Current status (1987) of geothermal exploration in Ethiopia

Geothermics, Vol. 17, No. 2/3, pp. 477-488, 1988. Printed in Great Britain.

0375-6505/88 $3.00 + 0.00 Pergamon Press plc © 1988 CNR.

C U R R E N T STATUS (1987) OF G E O T H E R M A L E X P L O R A T I O N IN E T H I O P I A ABEBAW ENDESHAW Geothermal Project, Ethiopian Institute of Geological Surveys, P.O. Box 2302, Addis Ababa, Ethiopia

Abstract--The Government of Ethiopia realized 18 years ago that the Ethiopian rift valley and the Afar depression were regions of high geothermal potential suitable to be exploited for electric power generation. Systematic geothermal investigations on a regional scale started in 1969 and since then continuous investigations have been conducted and several geothermal prospect areas identified. Eight deep exploration wells have been drilled in the Aluto-Langano volcanic complex and sufficient geothermal steam to fuel an economicallyviable power plant has been discovered. INTRODUCTION Ethiopia has significant geothermal resources in the main Ethiopian rift valley and the A f a r depression; both are part of the great East African rift system. G e o t h e r m a l exploration in Ethiopia dates back to 1969, when the first E t h i o p i a n - U n i t e d Nations D e v e l o p m e n t P r o g r a m m e ( U N D P ) geothermal reconnaissance investigations started (Ethiopia-United Nations D e v e l o p m e n t P r o g r a m m e , 1973). This work, consisting of geological, geochemical and hydrogeological surveys, as well as remote sensing (infrared imagery), covering the whole of the Ethiopian rift and Afar depression, resulted in the selection of potential areas for further detailed studies: Dallol in the Danakil depression, T e n d a h o in the northern A f a r depression, and the Lakes district in the main Ethiopian rift. The Lakes district and T e n d a h o areas were subjected to more detailed studies. Between 1979 and 1980 geothermal exploration was completed in the T e n d a h o area and drilling of three deep exploratory wells is planned to start in 1988. In the A l u t o - N o r t h Langano volcanic complex, eight deep exploration wells were drilled between 1981 and 1985 and a two-phase geothermal reservoir has been discovered. Well testing and feasibility studies have been completed. The project was financed by the G o v e r n m e n t of Socialist Ethiopia, the E u r o p e a n Economic C o m m u n i t y (EEC) and the United Nations Development Programme (UNDP). Towards the end of 1985 geothermal investigations began on eight selected prospect areas in the southern Afar regions, with the financial aid of the World Bank and the G o v e r n m e n t of Ethiopia. During the initial period of geothermal exploration and deep geothermal drilling, the Ethiopian Institute of Geological Surveys (EIGS) relied extensively on international experts. In order to become more self-sufficient the E I G S has made great efforts to train nationals in all fields of geothermal sciences and engineering as well as in deep geothermal drilling. Nowadays, the Institute has a strong geothermal unit capable of carrying out geothermal exploration, including drilling, with minimum consultancy of foreign experts. RECONNAISSANCE SURVEY OF THE ETHIOPIAN RIFT VALLEY Most of the geothermal manifestations are located in the Ethiopian rift system, which has high regional heat flow due to an underlying basic upper mantle intrusion beneath the thinned crust. 477

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Fig. 1. Map of Ethiopia showing geothermal project area and main hydrothermal manifestations.

Geothermal Exploration in Ethiopia

479

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Fig. 2. Location of known geothermal resources in Ethiopia. The objective of the geothermal project was to investigate all the thermal manifestations within an area of 150,000 km 2 of the Ethiopian rift valley and the Afar depression in order to identify geothermal resources (Fig. 1). The project included mapping of the geothermal features, and chemical analysis of gas and water samples (Table 1). Several areas of major interest were then selected, including Dallol in the Danakil depression, Tendaho in the North Afar and the Lakes district (Fig. 2). The Survey recommended further detailed studies in these areas. TENDAHO GEOTHERMAL AREA Tendaho geothermal area is located 600 km north-east of Addis Ababa on the main highway connecting the port of Assab to Addis Ababa. A series of studies have been conducted with the financial assistance of the G o v e r n m e n t of Italy. The area with important geothermal manifestations covers approximately 2500 km 2. All previous studies carried out in this area gave encouraging indications for the exploitation of geothermal resources to generate electricity and for non-electric uses. According to the technical and economical studies by Aquatet (1982), a 20

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Geothermal Exploration in Ethiopia

