should be takeninto consideration of the design of a HDRsystem. There will be no single HDR tech-nology but several concepts adapted to theregional conditions.

The development of HDR technology iswell described in Rummel et. al. (1992) andJung et al. (1997) and Baumgärtner & Jung(1998). The following compilation based onthese publications.

The HDR concept

The first Hot Dry Rock concept basedon the assumption that deep cristallinebasement rock formations are nearly dry andimpermeable for fluids due to the pressureof the overburden rock. Therefore, it was

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suggested to induce artifical fractures actingas heat exchange surfaces, through whichfluid is circulated via boreholes penetratingthese fractures. In comparison to hydro-thermal systems the fluid circulation is per-formed in a closed loop where the fluidpressure is maintained as such a level toprevent boiling. Steam for electric energyproduction is only produced in the second-dary loop at low pressure or by using se-condary fluid with low boiling temperature.Originally electric energy production wasthe main objective, which only can beachieved economically at temperatures abo-ve 140 centigrades. Today a combined useof heat and electricity is considered as beingmore attractive.

Model estimates demonstrate that aHDR system has to produce a thermalcapacity of 10 to 100 megawatts over aperiod of at least 20 years to get into therange of economy. Such size of a systemrequires heat exchange surfaces of 3 to 10square kilometers and circulation rates of 50to 100 liters per second. The critical fluidpressure tor subsurface system operation is afunction of the stress field at a depth whichvaries from site to site. For a HDR reservoirof a depth of 5 kilometers the minimumpumping pressure required is about 40megapascals. In addition, economic reasonsset an operation pressure limit. Today ist isestimated that the flow impedance in a HDRsystem (the difference between inlet andoutlet pressure divided by the flow rate atthe outlet) should be in the range of 0.1megapascal-seconds per liter (MPa s/I).Larger impedance values require a much tohigh pumping capacity for fluid circulation.lt is evident, that circulation on a pressurelevel higher than the hydrostatic rock pres-sure are associated with enourmous fluidlosses. Such losses may occur by fluidpenetration into the rock matrix.

During the past different types ofsystems had been proposed and wereexperimentally tested to some extent:

The initial concept that was investigatedand proposed by the Los Alamos ScientificLaboratory (1) conceives the crystalline rockof the deep underground as a homogeneous

impermeable block. The idea is to connecttwo boreholes by artificially created frac-tures. This fracture system is created by hyd-raulic fracturing which leads to approxi-mately planar vertical fractures evoked bystrain processes. The heat is extracted in aclosed loop system: water is pressed from aninjection borehole to a production boreholeand the required pressure keeps the fracturesurfaces open.

The concept of the Camborne School ofMines (2) (Cornwall), which was proposedsome years later emphasizes the fracture net-work in the underground: the existing jointsprevent the extension of huge artificial frac-tures. However, the existing joints are shear-ed and widened during the frac-experimentsthus causing a fracture network with increa-sed permeability. This process is called sti-mulation. The advatage of this idea compa-red to the single- and multiple-fracture con-cept is a more intensive volumetric flowthrough the rock formation resulting in amore regular cooling effect.

The recent concept of the European Re-search Project Soultz (3) uses natural exten-sive fracture- and fault-systems, which showa relatively high permeability. The aim offrac experiments according to this concept isto connect the discontinuities to the bore-holes. The main advantage of this conceptis, that it is not necessary to directly connecttwo or more drillholes by an artificiallycreated or stimulated fracture system, whichhas turned out to be one of the most chal-lenging problems. Each single borehole onlyhas to be hydraulically connect to the exten-sive fault-zone.

Moreover by integrating natural faultzones extensive circulation systems can becreated. Such a system, which normally isperipheral open, cannot be operated with anoverpressure, because this would result in astrong fluid loss. As a prerequisite it istherefore necessary, that the stimulatedfractures are sufficiently permeable at hyd-rostatic pressure conditions. This concepthas been shown to be very successful underthe conditions of the Upper Rhine Graben,where natural fault zones are present.

In each concept the circulation occurs

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within a closed loop at a pressure level,which avoids boiling of the fluid. Steam forgenerating electrical power arises first in the

secondary cycle. This operating systemsguarantees that no toxic material, fluids orgas are released.

Fig 1: Conceptual Models of HDR systems.

Due to good results of experinces duringthe last years the generation of multiplefracture systems was suggested. In such amodel several parallel vertical fractures areconnected by two deviated boreholes. Toavoid thermal interaction between adjectfractures, the distance between the fracturesshould be in the order of 200 meters. Thevertical distance between the fracturesboreholes may be in the order of 300 meters.The economic operation of such an HDRsystem requires about 10 single fractures tobe induced during separate hydraulicfracturing operations.

