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TRAINING ON SURFACE EXPLORATION STUDIES FOR GEOTHERMAL RESOURCES AND DEVELOPMENT OF CONCEPTUAL MODELS

UNDER THE AUSPICES OF INTERIM PROJECT COORDINATION UNIT OF THE AFRICA GEOTHERMAL CENTER OF EXCELLENCE

CONCEPTUAL MODEL OF THE ALUTO-LANGANO GEOTHERMAL

SYSTEM, ETHIOPIA

MESERET T. ZEMEDKUN

UN ENVIRONMENT

ETHIOPIA

• Ethiopia has actively been exploring for its geothermal resources since 1969

• Over 23 high temperature geothermal potential areas identified and only two geothermal prospects are subjected for drilling.

• Estimated potential of more than 10,000 MWe

• A larger number of areas have potential for medium to low temperature resources throughout the Ethiopian Rift System.

ALUTO - LANGANO

TENDAHO

TWO GEOTHERMAL FIELDS SUBJECTED FOR DRILLING

Aluto-Langano Field

• Drilling commenced in 1981 where a total of eight deep exploration wells were drilled.

• Four wells found to be productive with a max. temp. of 300 C.

• The 7.2 MWe Geothermal Combined Cycle Unit was commissioned in 1998.

• The plant had intermittent operation between 1998 and 2014 with challenges of maintenance

Ethiopia - Aluto− Drilling is in preparation to allow for

expansion of the field for 70 MWe

− Financing for drilling is from World

Bank, Iceland, JICA and Government of

Ethiopia

− Two wells were drilled between 2013

and 2015 (LA-9 &10)

− Progress has slowed since 2015 due to

various challenges

Legend Geological Mapping

1:50,000 scale

• LITHOLOGICAL AND

STRUCTURAL STUDIES

•HEAT SOURCE

Abundant silicic volcanoes,

recent volcanic products

•PERMEABILITY

WFB and other transverse

faults (production of

secondary Permeability)

GSE, 2018

W E

Total Alkali-Silica diagram for classification of

Aluto outcropping rocks.

SURACE MANIFESTATIONS

SOIL GAS AND TEMPERATURE ( 400 Radon and 200 CO2)

Consultancy Services for Geothermal Surface Exploration in Aluto Langano - Ethiopia

Classed post map of the activity of 222Rn in soil gases for the 430 stations surveyed during this work

Classed post map of soil temperature measured at 1 m depth in the 375 stations surveyed during this work

Areas of high CO2 fluxes are situated: (i)

along the Jawe fault zone and (ii) at Bobessa

and about 1.5 km to the south of it.

Soil temperature anomalies accompanied by CO2 flux anomalies

Geochemical Study ( Fluid, gas, isotopic, Radon, Co2 flux)

GSE, 2018

Geochemical Study ( Fluid, gas, isotope, Radon, Co2 flux)

Location map of the deep geothermal wells (greensymbols), Temperature gradient wells (whitesymbols), and surface manifestations of the Aluto-Langano geothermal field, including bothfumaroles and steaming grounds (yellow symbols)

GSE, 2018

Cl

SO4 HCO3

0 25 50 75 100

25

50

75

100

0

0

25

50

100

75

Figure 12A. Cl - HCO3 -SO4 PLOT (2003)

Bole SP

SP-84

SP-2

TG-31

TG-32

Shalla NE

Shalla SW

Chitu Sp

Wondo (Belle)

Wondo (Giorgis)

Water Classification

A-C BICARBONATE

Q

K/100Sqrt Mg

0 25 50 75 100

Figure 13A. Evaluation of Na-K-Mg temperatures (1973-2003). After Giggenbach (1988)

25

50

75

100

0

0

25

50

100

75

Na/1000

immature waters

80

100

120140160

180200

220

240

260

280

300

320340

Bole Sp

SP-84

SP-2

TG-32

TG-31

Shalla (SW)

Chitu HS

Wondo (Giorgis)

Wondo (Belle)

Evaluation of Na-K-Mg temperatures

• Geophysical Investigations with emphasis to

parameters that are sensitive to temperature

and fluid content of the Rocks ( gravity,

Magneto telluric and Microseismic Survey).

