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Renewable Energy 39 (2012) 490e495

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Renewable Energy

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Technical note

Development program of hot dry rock geothermal resource in the YangbajingBasin of China

Zijun Fenga, Yangsheng Zhaoa,b,*, Anchao Zhoub, Ning Zhanga

aMining Technology Institute, Taiyuan University of Technology, Taiyuan, Shanxi 030024, ChinabCollege of Mining Technology, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China

a r t i c l e i n f o

Article history:Received 6 October 2010Accepted 6 September 2011Available online 22 September 2011

Keywords:Tibet YangbajingHot dry rockArtificial reservoirIn situ stress fieldDevelopment program

* Corresponding author. Mining Technology InstTechnology, Taiyuan, Shanxi 030024, China. Tel.: þ86

E-mail addresses: fzj3893811@126.com (Z.J. Feng),

0960-1481/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.renene.2011.09.005

a b s t r a c t

Geothermal energy from hot dry rock (HDR), considered an almost inexhaustible source of “green”energy, was first developed and tested in the 1970s, leading to installations in America, Japan, Britain,France and other countries. In the present work, a liquating rock mass at a depth of 5e15 km in the TibetYangbajing region in China was subjected to detailed analysis. The temperature distribution of thegeothermal field in the region was determined by the finite element method. The results estimate thatthe HDR geothermal resource of the Yangbajing region is 5.4� 109 MWa, representing a huge potentialsource of HDR geothermal energy for China. Based on detailed research into the continental dynamics ofthe environment forming the HDR geothermal field of Tibet, along with the tectonic characteristics of thesouthern slope of Tanggula Mountain and the DangxiongeYangbajing Basin, and the magnitude andorientation of the in situ stresses in the region, the design of an arrangement for extracting these HDRgeothermal resources is proposed: taking the fault zone nearest the high-temperature liquating rockregion as the location of an artificial reservoir, a vertical injection well could be drilled at a low point onthe downdip side of the fault, and two dipping production wells drilled higher up. In this way, anartificial reservoir 3� 1011 m3 in volume would be created: 360 times the volume of the HDR geothermalreservoir in Cornwall, UK, which uses hydrofracturing. An investigation of the reservoir features,including seepage analysis of the heat exchange area, project implementation and investment analysis,indicates that a 104 MW capacity power station with a projected operating life of approximately 100years could be constructed. An analysis of a geothermal extraction system comprising one injection welland two production wells suggest that a power station of 1000 MW installed capacity could be con-structed initially to provide electricity production of 8.64� 109 kWh per year.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

High-temperature geothermal resources in rock at tempera-tures above 200 �C is termed “hot dry rock” by the National Labo-ratory in Los Alamos, USA. Although the total heat energy withinthe Earth is endless from a human perspective, only a small fractioncan be exploited due to the high extraction cost. Currently, explo-ration for HDR geothermal resources is mainly aimed at extractingnaturally occurring superheated steam fromhigh-temperature rockfor generating electricity, due to the stable temperature and longlifetime of hot dry rock.

A detailed evaluation of HDR geothermal resources worldwidehas been carried out by HDR geothermal experts in developed

itute, Taiyuan University of351 6014865.y-s-zhao@263.net (Y.S. Zhao).

All rights reserved.

countries on the basis of geothermal gradient [1]. In the USA, HDRgeothermal reserves within the most easily accessible zone lessthan 10 km deep (geothermal gradient> 45 �C/km) are about12.77 M quads (1 quad¼ 1015 BTU or 1.055�1018 J), which is morethan the total energy available from all fossil fuels worldwide [2]; inJapan, reserves are estimated to be 4e40 M quads [3]. The totalcrustal HDR geothermal reserves to a depth of 10 kmworldwide are40e400 M quads, equal to 100e1000 times the total available fossilfuel energy [4]. Clearly, HDR geothermal resources potentiallyrepresent an enormous untapped, safe, long-term supply of “green”energy. It should be vigorously exploited worldwide in view of theexhaustion of fossil fuel energy, and in terms of the diversity andbalanced development of available energy.