Results in the European HDR projectenable to run the system with submersiblepumps using the natural fracture networkafter massive hydraulic stimulation. Thisavoids/ reduces the water losses by high

injection pressure during circulation.

Generation of fracs by hydraulicstimulation

The aim of a HDR system is to createlarge heat exchanger fracture systems.Presently, the only method by which suchfractures can be generated is the method ofhydraulic - fracturing. This was first used inthe oil industry. Fracturing is achieved byinjection of fluid into the borehole by highcapacity pumps. When the fluid pressurereaches a critical value an axial tensionfracture will originate at the borehole walland will propagate into the rock with anorientation perpendicular with respect to theminor principal stress direction. Resultsshow that the induced fractures reach alength of several hundred meters and a

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vertical orientation.In comparison to sedimentary rock

formation of oil and gas reservoirs,crystalline rocks of HDR systems arecharacterized by extensive natural joints andfractures which certainly may play adominant role in the fluid circulationsystem. Results of experiences fromhydrofrac operations show that the case ofcrystalline rocks water can be used asstimulation fluid to enhance rock masspermeability. The use of propants is notnecessery.

Image tools for fracture detection

The development of Borehole Loggingtools enables to measure the orientation ofsingle fractures on the borehole wall and todetermine the fracture network in the rockmass.

Most common high resolution tools arethe Acoustic Borehole Televiewer (BHTV)and the Formation Imager (FMI). TheBHTV produces an acoustic image of thehole circumferential borehole wall. Theborhole wall is a good reflector. Fracturesintersecting the borehole absorb acousitcenergy and show a reduced reflectivity.

The FMI measure the joints withelectrical sensors on 4 pads. While the intactrock mass is an electric insulator, fluid filledjoints show low electric resistance. Inhydrothermal altered zones the FMI toolshows

large dark sections. The BHTV tool ismore advantage in these zones.

The development of high temperaturetools (>200 °C) is needed.

Seismic Monitoring

A comprehensive understanding of thefracture distribution and hydrogeomechanical processes occuring durignoperation provides valuable information forreservoir development and optimization ofproduction. Some of this information can be

obtained from well logs, but they onlyprovide direct information about conditionsnear the well. Microseismic (MS)monitoring techniques can be primarymethods for obtaining detailed informationabout reservoirs and fracture systems as faras 1 km from boreholes (Fig. 2). Theopening of fractues during hydraulicstimulation induce microearthquakes. Theorigin of this events can be located.

The events are frequently distributed asa seismic “cloud” and it is possible to visu-alize a blurred image of the seismicallyactive regionfrom this cloud.

New processing methods like “col-lapsing” enables to improve abolute andrelative locations of microseismic events.Collapsing uses th eexistence of neighboringseismic events to obtein better locations ofall events. The collapsing mehtod is basedon the observation that a cluster of pointsoccupies more volume when each point isperturbed by random errors than it does if noerrors are present.This method wasdeveloped in a interantional collaborationknown as “MTC More than Cloud” –Pro-ject. Using the collapsing method afterstimulation tests and microseismic observa-tions at Soultz it can be shown that onlyseismicity that occupies the volume of rockthat is experienced critically elevated pres-sures during stimulation from each well(intersection) on Figure 2.

Additionally seismic waveforms fromactive seismic sources are used extensivelyfor seismic imaging. method (Reflectionmethod) and multicomponent signal pro-cesssing was improoved.

Improved measurements of the overalldistribution and relative location of inducedseismic events will allow construction andtesting of modells that include locations ofpotentially permeable fractures that adrseismically active. Such models will help usmake quantitative predictions about therelation between seismic events an reservoirperformance.

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Fig 2. Seismic cloud

HDR - historyThe table show the technical milesones ofthe development of HDR systems

1970 Proposal of the Los Alamos Scien-tific Laboraty in New Mexico 1USA to use heat from the hot dryrocks of the crystalline crust.

1973 First HDR experiments at FentonHill on the east - side of the Valles -Caldera near Los Alamos.

until 1979 Phase 1 of the LASL - HDRproject with circulation experimentsat 3 km depth at temperatures of

about 195 °C.since 1980 Phase II of the LASL - HDR

project at a depth of 4.5 km at

temperatures of 330 °C withcirculation experiments at about 3.6km depth.

1980-1986 Participation of FRG and Japanin the LASL - HDR project.