• GRAVITY Used to provide information on

subsurface structures and intrusive bodies

• Resistivity Methods Used to Map low resistivity

of the areas Abundance and distribution of

hydrothermal clay over active systems is used

to delineate the target areas

• Cap rock, normaly smectite clay ( Low

resitivity) The underlining reservoir ( Chlorite

clay) higher resistivity

Geophysical Study: MT-TEM, Gravity and Microseismic survey

GEOPHYSICS

• ELECTRICAL RESISTIVITY

•MAGNETICS

•GRAVITY

• MICROSEISMIC

• TG WELLS WERE DRILLED

MT SURVEY: 1D INVERSE MODELLING

Consultancy Services for Geothermal Surface Exploration in Aluto Langano - Ethiopia

In order to have a better picture of the clay cap distribution, all the

1D smooth models were interpolated with a three-dimensional

gridding software to obtain a 3D pseudo model of the resistivity

distribution. The 20 Ohm m resistivity surface was then

extrapolated, assuming that it can delineate the clay cap

distribution, and three maps were elaborated

Elevation a.s.l. of the bottom of the clay cap, assumed to becoincident with iso-resistive surface 20 Ohm m

➢ Bottom Elevation of the Clay Cap:

The bottom of the clay cap shows three

discontinuities (dashed white lines), that delineate

a region of up-doming labelled D1. This region

well correlates with the isotherm updoming

detected in wells LA-03 to LA-08. A second region

of updoming labelled D2 is imaged in the plain

west of the AVC

Geophysical Study: MT

GSE, 2018

West-east 2D resistivity profiles

Geophysical Study: MT

GSE, 2018

Negative anomaly N3. It extends over the south-

eastern lower portion of the AVC, mostly in

pumiceous pyroclastic products of both pre-

calderic and syn-post-calderic age. Recent rhyolite

lava flows outcrop to the west and north of the

anomaly. Similarly to anomaly N1, here too many

crateric structures are developed within the

anomaly

Residual Bouguer Anomaly Map at 2,560 kg/m3

Negative anomaly N4. It is located in

correspondence of the alluvial deposits which

occupy the very center of the caldera

Positive anomaly P1. It extends in the eastern

part of the caldera, being centered around a

volcanic edifice of post-caldera rhyolitic lava

flows

Gravity

GSE, 2018

HORIZONTAL DERIVATIVES

The gradient map shows rectilinear

features L1, L2 and L3, trending NNE-

SSW and reflecting the faults of the

Wonji system, which crosses the whole

AVC: in particular, L1 corresponds to the

Worbota-Adonsha lineament, L2 to the

Artu Jawe-Oitu Artu lineament and L3 to

the Bobessa-Gebiba lineament

(although slightly displaced to the east).

Rectilinear features L4 and L5, have,

NNW-SSE trend

Horizontal gradient magnitude

Geophysical Study: Gravity

ANALYSIS OF THE RESULTS

Due to the low number of registered events, for a better reconstruction of the seismological setting the

ALMS data were combined with the data collected in 2012-2014 by the LMS

The Figures show the interpolation of hypocentral depths greater than 2 km b.g.l. It can be observed

that the interpolation suggests the existence of a dome-like structure reaching its top at a depth of

about 5 km b.g.l. and slightly displaced to the east in relation with the centre of the caldera

Interpolation of depth events, of integrated ALMS+LMS catalogue, greaterthan 2 km depth. Left: SN section, Right: WE section. The dashed blacklines are the projected caldera rims