There are abundant HDR geothermal resources in China.Abnormally high-temperature geothermal zones of HDR arepresent in the TsinghaieTibet Plateau by the subduction of theIndian Ocean Plate beneath continental Europe in the south-west of

Fig. 1. The crustal structure and tectonic model of the DangxiongeYangbajing Basin[7]. PW: production well; IW: injection well.

Fig. 2. Yangbajing geothermal field and hot water cycle [9]. 1dmake-up water fromatmosphere; 2dthermal upflowwater; 3dtemperature isoline; 4dQuaternary systemporous-type heat reservoir; 5dbedrock porous-type superficial heat reservoir;6ddeep stratoid bedrock fracture-type high-temperature heat reservoir; 7dboilingspring; 8dvapour ground; 9dgeological boundary; 10dslide-disunion fault plane;11dnormal fault; 12dblind fault.

Z. Feng et al. / Renewable Energy 39 (2012) 490e495 491

China, one example being the anomalous geothermal zones in theTibeteYangbajing Basin and at Tengchong in Yunnan Province. Inthe south-east of China, high geothermal gradient zones areconcentrated in Taiwan, Hainan and the south-eastern coastal areaas a result of the tectonism of the Philippine Plate. In addition, therealso exist dormant volcanoes or volcanic eruption zones such asChangbai Mountain, Wudalianchi, etc., and zones of highgeothermal gradient in regions such as Tianjin, Peking and Shan-dong Province and so on [5]; such zones are all potential HDRgeothermal resources in China.

To date, HDR geothermal resources have not been extensivelyexploited anywhere in the world, so far representing only a smallproportion of the available energy structure. This is a question ofthe level of development of extraction technology, particularlyregarding the difficulty of drilling a horizontal well in high-temperature rock on a scale as large as the proposed artificialreservoir, and also the high extraction costs where a lowgeothermal gradient is involved. As a result, the technologyrequired to exploit HDR geothermal resources in China must beinvestigated, based on the geological structure characteristics ofknown HDR geothermal resources and taking into account theinteraction of temperature, stress and seepage conditions.

The natural hot water geothermal resources in the Yangbajingarea have shown obvious depletion following 20 years’ use, and thetemperature, pressure and flow rate of the production well have alldeclined accordingly. At present it provides a maximum of only16 MW at the Yangbajing geothermal power station, one of thebest-known geothermal power plants in China.

Geophysical exploration has located a liquation magmachamber with an outer temperature of about 500 �C at a depth of5e15 km in the Yangbajing geothermal zone [6]; the geothermalgradient is about 45 �C/km. The reservoir occurs in granite, makingit a high-grade HDR geothermal resource. The resource beingcurrently exploited is a superficial hot water layer, which repre-sents only a small proportion of the overall geothermal resource;exploration and extraction of the HDR geothermal resource in thedeep Yangbajing geothermal field is an inevitable choice for even-tually replacing the hot water resource and improving the gener-ating capacity of the Yangbajing plant.

This paper presents a new program for exploiting this resource,based on an analysis of large-scale rock mass structural character-istics of the region together with the in situ rock stresses,temperature and seepage properties.

2. Temperature distribution and geothermal resourceevaluation in the Yangbajing HDR geothermal zone

The DangxiongeYangbajing Basin located to the south-east ofNyainqêntanglha is a long, narrow downfaulted basin with north-easterly spreading orientation and extensional fault tectoniccontrolling the strike.

Artificial seismic data shows a low-velocity layer, which may bemagma, at a depth of 22 km. Magneto-electrotelluric exploration(METE) indicates the presence of a low-resistance layer of 5 Um,which is probably a cooling high-temperature melt mass (Fig. 1) atabout 5 km depth in the north of the Yangbajing geothermal field[7]. Deep seismic reflection data indicates a local liquating rockmass at a depth of 13e22 km in supracrustal rocks located in thenorth of the Yangbajing geothermal field [8]. These results allconfirm the presence of a heat-producing body, probably high-temperature liquating magma at depth in the Yangbajinggeothermal field.