1974-1977 HDR feasibility studies in

Japan.1975 Start of preparations for the Urach

research drillholesince 19771977 HDR feasibility studies at shallow

depth (500 m) in Falkenberg,Germany (until 1986), Cornwall,Camborne School of Mines (CSM),UK and Massif Central, France

- Start of the Urach geothermaldrillhole Urach 3 to a depth of 3.334meters (143°C) and hydraulkic test-ing programme.

1978-1979 The MAG ES - study (use ofman – made - geothermal - energy -systems) initiated by the Interna-tional Energy Agency, operated byKFA Jülich and carried out byPreussag A.G.

1977-1981 HDR studies in the formerGerman Democratic Republic atMellin, Altmark.

1980 Deepening of the Cornwall drill-lholes to 2.6 km depth (80°C)

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1980-1986 Deepening of the Urachborehole to 3.488 km (147°C) inPhase III and hydraulic testingprogramme (single borhole system)

1984-1989 Fundamental investigations inthe Le Mayet de Montagne - projectin France at a depth of 800 meters(University ot Paris).

1986 Start of the German - French HDRproject at Soultz - sous - Forêts inNorthern Alsace (F).

1987 Phase 1 of the Soultz project bydrilling a first 2 km deep hole (140°C) and investigation of thecrystalline basement in the Rhine-Graben.

1986-1991 HDR experiments in Japan inHijion (NRIPR) and other locations(Sendai University).

1989 The Camborne School of Mines (theEnglish HDR project group) joinsthe Soultz HDR project. Foundationof an industrial consortium toorganize planing and technicaloperation of an European HDR -project.

1990-1993 Phase II of the Soultz project:drilling a second 2 km deep anddeepening the first drillhole to 3.5km depth to a temperature of 160°C and geothermal reservoiridentification. The second drillholetha was used as a seismic obervationhole

1994-1995 Phase III of the Soultz project:Drilling of the second main hole to adepth of 3876 m (170°C)Production tests, first steamproduction im Middle Europe fromcrystalline rocksMassive stimulation and circulationtests with seismic monitoring,developmernt of the heat exchangerstage I, thermal power of 8 MW wasachived.

1991-1996 Phase III of the UrachProject. Extension of the drillhole toa depth of 4445 m (172°C) and in-tense borhole measurement prog-ramme

1996-1997 Development of the

downhole heat exchanger by mas-sive hydraulic testing, worldwiodethe largest HDR system was created.Hydraulic longterm 4 month circula-tion test, thermal power of 11 MWwas achievd.

1998-2000 Extension of the secondmain hole an Soultz to a depth of5060 m (201°C) Hydraulic stimula-tion and seismic moitoring

2001 Planning and preparation of a firstscientific HDR pilot plant at Soultzsous Forêts Planning and prepara-tion of a first HDR research pilotplant a Urach

Examples of various HDR projects:milestones of HDR development

The first HDR projectLos Alamos Scientific Labratory (LASL),USA

The first proposal to use geothermalenergy from deep hot rocks of the crystallinebasement came from physicists at the LASLin 1970. The active phase started in 1973, atcertain times about 110 scientists and engi-neers were employed. The total financialexpenditure until 1990 is estimated at DM300 million. Germany and Japan contributedto the project financially and personnel withan amount of about DM 60 million.

Fenton Hill is located east of a Calderaabout 40 kilometers west of Los Alamos onthe west side of the Rio Grande Graben inNew Mexico. Due to volcanism the thermalheat flow is about 250 milliwatts persquaremeter or about three times the averageheat flow at the Earth's surface.

During the first phase of the project(1973 - 1979) LASL tested the basic HDRconcept: connecting 2 boreholes by a singlefracture at a depth of 3 kilometers in grano-diorite rock at a bottom hole temperature of195 centigrades. The fracture was generatedin the first drillhole (GT-2) by hydraulicfracturing, and was identified by seismo -acoustic measurements. The subsurface flowpath system consisted of several natural fra-ctures in addition to the induced hydrofrac.

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The distance between inlet and outlet wasabout 90 meters. Circulation experimentswere carried out over a total period of 100days and demonstrated rather low waterlosses and low flow resistance of the system.This first result was beyond previousexpectations and also showed that proppantsare not required to keep fractures open incrystalline rock. This propping effect due torough and uneven fracture surfaces is ofgreat significance for HDR technique.

The mean thermal capacity of this firstsmall circulation system was about 3megawatts. Based on the data from thecirculation experiment the effectivesubsurface heat exchange area wasestimated as only 8.000 squaremeters, anarea much smaller than the induced orstimulated fracture area. During theexperiments a geothermal energy of 5gigawatthours was extracted trom the rockmass, an amount to cover the yearly energyconsumption of several hundred households.