Geophysical Study, Microseismic

ANALYSIS OF THE RESULTS

Aluto, Geophysical Study: MT, Gravity and Microseismics

W-E Cross Section 1 of 10

ALUTO-VOLCANIC PRODUCTS

Lake sediments

Bofa basalt

Tertiary ignimbrite

GSE, 2018

FURTHER INFORMATION FROM THE DRILLED WELLS

LA-3

INFORMATION FROM THE DRILLED WELLS TO DEVELOP CONCEPTUAL MODEL

0 50 100 150 200 250 300 350

Temperature (°C)

-500

0

500

1000

1500

2000

Ele

vatio

n(m

.a.s

.l)

Legend

LA1-T16

LA2-T13

LA3-T55

LA4-T33

LA5-T18

LA6-T15

LA7-T27

LA8-T10

Figure 1: Downhole Temperature Profiles (Aluto wells)

File:Tempelveation

1

(

⊥ ⊥ ⊥ ⊥

0 1 2 3 Km

Lake Ziway

LA-7LA-6

LA-4

LA-8

LA-3

LA-5

LA-1

Lake Langano

LA-2

DOWNHOLE TEMPERATURE PROFILES

CLAY MINERAL STUDIES

Distribution of clay minerals with temperature

Mixed Layer Illite smectite

Illite-chlorite

illite

chlorite

Chlorite intergrade

Chlorite intergrade

FLUID CHEMISTRY

GEOCHEMISTRY

• Water and gas sampling from thermal

manifestations and the wells

•Analyse for major cations and anions

•Data processing using various geochemical

softwares (e.g. WATCH, SOLMINEQ 88 etc..)

•Determine the reservoir temperature using various

chemical geothermomters (e.g. Si-geothermometer,

Na-k, Na-K-Ca geothermometers)

AASGC

Cl

SO4HCO3

0 25 50 75 100

FIGURE 2: A plot of Cl-HCO3-SO4.

LA-6 (1985-2000)

LA-3 (1985-2000)

FLUID CHEMISTRY

120 160 200 240 280 320 Temperature ( C)

-0.80

-0.40

0.00

0.40

0.80

1.20

S a t u r a t i o n I n d e x ( S I ) Quartz

Amorphous silica

Calcite

FIGURE 6: A plot of SI vs temperature (well LA-3)

80 120 160 200 240Temperature ( C)

-0.80

-0.40

0.00

0.40

0.80

1.20

S

atur

atio

n In

dex

(SI)

Quartz

Calcite

Amorphous silica

FIGURE 8: A plot of SI vs temperature (well LA-4)

SATURATION INDICES OF MINERALS

FLUID INCLUSION GEOTHERMOMETRY

CHAIXMECA H-F STAGE

V-RICH INC

P-INCLUSIONE

S-INCLUSIONE

TYPES OF INCLUSIONS

THE ALUTO-LANGANO GEOTHERMAL FIELD

M.T

LA-7

THE ALUTO-LANGANO GEOTHERMAL FIELD

Interpretation of Fluid inclusion Results

M.T

LA-7

Mixed Layer Illite smectite

Illite-chlorite

illite

chlorite

Chlorite intergradeChlorite intergrade

Distribution of clay minerals vs temperature

The Aluto-Langano Geothermal field, Ethiopia

THE ALUTO-LANGANO

GEOTHERMAL FIELD

41

BOBESA to be explored by

drilling in parallel with Aluto

for additional 35 MW

3 well sites are selected

within 5 sq km area

Main Reasons

1. Shallower depth

2. Better permeability

3. Upflow zone

4. Potential 35 MW

Conceptual modelling, Aluto Integrated results

42

The areal extent of the reservoir is inferred on the base of the geoelectrical anomaly

Top of reservoir is expected at shallow depth 600m

thickness of the reservoir is assumed to amount to 1,200 m.

The reservoir is expected to be two-phase in the upflow zone and liquid dominated in the peripheral one

Conceptual modelling, Aluto, Integerated results

THANK YOU