Isotope analysis of hydrogen and oxygen from the thermalgroundwater shows that the general replenishment level of the hotwater, which consists of modern atmospheric precipitation and

surface water infiltration, is 4860 m e equal to the elevation of thelocal snowline and the original distribution of the surface watersystem. Large amounts of thawy water from the NyainqêntanglhaRange, along with atmospheric precipitation, permeate under-ground along the fault belts, and deep water-bearing strata arecontinually replenished as heat is transferred from the rock to thewater in the cycle. The lower-density hot water produces naturalupwelling along the faults, forming a high-temperature thermalstorage contained within the relatively closed fissure system. If thisflow is hindered, the cycle pressure spreads towards the south-east,so that the main flow moves from north-west to south-east,forming the shallow thermal storage layer shown in Fig. 2 [9].

Taking account of the vertical distribution morphology and thecharacteristics of the deep-seated liquating mass in the Yangbajinggeothermal field, we used the finite element method to analyze thezonal temperature distribution [10] as shown in Fig. 3, based on thetemperature of the deep-seated liquating rock mass as more than500 �C. The zone extends for about 180 km horizontally and about20 km vertically. The geothermal gradient is highe about 45 �C/kmin the vertical direction.

Fig. 3. Finite element analysis results for vertical cross-section temperature distribu-tion in the DangxiongeYangbajing Basin. Fig. 4. In situ stress directions in the Yangbajing region [12].

Z. Feng et al. / Renewable Energy 39 (2012) 490e495492

According a study by Wu et al. [11], the north-easterly extent ofthe deep-seated liquating mass is more than about 150 km. Bydividing the section area into a large number of small elements atthe depth of 7e18 km, the cross-sectional area is calculated to be1200 km2 and the total volume is therefore 180,000 km3, and itsaverage temperature is 500 �C. Taking the lowest extractabletemperature as 150 �C, the total geothermal reserve is5.4�109 MW per year. Assuming an electrical efficiency of 0.17, theenergy output is then 0.92�109 MW per year, which would supplya power station of 50 GW installed capacity for 1.8� 104 years. It isobviously a huge potential “green” energy source that couldaddress our urgent need for substitution energy in China.

3. Characteristics of the recent tectonic stress field in theYangbajing geothermal field

Future exploration and use of geothermal resources from thedeep-seated thermal reservoir in the Yangbajing geothermal zonewill be immediately influenced by the crustal stress state; it wastherefore important to study the characteristics of the crustal stressfield. Our project group teams collected the results of crustal stressmeasurements conducted by Zhang et al. [12] using the piezo-magnetic stress-relief method. They set four test points in theYangbajing geothermal zone, two of which, marked as Ybj1 andYbj2, were located on the south bank of Duilongqu and the othertwo (Ybj3, Ybj4) on the other side, to the left of the G109 road. Thelithology of the tested strata comprises medium-to-coarse-grainedgranite, jointed and fissured at ground surface level and completelyintact at depth. The results are listed in Table 1 and illustrated inFig. 4. The stress measurements indicate that the maximum hori-zontal principal stress in Yangbajing lies between 3.3 and 10.4 MPain a NEeNEE direction, and the minimum horizontal principalstress ranges between 2.5 and 8.4 MPa.

Based on focal mechanism calculations (Figs. 5 and 6) [13], thedirections of the compressive and tensile principal stresses in the

Table 1Crustal stress measurement results for Yangbajing District [12].