After these first promising tests, the twoboreholes were connected by a large fractureover a distance of 300 meters. A one yearcirculation test phase follows which,however, only yielded a production rate of6.4 liters per second due to much higherflow resistance. A rather poor result,although the water loss again was small. Theeffective heat exchange area was estimatedas 50.000 square meters, confirming theprevious result that the etfective flow path islimited to the direct connection between thetwo boreholes. It is interesting to mentionthat a part of the thermal capacity of 2.3megawatt was used to drive a 50 kilowattturbine generator and to produce electricenergy.

The second phase ot the LASL- HDRproject started in 1980. Objective of thisproject is to test the multifrac HDR concept.The system consists of two 4.500 meter deepboreholes, which are deviated on the last1.000 meters to an angle of 30 degrees withrespect to the vertical. The bottom holetemperature is 327°C. This high temperaturein the vicinity of the Valles Caldera, how-ever, induced various problems for drillingand borehole measurements and hydraulic

tests. A hydraulic connection between thetwo boreholes could not be achieved in thedeeper part. Therefore, it was decided to fillthis part ot the boreholes with sand, and tocarry out hydraulic fracturing experiments ata depth of 3.600 meters. This test was thelargest hydrofrac operation ever conductedin the U.S., but failed with respect toconnect both boreholes hydraulically. Thespatial distribution ot microseismic eventsmonitored during the injection test showsthat a large complex fracture system wasstimulated over an extension of 800 metersand with a thickness of about 150 meters.lnstead of being vertical the system isinclined by about 30 degrees. According tothis information the second borehole wasdirectionally deviated to penetrate thisfracture zone. Thus, in 1986 - seven yearsafter the start of the project phase - thesecond HDR circulation system could becompleted. A one month circulation testindicates that the system may becharacterized by rather favorable hydraulicproperties. The thermal system capacity is inthe range of 10 megawatts

In 2001 the U.S. Department of Energy(DOE) has closed its Fenton Hill Hot-Dry-Rock project. In order to be more responsiveto US industry, DOE now has the EnhancedGeothermal Systems (EGS) program thatwill provide near term benefit to the geo-thermal electric power generation industry,and a long term benefit to the nation’selectric power consumers. Initial focus ofthe EGS program is on transiting from anational lab-based R&D Program (HDR), toa highly industry-responsive program. Shortterm objectives will seek to increase steamproduction from a commercially developedhydrothermal field, while longer term objec-tives will seek to create a man-made reser-voir that can produce steam, and electricalpower, at competitive rates. The knowledgeachieced from the Fenton hill experimentsshould now transfered to the geysirs.

The HDR project at the Camborne Schoolof Mines (CSM), UK

The second large HDR research project

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was carried out by the Camborne School ofMines in Cornwall. First experiments inshallow boreholes drilled to a depth of 300meters started in 1977. Since 1980 experi-ments have been conducted at a depth of2.000 meters, at a rock temperature of 80centigrades. The objective within this pro-ject phase was, first, to solve HDR - relateddrilling, hydraulic and rockmechanical prob-lems, before attacking the problem of rocktemperatures by drilling to great depth du-ring a third project phase scheduled for1992.

The location is the Rosemanowes quarrynear Penryn in Cornwall, in the middle ofthe Carmenellis granite massif which ischaracterized by the highest heat flow inEngland (120 milliwatt per squaremeter).The average geothermal gradient is about 35degrees Celsius per kilometer depth.

The English scientists intended to createa multifracture system by stimulating thenetwork of existing fractures both byblasting and hydraulically. Since naturalfractures are essentially vertical, boreholeswere planned as inclined holes at depth inorder to intersect as many as possible.

In 1980, two 2.000 meter deepboreholes were drilled with the inclinedsections 300 meters apart. During thesubsequent circulation test over a period ofhalf a year only a poor hydraulic connectionbetween the two boreholes could be achi-eved. Therefore, a third borehole was drilledto a depth below the other boreholes. In1984 the borehole drilled by directionaldrilling technology reached the target areawith high precision. Hydraulic connectionwith the existing borehole could only becreated by a massive hydraulic fracturingstimulation test - the largest ever conductedin Europe. Over the following 3 1/2 yearsthe 3 - borehole system was intensivelyinvestigated by hydraulic circulation testsfrom one borehole (injection hole). Afterabout three years circulation in one of theflow path a thermal short circuit occurred,i.e. the circulating water was not heated anymore along this flow path. Although only 20per cent of the total flow passed through thischannel, it resulted in a drastic reduction of

the water temperature at the outlet.Presently, this short circuit is explained byassuming channel flow instead of water flowover a wide heat exchange area. This is aproblem which must be generally consideredin future HDR system planning.