481

MW plant seemed initially the most appropriate; according to power demand it could be extended to 100 MW. The supply of geothermal energy at competitive prices could make a significant contribution to the socio-economic development of the area. Based on the results of previous studies by Aquater, deep drilling is planned to commence in 1988 with the financial assistance of the government of Italy. LAKES DISTRICT GEOTHERMAL AREA Based on the recommendations and results of the reconnaissance survey of the 1969-1973 E t h i o p i a n - U N D P Geothermal Project, detailed geologic, geochemical and geophysical surveys were carried out in most of the Lakes district, particularly Aluto-Langano, Shalla, Corbetti and Abaya areas. The studies have indicated the existence of heat sources in the northern, central and southern areas of the Lakes district that are associated with a shallow magma chamber beneath the Aluto, Corbetti and Hobecha volcanic centres. Deep exploration was also recommended in the above areas. ALUTO-LANGANO GEOTHERMAL AREA The Aluto-Langano geothermal area is located on the floor of the Ethiopian rift valley about 200 km south-east of Addis Ababa. In 1980 a Technical Review Committee, with the participation of United Nations experts, was convened to review and assess the available data on the Lakes district areas and to make recommendations on further activity, such as selection of priority areas for initial drilling and well siting. As a result, Aluto-North Langano area was recommended as first priority. Corbetti and Lake Abaya geothermal areas were recommended as second and third priorities. Subsequent detailed investigations, including geophysics, were mainly concentrated in the Aluto area. The priority given to this area was partly influenced by its proximity to the existing National Electric Grid. In 1978 the Ethiopian Government reached an agreement with the European Economic Community (EEC) and the United Nations Development Programme (UNDP) to jointly finance a deep geothermal exploration drilling programme. The objective of the project was to verify by drilling the existence of sufficient quantities of geothermal steam to fuel an economically viable power plant in the area (Bodvarsson, 1986; Hole, 1986; Mahon et al., 1984; Molla Belaineh, 1986, 1987). DEEP EXPLORATION WELLS From 1981 to 1985 eight exploration wells were drilled in Aluto-Langano geothermal area. Two of the wells, LA-1 and LA-2, were drilled outside the Aluto volcanic complex, while the other six wells, LA-3 to LA-8, were drilled within the Aluto complex (Fig. 3). The first well (LA-1) was drilled on the southern flank of the Aluto volcanic complex, on the northern extension of the East Basuma fault. Subsurface conditions encountered were characterized by poor permeability and low temperature. Well LA-2 was located on the western flank of the Aluto volcanic complex and on the northern extension of the West Langano Wonji fault. Subsurface conditions were similar to those encountered in LA-1. Six wells were drilled in the south-eastern part of the Aluto complex, LA-3 and LA-6 being located on the N N E - S S W trending fault zone, LA-4 and LA-5 east of the fault zone, and LA-7 and LA-8 to the west (Fig. 4).

Depth (m)

1317 1602 2144 2062 1867 2200.8 2448.5 2500

Wells

LA-1 LA-2 LA-3 LA-4 LA-5 LA-6 LA-7 LA-8

Na 563 85 675 758 1060 934 684 670

pH 9.6 9.2 9.3 9.5 9.0 9.0 8.8 9.1

Maximum temperature (°C)

88 117 315 231 208 335 225 271

39 20 157 230 148 150 81 53

K 1 1 1 5 6 6 8 6

Ca 0.1 0.1 0.1 0.5 0.5 0.2 0.9 0.4

Mg 162 18 163 199 157 175 99 107

Co3 744 177 830 375 1300 1442 1489 536

HCO3 230 206 310 479 720 454 325 550

CI

Down-hole water composition (ppm)

19 20 282 473 168 204 135 73

SO4 24 2 38 37 28 46 23 22

F 5 0.5 14 53 27 30 23 31

HBO2

84 29 556 558 317 418 210 186

SiO2

1871.1 366.6 3026.1 3167.5 3931.5 3859.2 3257.9 2234.4

TDS

--1650 1000 -1650 970 1150

Enthalpy ( k J k g l)

--3.0 4.2 -5.4 1.2 2.0

Sampling pressure (barsgauge)

28 2.6 5.5

2500 2163 2376

m

m

20 2.4

_

_

H2S

2052 5432

_

_

CO,

Gas composition in discharging wells (mmol (100 mol) -t steam)

Table 2. Water and gas composition in the deep exploratory wells of A l u t o - L a n g a n o geothermal field

1",3

Geothermal Exploration in Ethiopia 38~45'

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Fig. 3. Aluto-North Langano geothermal field and location of exploration wells.