Pointnumber

Depth/m Maximumhorizontalprincipalstress/MPa

Minimumhorizontalprincipalstress/MPa

Maximumhorizontalprincipalstress direction

Ybj1 13 10.4 8.4 N70�EYbj2 12 5.7 2.8 N81�EYbj3 12 6.6 4.6 N45�EYbj4 11 3.3 2.5 N45�E

central southern QinghaieTibet Plateau correspond to thosethroughout the entire Tibetan Plateau. Normal fault-type earth-quakes induced by the action of eastewest tensile forces differ fromthe extrusive reverse faults surrounding the plateau, which wereinduced by compressive force. Near the Yangbajing high heat-flowzone in particular, the eastewest tensile stress field predominatesto an inferred depth of more than 100 km. A series of large normalfaults induced by expansion in an eastewest direction caused bythe eastewest tensile tectonic stress in the QinghaieTibet Plateauhas resulted in a high-temperature heat flux flowing upwards fromthe deep asthenosphere to ground surface via the active normalfaults and associated fractured rock, and are the cause of theanomalous geothermal field.

The results both of crustal stress measurements and focalmechanism solution analyses show that the orientation of theminimum horizontal principal stress in the Yangbajing geothermalfield is normal to the strike of the basin, and the maximum hori-zontal principal stress is parallel to the strike. The magnitudes anddirections of the present-day stresses are consistent with theevidence of the fractured structure of the rock mass south-east ofNyainqêntanglha. This indicates that the early and modern tectonicstress fields are unchanged, supplying a sound basis for the designand development of the deep HDR geothermal resource.

4. Extraction project design of the HDR geothermal resourcein the deep-seated Yangbajing Basin

The mode of storage of HDR geothermal energy in theYangbajing Basin shown in Fig. 1 is based on the tectonic “stepped”development of the large rock mass lying between the southernslope of Nyainqêntanglha and the DangxiongeYangbajing Basin,characterised by five very large stepped normal faults with steepdips, between 55� and 70�, to less than 5e6 km depth from themountaintop of Nyainqêntanglha to DangxiongeYangbajing overa horizontal distance of 4e6 km, together with a number of smallfractures with the same dip whose fissure planes extend along thestrike of the DangxiongeYangbajing Basin in a north-easterlydirection. The results both of stress measurements and focalmechanism solution analyses show that the minimum horizontalprincipal stress is normal to these fault and fissure planes.

At depths exceeding 6 km, the faults show a gradual transitionto become a large number of shear zones or slide bands withmoderate dips of 15�e20�, striking parallel to the outline of themagma chamber. The distance between the fault plane and groundsurface gradually increases from Nyainqêntanglha to the

Fig. 6. Earthquake occurrences in the area e focal mechanism solution (verticalsection) [13]. (a) Wu er nets planar projection. (b) Principal stress directions. (Solidline¼ compressive stress, dashed line¼ tensile stress).

Fig. 5. Yangbajing region focal mechanism solution (plane map) [13]. (a) Wu er netsplanar projection. (b) Principal stress directions. (Solid line¼ compressive stress,dashed line¼ tensile stress).

Z. Feng et al. / Renewable Energy 39 (2012) 490e495 493

DangxiongeYangbajing Basin and then to Pangduo Mountain (seeFig. 7): the fault located in the centre of the Yangbajing Basin isnamed F5, and the others are F4, F3, F2 and F1 respectively, goingfrom the Yangbajing Basin to the southern side of Nyainqêntanglha.The buried depth of faults F1eF5 increases and the vertical over-burden stress also increases in sequence. The coefficient of seepagein the fissures was calculated from the following [14]:

kf ¼ kf0exp�� b

�sn � bp

kn

��; (1)

where kf0 and kf are the coefficients of seepage in the fissurewithout and with the applied stress; b is an influence coefficient ofnormal stress on the fissure; b is a coefficient related to theconnectivity of the fissures; kn is the normal stiffness of thefissures; sn is the normal stress applied to the fissures; and p iswater pressure in fissures.

The stress normal to the fissures sn, is roughly equal to theoverburden stress. The stress decreases, and the coefficient of

seepage increases, as the fluid flows upwards along the dip of thefaults, as inferred from Fig. 8.