HDR research in France

After first preliminary investigations inthe Massif Central since 1978, from 1984 to1986 more fundamental investigationsrelated to the HDR concept were carried outat a granite location near the village LeMayet de Montagne in the Massif Central.The project was funded by the EuropeanCommunity, the Agence France pour laMaitrisse de l'Energy and the CentreNational de la Recherche Scientifique.Besides several shallow boreholes for seis-mo-acoustic observations, two 800 metersdeep vertical boreholes were drilled. Dif-ferent in comparison with other HDR drill-ling projects where borehole sections ofseveral hundred meter length were stimu-lated, in Le Mayet single existing fractureswere selected and then hydraulically openedand propagated by the use of boreholepackers. During circulation tests over aperiod of several months the hydraulicproperties of the fracture system and theheat exchanger were investigated. An impor-tant technical innovation was the develop-ment of an electrical borehole imaging de-vice which allows to observe the generationof fractures during hydraulic fracturing inthe pressurized borehole interval.

HDR activities in Japan

In Japan the development of HDRsystems is systematically pushed aheadsince 1970. At present, four HDR projectsexist; field experiments are essentiallycarried out in the following three projects:

Hijiori - Project (Yamagata Province):

Since about 1980 the Ministry ofInternational Trade and lndustry (MITI)supports a technical feasibility study of the

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HDR principle in four boreholes of about2000 –2200 meter depth and rocktemperatures of about 250 centigrades atHijiori. A multi well system was developed.Short term hydraulic tests were performed in1991, 1995 and 1996. Two reservoirs at1800 and 2200 m in the hot granitebasement were created. In 1991 a three-month circulation test was conducted in theupper reservoir. End of 2000 a longterm (2year) circulation test started in the lowerreservoir. The main objective is toinvestigate the lifetime of the artificialreservoir. Unexpected high impedance ofinjection at the beginning was observed. Ahigher recovery and a very low pressure ofthe shallow reservoir were determined.

In the second year active controll of theshallow reservoir is planned.

The scientific responsiblity is with theNew Energy and Industrial TechnologyDevelopment Organization (NEDO) andwith the National Research Institute forPollution and Ressources (NRIPR).

Yunomori Experimetal Field (IwateProvince):

Following the more fundamental -oriented Gamma - project at the TohokuUniversity the so-called Hot - Wet - Rock(HWR) project was started with anexperimental field in Yunomori in 1988.Objective of the project is to study the waterrock - interaction at a depth of 1.5kilometers at a rock temperature of 200 °C.The project is funded by the Ministry ofEducation and by several privateinstitutions.

At Yunomori also the in-situexperiments of the TIGER project arecarried out scientifically operated by NEDOand funded by MITI.

Ogachi - Project:

The Ogachi project is supported by theelectric power industry and is operated bythe Central Research Institute of ElectricPower lndustry (CRIEPI). Subsequently tonumerous fundamental research works in the

Akinomiya experimental field between 1986and 1988 (Phase I) circulation tests inshallow depth were performed. Thereservoir in about 1000 m depth was deve-loped with two new bore holes betwenn1990 and 1992. Large scale hydraulicfracturing and circulation test has beenconducted since 1990 in Phase II attemperatures of 228 °C. In Phase III 1993 acirculation test of 22 days was conducted. Inthe following year of 1993 a 5 monthcirculationtest was performedand in 1995 a1 month circulation test followed. Aproduction temperature of 160 °C and afluid recovery of 25 % were achived.Acoustic emission monitoring and tracertests were carried out.

In addition to its own HDR researchactivities Japan also participated in the HDRproject of the Los Alamos ScientificLaboratory in the US. During 1980 to 1986.Also a scientific cooperation with theEuropean projects is in progress.

Summarizing it may be concluded thatin Japan HDR technology and its relatedscientific and technical research is supportedon a large scale financial basis. In additionto govemmental funding the development ofHDR technology is either partially fundedby the electric power industry, or directlycarried ahead like in the Ogachi project. Inthe so-called TIGER project all HDRresearch groups collaborated with industry.Japan also seeks exchange of know-howwith European HDR research groups.

Australian HDR activities

A large potential for HDR geothermalenergy exists in Australia. Investgationsstarted arpound 1995. A comercial companywas formed in 2000 “Geodynamics Limi-ted”. The mission of the company is to deve-lop Australia´s unique potential for theprofitalbe generation of clean and renewablegeothermal energy from kown Hot DryRock resources.