The temperatures recorded in the Aluto wells were high (180°C in LA-5-335°C in LA-6); very low permeability was found in LA-5 and moderate permeability in the other wells (Table 2). Wells LA-1 and LA-2 drilled outside the Aluto volcanic complex are non-productive. The wells drilled within the Aluto volcanic complex are productive with the exception of well LA-5. Each of the productive wells produce steam equivalent to approximately 2 MW c. According to the feasibility study by ELC-Electroconsult (1985, 1986) and ELC-Electroconsuit and Geotermica Italiana (1986), the Aluto geothermal field is related to a hydrothermal system controlled by a recent fault (Wonji fault) that acts as an upflow area for the hot geothermal fluids. The lateral flow occurs through the Bofa basalt and crystalline ignimbrite, which acts as the geothermal reservoir. The wells located along the upflow area (LA-3 and LA-6) produce fluids of high enthalpy (1500-1800 kJ kg-1), while the lateral wells (LA-4 and LA-8) produce fluids with a low enthalpy (1000-1100 kJ kg -1) and a high content of non-condensable gases. Permeability in the reservoir is rather low, ranging from 3 to 5 millidarcy (Table 3).

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Fig. 4. Deep exploratory wells and geothermal manifestations in Aluto-North Langano geothermal field.

Simulations of field performance indicate that, at a fluid extraction rate sufficient to feed a 30 MW~ unit, the reservoir could produce for more than 30 yr. E X P L O I T A T I O N OF R E S O U R C E S Aluto-Langano geothermal resources can be utilized to supply electricity and/or heat for various purposes. According to the feasibility study, the development of Aluto geothermal resources for electricity generation should be conducted in three consecutive phases to reach a full capacity of 30 MW. Phase 1. Early installation of a 3.5 MW,, back-pressure unit utilizing the existing productive wells.

Table 3. Well characteristics in Aluto Langano geothermal field (ELC-Electroconsu[t, 1986)

Well

Maximum temperature (°C)

Total depth (m)

Total flow-rate (kgs f)

Water flow-rate (kgs ')

Steam flow-rate (kgs l)

Enthalpy (kJkg t)

Wellhead pressure (barabs.)

LA-3 LA-4 LA-6 LA-8 LA-7

315 23(1 335 271 225

2143 2062 220(i 250(1 2448

10.4 25.6 12.8 14.8 21.6

6.2 22.5 7.1 11.6 19.0

4.2 3.1 5.7 3.2 2.6

1600 98[1 1650 1140 850

7 7 7 7 3

Geothermal Exploration in Ethiopia

485

P h a s e 2. I n s t a l l a t i o n o f a 15 M W e c o n d e n s i n g unit to be r e a d y by t h e e n d o f 1997. S e v e n a d d i t i o n a l p r o d u c t i o n wells will h a v e to be drilled. P h a s e 3. I n s t a l l a t i o n o f a s e c o n d 15 M W e c o n d e n s i n g unit a f t e r a few y e a r s o p e r a t i o n o f unit o n e . N i n e a d d i t i o n a l wells will h a v e to be drilled. A p a r t f r o m electricity g e n e r a t i o n , t h e m o s t c o n v e n i e n t utilization of A l u t o g e o t h e r m a l s t e a m is its d i r e c t use in the c o m m e r c i a l e x t r a c t i o n of s o d a ash f r o m L a k e s A b i y a t a a n d Shalla.

CORBETTI

GEOTHERMAL

AREA

T h e C o r b e t t i C a l d e r a a p p e a r s to be o n e o f the m o s t p r o m i s i n g g e o t h e r m a l a r e a s within the E t h i o p i a n rift, as i n d i c a t e d by the large q u a n t i t y o f d a t a c o l l e c t e d in this a r e a since 1969. T h e a r e a is l o c a t e d on the rift floor a b o u t 250 k m south of A d d i s A b a b a b e t w e e n L a k e S h a l l a to t h e n o r t h a n d L a k e A w a s a to the south. T h e C a l d e r a is 12 k m in d i a m e t e r . J u d g i n g f r o m its w e l l - p r e s e r v e d m o r p h o l o g y , the C a l d e r a m u s t be c o n s i d e r e d v e r y recent. T h e C a l d e r a c o l l a p s e is likely to h a v e b e e n the result o f a h u g e e r u p t i o n of p a n t e l l e r i t i c p y r o c l a s t i c s a n d i g n i m b r i t e s , which a r e f o u n d b o t h a l o n g the i n n e r C a l d e r a walls a n d in the a r e a a r o u n d it. A large n u m b e r o f f u m a r o l e s a n d s t e a m v e n t s can b e f o u n d within the C a l d e r a . Six g r a d i e n t wells w e r e d r i l l e d to d e p t h s of 93-178 m within the C a l d e r a (Fig. 5). S o m e o f t h e wells r e a c h e d t h e w a t e r - t a b l e a n d f o u n d t e m p e r a t u r e s a b o v e 90°C ( T a b l e 4). D r i l l i n g o f t e m p e r a t u r e g r a d i e n t wells will c o n t i n u e until the e n d of J u n e 1987.