4.1. Design of an extraction project for the HDR geothermal resourcein the Yangbajing Basin

Based on the analysis of the geological structure in theYangbajiing Basin, the extraction program proposes a 9000 m-deepvertical water-injection well to be drilled into fault F1 centrallybetween faults F4 and F5 at a distance of 27e28 km fromwhere F1meets the surface, as shown in Fig. 7.

Concrete lining would be used from ground surface to 8500 m.The deepest 500 m (from 8500 to 9000 m) would be unlined or, ifnecessary, would have shock tubing installed to protect the welland keep it open. Two inclined production wells would then bedrilled to the north of the water-injection well (see Fig. 7), inter-secting faults F1eF5, and creating a closed-circuit system

Fig. 7. Depths of faults, and arrangement of injection and production wells.Fig. 9. Equi-flow line distribution for water infusion for 60 days; q in m3/m2/d.

Z. Feng et al. / Renewable Energy 39 (2012) 490e495494

consisting of a water-injection well, two production wells and thefive faults. As shown in Figs. 1 and 7, at the points where the wellswould intersect the fault shear zones the temperature rangesbetween 350 �C and 450 �C, the liquating magma pocket being only500e1000 m distant.

4.2. Characteristics of the fluid field during extraction of HDRgeothermal energy

When water is injected into hot dry rock via the injection well,the average pressure acting on the rock in the lower 500 m of thewell would be approximately 130 MPa, made up of the overburdenrock stress in the 8500e9000 m region (210e225 MPa) counter-acted by the hydrostatic pressure due to the head of water(90 MPa). Thus, thewater pressure before injection is only 130 MPa.The resistance to seepage increases with increased overburden rockstress acting on the fault zones at greater depth. The main flowdirection of the heated water would be upwards along the faults,whereas the injected water would flow from the bottom of theinjection well along the strike of the fault in a horizontal direction.

The fluid field is shown in Fig. 9 for awater-injection time of twomonths [15]. The flow gradient is greater from the bottom of theinjection well into deeper rock along the dip of the fault than fromthere to the surface, implying that the fluid flow would tend to be

Fig. 8. Permeability curve for faults vs depth below surface from the injection well toproduction wells.

upslope towards the production wells rather than downwards intodeeper rock.

4.3. Construction of an artificial reservoir and evaluation of an HDRgeothermal resource

As Fig. 1 shows, the vertical distance of the geothermal energyextraction scheme between the injection well and production wellis 4 km, corresponding to an inclined distance of 25 km anda horizontal spreading range of about 3 km. On this basis, the totalvolume of fractured rock which could be considered to be thereservoir of HDR geothermal resources amounts to 3�1011 m3. Forcomparison, such a reservoir would be 360 times larger than the8.25�108 m3 hydrofractured reservoir in Cornwall (UK) [16], and973 times the volume of that in Soultz (France). The artificialreservoir of geothermal energy formed by a naturally occurringtectonic shear zone has distinct advantages over that by artificialhydrofracturing, and it is the basis of the extraction designproposed here.

Moreover, since the minimum horizontal principal stress isapproximately normal to the fault plane, the expansion of fissures isalso in that direction. Therefore, both existing and the newlyformed fissures would intersect the proposed production wells,which would be favourable to the discharge of high-temperaturesuperheated steam at ground surface.

The resources contained in this system can be calculated from:

Q ¼ r$V$c$T ; (2)

where Q is the extractable resource, kWa; r is the density of granite(2700 kg/m3); V is the volume of artificial reservoir, m3; c is thespecific heat capacity of granite (1000 J/kg/�C); and T is thetemperature of extracted thermal energy, �C.