Several regions in the central, southernand eastern part of Australia will be inves-tigated. In significant parts the estimatedsubsurvface temperature at 5 km depth is >

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250°C. This was extrapolated from adatabase of temperatures recorded on 3291wells. The deepest well is 5594 m while theaverage is 1669 m.

Studies started to investigate some po-tential sites. An excellent economic potentialwas estimated -.. operating costs of 1c-2c/kwh and total costs of 6c-8c/kWh. A Studyof a 360 MW (electric ) power plant with 25injection wells an 36 production wells isunder work.

HDR - activities in Germany

Several German researchers wereactively involved in the first HDR activitiesat Los Alamos since 1972. Such associationsautomatically resulted in the formulation ofown concepts and contributions related toHDR research:- the Falkenberg Frac Project had theobjective to investigate hydraulic fracturingin a tectonically rather intact crystalline rockat shallow depth (boreholes to 500 metersdepth);- the Urach Geothermal - Projectintented to explorate the geothermalanomaly Urach by a borehole drilled to3.500 meters depth and to test the possibilityof geothermal energy extraction by oneborehole only;- participation in the HDR - Project of theLos Alamos Scientific Laboratory (1980-1986);- participation in the German - FrenchHDR - Project at Soultz - sous - Forets inthe French Alsace since 1986;- conducting the MAGES - study for theInternational Energy Agency (IEA) during1978 to 1980.

In the following only the projectsconducted at Falkenberg and Urach arepresented in more detail.

Falkenberg in North East Bavaria

Considering the enormous technicalexpenditures for deep drilling at high tem-perature convinced some of the Germanresearch groups to conduct a common HDRresearch project at shallow depth to study

some principal problems of fracture growthin a natural jointed rock mass and fluidtransport in hydraulically induced farcturesin shallow experiments. The Falkenbergproject was conducted during 1977 to 1986and was funded with about 15 million marksby the German Federal Ministry of Researchand Technology and - to some extent - bythe European Community.

The test site was about 2 kilometerswest of the village of Falkenberg in thenorthern part of the Falkenberg granit massifin northeast Bavaria. The test site seemed tobe suitable due to a low density jointnetwork and due to rock homogeneity.Seven boreholes were drilled on 100 by 100meter surface area and gave an excellentopportunity to conduct a subsurface inves-tigation of the joint network by geophysicalborehole explorations prior to a generationof a large artificial fracture. In comparisionto massive hydrofrac operations for oilexploitations and also used in other deepHDR projects, the Falkenberg fracturestimulation work was limited to smallinjection rates and only small fluid injectionvolumes. Inspite of this an artificial fractureplane of an area of 15.000 squaremeters wasinduced which also connected all the sevenboreholes over a distance of about 70meters.

Numerous hydraulic and rockmechani-cal tests were conducted on this fracture. Forthe first time the relation between fracturewidth and fluid pressure within the fracturewas systematically investigated. The measu-rements demonstrated that fractures can bekept open without the use of proppants dueto their natural uneveness. The fracture wasintersected by several boreholes drilled to450 meters depth. Subsequently its hydraulicbehavior was tested. These tests demon-strated that the actual fracture surface had tobe several squarekilometers and its trans-missivity was much larger than required foran HDR system. This result demonstratedthat crystalline basement rock may containlarge natural fracture zones suitable forextraction of geothermal energy.

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Research work at Urach Spa

The first phase of the Hot Dry Rock(HDR) Project in Urach Spa begann in1977/78 with the Urach 3 drilling hole. Adepth of 3334 m within a metamorphicgneiss rock was achieved. An extension ofthe drill hole to 3488 m depth followed in asecond phase in the year 1982/83, where atemperature of 147° C was attained. Asingle hole circulation system was tested.

Basic results concerning the temperaturefield, joint system, stress field and hydraulicbehaviour of the rock were achieved. Due tohigh flow impedance the concept was turnedback to a doublette system. To reach highertemperatures for realizing a HDR pilot plant,futher investigations had to be obtained. TheUrach 3 drill-hole had to be extended from3488 m to higher depths, where the requiredrock temperature of > 170°C was expected.The final depth was reached in a third phase,at 4445 m. The bottom hole temperature attrue vertical depth of 4394,72 m wasdetermined with 170°C. It can be provedthat the temperature gradient is constantwith 2.9 °K/100 m depth. Temperatures ex-pected at 4500 m depth are in the range of175°C. As main lithological units, meta-morphic rocks such as biotite-gneiss, ana-texite and diatexite were determined in theextended drill hole. The different crystallineunits are effected by brittle deformation. Theresulting fracture system is sealed byhydrothermal products (clays, carbonates,sulphates). The aperture of fractures is in therange of some tenth to ten millimeters.Subvertical sinistral strike-slip shears andfaults which correspond to the most intensecataclastic structures, strike N 170° E.Televiewer and FMI logs show a generalNorth-South orientated joint system andborehol e breakouts around N 80-120° E.The orientation of maximum horizontalstress direction was determined with N 170°E. Wireline Hydrofrac stress measurementsat 3352 m depth yields values of S hminbetween 41-50 MPa. Estimated stress mag-nitudes of Anelastic Strain Recovery (ASR)measurements on cores from around 4425 mdepth yields values of S h = 63 MPa. Within