Table 4. Well characteristics in Corbetti geothermal area

Total depth (m) Water-level (m) Bottom hole temperature (°C)

CTG-1

CTG-2

148 -80.4

178 162.7 90.2

Depth (m) 5 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

152 134.2 92.3

93 -92.8

CTG-5

CTG-6

96 91 25.6

162 -94.1

Temperature (°C) 31.8 36.7 43.0 50.3 55.9 60.2 62.7 64.4 67.3 69.3 72.3 74.3 76.4 77.8 79.0

31.1 40.8 84.1 93.5 93.7 93.9 93.9 93.9 94.0 94.0 94.0 94.1 94.1 94.2 94.2 94.2 93.5 92.1

90.6 90.7 90.8 90.8 90.9 91.0 91.0 91.0 91.1 91.1 91.1 91.1 91.2 91.1 91.2

27.7 32.0 39.5 50.2 68.0 77.8 83.7 85.9 86.1

24.2 24.2 24.1 24.0 24.0 24.1 24.2 24.4 24.6 24.7

93.8 93.8 93.8 93.8 93.9 93.9 93.9 93.9 94.0 94.0 94.0 94.0 94.0 94.1 94.1 94.1

24.5

24.5

28.2

24.5

22

25.2

170

Ambient temperature (°C)

Wells CTG-3 CTG-4

486

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( ~ 1572ml

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0

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Road

Q

Drilled bore hole

1665m •Jezomltri¢ leval ~ Ground water tempOc

2Kin

Sca~e

(~TGI

7Ol5 '

7°15 Q

kJ

7o10 ' 7°1( 140

i

"6&" f f - -

~.

y-

-

Fig. 5. Corbeui caldera.

SOUTH AFAR GEOTHERMAL AREA There are many geothermal areas in the southern Afar between Tendaho to the north and Aluto-Langano to the south (Fig. 6). On the basis of available information, the areas of Lake Abe, Teo, Danab, Meteka, Dofan, Fantale, Tulu-Moye and Gedemsa have been selected for reconnaissance and semi-detailed studies. Since 1985 geothermal studies have been conducted by ELC-Electroconsult of Milan under a contract from the Ethiopian Institute of Geological Surveys. The project is financed by the Ethiopian Government and the World Bank. The exploration work was conducted in two phases. In the first phase geological, volcanological and fluid geochemical surveys were carried out. Based on these results, four priority areas, Dofan, Gedemsa, Fantale and Tulu-Moye, were selected for further geophysical and geochemical studies. The latter have already been carried out during the second stage of the project.

Geothermal Exploration in Ethiopia

487

to ASSAB / TENDAHO

f

T,O

'"

I

DANABD

,,

METEKA DIRE~ DAWA~.~ FANTALE to~.ADDISA B A B A ~ ~~GE(EMSA ~

0

TULLUMOYE

20

40

60Kin

Fig. 6. Location of geothermal areas between Tendaho and Langano, in the southern Afar.

REFERENCES AQUATER (1982) Technico-economic study for Tendaho geothermal resources development. Economic Report, Comm. 318700, IDRO A/1123, Ethiopian Institute of Geological Surveys, Geothermal Exploration Project. Bodvarsson, G. S. (1986) Independent review of reservoir engineering work at the Aluto-Langano geothermal field, Ethiopia. U.N. Department of Technical Cooperation for Development, New York. ELC-Electroconsult (1985) Geological report of Aluto Volcanic Complex. Ethiopian Institute of Geological Surveys, Geothermal Exploration Project. ELC-Electroconsult (1986) Exploration of Langano-Aluto geothermal resources. Feasibility Report, Ethiopian Institute of Geological Surveys, Geothermal Exploration Project. ELC-Electroconsult and Geotermica Italiana (1986) Geothermal reconnaissance study of selected sites of the Ethiopian rift system. Second Interim Report, Ethiopian Institute of Geological Surveys, Geothermal Exploration Project. Ethiopia-United Nations Development Programme (1973) Geology, geochemistry and hydrogeology of hot springs of the east African rift system within Ethiopia. Technical Report DP/SF/UN/116, United Nations, New York. Hole, H. M. (1986) Mission report on drilling and related activities. Technical Review Meeting, U.N. Department of Technical Cooperation and Development. Ethiopian Institute of Geological Surveys, Geothermal Exploration Project.

488

A. Endeshaw

Mahon, W. A. J., Di Paola~ G. M., Hochstein, M. P. and Dench, N. D. (1984) Technical Review Meeting. Mission Report. Ethiopian Institute of Geological Surveys, Geothermal Exploration Project. Molla Belaineh (1986) Summary of scientific investigations of Aluto-Langano geothermal field. Ethiopian Institute of Geological Surveys, Geothermal Exploration Project. Molla Belaineh (1987) Geothermal energy in Ethiopia. Ethiopian Institute of Geological Surveys, Geothermal Exploration Project.