Using these figures, the calculated total energy is6.42�109 kWa. Assuming a thermal use ratio of 17% [17], elec-tricity productionwould reach 1.09�109 kWa, and a power stationwith an installed capacity of 10,000 MW could operate for 109years. If the transport of HDR geothermal energy from the liquatingmagma chamber during extraction is considered, the geothermalresources extracted from this zone would increase to at least threetimes that amount. However, at the beginning of construction, it isrecommended that a power station of 1000 MW installed capacitybe built to keep initial investment and construction time low, andassess the initial viability of the extracted geothermal energy.

Z. Feng et al. / Renewable Energy 39 (2012) 490e495 495

4.4. Investment analysis

The traditional technology for exploiting a deep HDRgeothermal resource includes drilling a horizontal borehole, andthis is the highest drilling cost. In the present proposal, however,the natural faults and tectonic features would replace the hori-zontal well and artificial reservoir, resolving the problems associ-ated with horizontal drilling at depth as well as minimizing cost.The project could be completed using available drilling equipmentand technology.

The cost of this project consists of three parts, as follows:

1) drilling cost

The drilling costs are for a vertical well and two inclined wells,calculated as follows:

C ¼ ðHv þ Hc1 þ Hc2Þ$P; (3)

where C is the total cost of drilling, yuan; Hv is the length of verticalwell, m; Hc1 and Hc2 are the lengths of the inclined wells, m; P is theprice per metre of drilling, yuan/m. For Hv¼ 10,000,Hc1¼Hc2¼12,500 and P¼ 8000, the total estimated cost from eq.(3) is 0.28 billion yuan.

2) cost of constructing power station

The construction cost per kilowatt of power plant capacity isapproximately 616 yuan/kW, so the total cost for a power plant of1000 MW capacity is about 0.616 billion yuan.

3) other costs

Since the project is large, some risk is involved and otheruncertain costs amount to 0.1 billion yuan.

According to the above calculations, the total cost is estimated atabout 1 billion yuan to construct a 1000 MW-capacity HDRgeothermal power station with generated output of 8.64 billionkWh per year.

5. Conclusions

To design the development of the HDR geothermal resource inthe Yangbajing Basin, the deep-seated characteristics of a liquatingmagma chamber and tectonic faulting between the south ofNyainqêntanglha and the Yangbajing Basin are discussed. Based onthe orientation and magnitude of in situ ground stresses, anextraction program of HDR geothermal energy is proposed. Theconclusions are as follows:

1) Avery large deep-seated liquatingmagma chamber lies betweenNyainqêntanglha and the Yangbajing Basin. A finite elementanalysis of the temperature field distribution of the HDRgeothermal field in this region shows that the ground tempera-ture gradient is 45/km in the rock above the liquating magmachamber, and the estimated HDR geothermal resource in theYangbajing region totals 5.4�109 MWa. This has enormouspotential as a future HDR geothermal energy resource for China.

2) The results both of crustal stress measurements and focalmechanism analyses show that the minimum horizontal

principal stress in the Yangbajing geothermal field is normal tothe strike of the basin and approximately to the fault plane, andthe maximum horizontal principal stress is parallel to thestrike.

3) Based on detailed research into the continental dynamicenvironment of the HDR geothermal field of Tibet, the tectoniccharacteristics of the southern slope of Tanggula Mountain andthe DangxiongeYangbajing Basin, and in situ stress magni-tudes and orientations, a program for extracting HDRgeothermal resources is proposed. The proposed schemeconsiders the fault sliding zone close to the magma chamber asa potential artificial reservoir which could be intersected bya vertical water-injection well and two inclined productionwells to create a geothermal reservoir 3�1011 m3 in volume.This approach would significantly reduce the cost of invest-ment and the difficulty of extracting HDR geothermal energy.

4) An HDR geothermal power station of 10,000 MW capacity,which could operate for at least 100 years, could be built toexploit this huge natural energy resource. Initially, an extrac-tion system comprising the three wells and a 1000 MW powerstation costing approximately 1 billion yuan would produce8.64 billion kWh per year. It is clear that such a project couldprovide significant economic and social benefits.

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