a carried out study (1990-1996), local boun-dary conditions, infrastructure, user potentialand a preliminary utilisation concept (3MWel, 17 MWth) were evaluated. Due tothe results of the investigations, it isproposed that the Urach site located in awidespread tectonic horizontal strike-slipsystem is suitable for a HDR demonstrationproject. The results can be applied in South-ern German and Northern Swiss regions andin other areas of Europe. Many potentialconsumers of geothermal energy producedby the HDR concept are situated closearound the Urach 3 drill site. In 2001 it isplanned to continue the research work and todevelop a research pilot plant.

The European HDR research project atSoultz-sous–Forêts, Alsace (F)

In 1986 some of the German and Frenchresearch groups conducting HDR researchjoined to concentrate on an HDR researchproject in the Upper Rhine Graben.

The project was funded by the EuropeanCommunity, the German Ministry of Re-search and Technology, ( followed by theMinistry of Environmental Affairs) and theFrench Maitrise de I'Energie.

Soultz is located about 50 kilometersnorth of Strasbourg in the centre of the high-est heat flow anomaly of Central Europe.Old oil boreholes indicate a temperaturegradient of 60 centigrades per kilometer ornearly 100 centigrades per kilometer atSoultz for the subsurface. The geologicalconditions are typical for most locations inthe Rhine - Graben (e.g. the Landau area).

During the first project phase (1987-1989) the objective was to test the crys-talline basement for its suitability for HDRenergy extraction. First, a 2 kilometer deepwell penetrating the subsurface granite to adepth of 2000 meters was drilled. Theborehole was used for numerous geophy-sical logging and hydraulic tests. Prelimin-ary results demonstrate, that the subsurfacegranite inspite of the existence of naturalfracture systems is suitable for planned HDRexperiments. The rock temperature is 120degrees Celsius at a depth of 1.000 meters,

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but increases much less at greater depth. In2.000 m depth the temperature is only 141degrees Celsius. This is less than 40 °Cexpected from extrapolations from thesubsurface gradient. This suggests that thegeothermal anomaly originates not by a highheat flow from the crystalline basement, butrather from water circulation in theoverburden sediments. The tempeaturegradientand the heat flow values are high inthe sedimentary cover (10,5°C / 100m, 176mW/M3) and quasi normal in thegranitebasesment ( 2,8°C/100 m; 82 mW/m3).

The hydraulic tests also demonstrate thateast - west oriented stesses are extremelylow due to the graben tectonics of the Rhine-Graben. The favorable north - south orientednatural joint systems therefore offer goodconditions for hydraulic circulationexperiments.

In the phase one by stimulation tests afracture was opened with an area of morethan 10.000 squaremeters. Hydraulic testsdemonstrated that the induced fractureintersected a natural fracture zone wheregreat water losses occurred and the fracturegrowth was stopped. This interconnectionopened the possibility to produce hot waterfrom this fracture zone at significant rates.Therefore today, the possibility forgeothermal heat extraction from such naturafracture systems at graben conditions (like inthe Upper Rhine - Graben) becomespossible.

During 1989-1991 a contiuous coredwell was drilled to 2280 m depth. Tehwellgave useful information on th ejointnetwork and mineralogy, and provided tehbasis for subsequent interpretation ofcuttings and geophysical logs.

Main objective of the second projectphase starting in 1992-1993 was todeepening the first main hole into a reservoirdepth from 2000 m to 3590 m depth with atemperature of about 160 °C.

Large scale hydraulic tests were carriedout to characterise the rockmass.Supportingactivities during the injection tests includedmicroseismic monitoring, productionlogging, fluid sampling.

The existing information available was

then used to target a second Well to a depthof 3890 m aproximately 450 m south of thefirst well.

In 1995 and subsequently 1996 this borehole was stimulated with a maximum flowof 78I/s to improve the injectivity as afunction of flow rate. During the test in 1995a total of 30,000 m_ of fluid was injectedand during the 1996 injection a total of28,000 m_ of fluid was injected betwenn3200 and 3600 m in three flow steps (24,45,and 78 l/s) with a maximum well headpressure of 13 MPa for 78l/s.

Productivity (and injectivity) of bothdrillholes is proportional to the appliedinjection rate during the frac-experiments(Fig. 3). This result is essential especiallybecause it makes the effect of frac-experiments predictable. Despite ofturbulent flow in the fracture system thepost-frac-productivity was abundant to attaina production rate of more than 10 l/s. Thiswas achieved solely by using a low artesianpressure and buoyancy effect without activeproduction. The production rate could beincreased to more than 20 l/s by a downholepump and a simultaneous reinjection.

During a four month circulation test in1997 8 MW thermal power were extractedcontinuously and the general functioning ofa HDR could be evidenced.

The circulation test was carried outbetween the two deep wells at around 25 l/s.The result of the test showed that brine canbe circulated through the enhanced fracturerock mass at great depth with a separation inexcess of around 450 m, with low powerconsumption for circulation, no geochemicalproblems and no water losses. Totally244,000 tons of hot brine were produced

In 1999 the second drill hole wasextended to 5060 m depth with temperaturesof 201 °C. Injectin tests and seismicobservation indicates that a reservoirindependend from the testing zone of 3200–3600 m was reached. The first hydraulictests showed very promising results for thedevelopment of a heat exchanger and ascientific pilot plant at depth around 5 km .

Results of the last years made Soultz tothe most successful HDR-project worldwide

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concerning size of fracture system, flowrate,flow impedance and fluid loss. The goal ofthe next project phase to construct a HDR-pilot-plant with a thermal power of 30 MW,a final depth of 5000 m and a rock tempe-rature of 200 °C therefore seems to befeasible. This project phase will be carriedout with a stronger industrial participationespecially together with electrical energysupplying companies. An according agreem-ent has been made.

Outlook

During the last 25 years of worldwideHDR research the initial idea to develop amethod for tapping geothermal energy fromdeep crystalline rocks, which can be used forpower generation at any localities, hasturned out to be unrealistic. Not only oneHDR method but several different methodswill be realised, which will use and improvethe individual local conditions thus enablingan economic heat production. Scientifictools to achieve this goal were developed

and can be applied in all depth- andtemperature-ranges which can be drilled.

An extensive experience of the effect-tiveness of these methods under varyinglocal conditions is lacking. Therefore inaddition to basic research in HDR projects,which has to be continued in the future,HDR-technology should now be realised.Possibilities and demand exist in the mar-ginal high enthalpy storage field and lowenthalpy systems as well as in the tapping ofhot water from fractured and porous-permeable aquifers that show a deficiancy ofan adequate flowrate.

Literature

Baria, R., Baumgärtner, J., Gerard, A., Garnish,J.: The European HDR Programme: MainTargets and results of the deepening og thewell GPK2 to 5000 m, Proceedings WorldGeothermal Congress 2000,Kyushu-Tohoku,May 28-June 10, International Summerschool on Direct Application (Ed.) Skopje,Macedonia, 2000

Baumgärtner, J., Jung, R.: in Kaltschmitt &

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Huengens (Ed.) ,Enrgiegewinnung ausErdwärme, Deutscher Verlag fürGrundstoffindustrie, 1998

Fehler, M., Jupe, A., Asanuma, H.: More ThanCloud: New techniques for characterisingreservoir struvture using induced seismicity,The Leading End, 2001-08-09

Jung, R., Baumgärtner, J., Kappelmeyer, O.,Rummel, F., Tenzer, H.: HDR-Technologie–geothermische Energiegewinnung derZukunft., Geowissenschaften 15, Heft 8,Ernst & Sohn, 1997

Okabe, T., Kiorihara, K., Hayashi,K., Karasawa,K., Swenson, D. Schroeder, R.: Analysis inpreparation for long Hijiori long termcirculation test, Proceedings World

Geothermal Congress 2000,Kyushu-Tohoku,May 28-June 10, International Summerschool on Direct Application (Ed.) Skopje,Macedonia, 2000

Rummel; F., Kappelmeyer, O., Herde, O:A.:Geothermal Energy, Future Energy Source?Facts-Research-Future, MesyGmbH (Ed.)Bochum, 1992

Swenson, S., Chopra, P., Wyborne,. D.: Initialcalculations of performance for anAustralian Hot Dry Rock reservoir,Proceedings World Geothermal Congress2000,Kyushu-Tohoku, May 28-June 10,International Summer school on DirectApplication (Ed.), Skopje, Macedonia, 2000