ORKUSTOFNUN NATIO N AL ENER GY AUTHO RITY

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THE UNITED NATIONS UNIVERSITY

BOREHOLE GEOLOGY OF WELL ASAL-5, ASAL GEOTHERMAL FIELD, DJIBOUTI

Ali Elmi Khaireh

UNU Geothermal Training Programme

Reykjavik, Iceland Report 6, 1989

Report 6, 1989

BOREHOLE GEOLOGY OF WELL ASAL-S, ASAL GEOTHERHAL FIELD, DJIBOUTI.

Ali Elmi Khaireh, * UNU Geothermal Training Programme National Energy Authority, Grensasvegur 9, 108 Reykjavik, Iceland.

* Permanent address: ISERST , P . O. Box . 486, Djibouti Republic of Djibouti

ABSTRACT

This report describes the borehole geology of the well Asal-S in the Asal geothermal field in Djibouti. This well was sunk down to 2105 m depth in the "inner rift" in order to explore there the temperature, permeability and the fluid characteristics of the area. The well penetrated a lava succession of olivine-, ferroand trachybasalts as well as trachytes . Intrusive rocks are inferred at four depth intervals. The well showed a very low permeability and could not be discharged. Three aquifers were located above the production casing, the largest and the hottest one being at 500-550 m. Three minor aquifers were found in the production part of the well below 1600 m depth. The rocks show a n

extensive water/ rock interaction where the intensity as well as rank of alteration increases with depth. The hydrothermal assemblage above ca. 1700 m depth indicates a much higher temperature condition than presently measured which clearly shows that the geothermal system has cooled down by as much as 200 · C at 1000 m depth. A v ague indication of retrograde mineralization, by the common occurrence of smectite down to 1000 m depth and laumontite presence at 600 m and 800 m depth , may indicate a lowering of temperature to below 200 · C.

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TABLE OF CONTENTS

Page ABSTRACT

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TABLE OF CONTENTS . . . . . . LIST OF TABLES

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1 I NTRODUCTION

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2 REVIEW OF THE PREVIOUS WORK . . . . . • .

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2 . 1 Geological Outline of Asal Geotherma l Field . . . 2.2 The Explorat i on History of Asal and Hanle Geothermal Fields . . . . . . . . . . 2.3 Well Asal-5 • • • .

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3 BOREHOLE GEOLOGY . . . . . • . . . . . . . . . . . . . . . .

3.1 Classification of the Rocks 3.2 Overvi ew of we ll Asal - 5 4 HYDROTHERMAL ALTERATION .

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4.1 Definition 4 . 2 Analytical Techniques . . . . . . . . . . . . . . . .

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4.3 The Al teration of the Primary Minerals . . . . .

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4 . 4 Distribution of Hydrothermal Altera tion Minerals 5 DISCUSSION 6 CONCLUSION ACKNOWLEDGEMENT REFERRENCES • .

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LIST OF FIGURES

Page Figure 1 - Location of geothermal resource and the centre of consumption . . .. . .. . Figure 2 - Location of exploratory wells i n Asal geothermal field . . . . . . . . Figure 3 - Lithological log of well Asal-S . . Figure 4 - Distribution of alteration minerals and alteration zones . . . . . Figure 5 (a) : X-ray diffraction pattern of smectite Figure 5 (b) : X-ray diffraction pattern of mi xed

layered clays and illite

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Figure 5 (c) : X-ray diffraction pattern of chlorite . . and actinolite Figure 6 - Measured temperature , mineral temperature and the boiling point curve of the well

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1 INTRODUCTION

This report is the last stage of the six months training course conducted by the UNU Geothermal Training Programme at the National Energy Authority in Reykjavik, Iceland . The training programme consisted of five weeks of introductory lectures dealing with various aspects of geothermal research , such as exploration and utilization; two weeks excursion to some of the geothermal fields in Iceland . Apart from these weeks the author studied borehole geology .

The report summarizes first the geology of the Asal field in the Republic of Djibouti followed by the exploration history of Asal and Hanle fields . The main topic deals with the lithology and the mineralogy of the well Asal-5. Binocular, petrographic microscopes and X-Ray diffraction (XRO) are the main techniques applied in the identification of the hydrothermal minerals using the cuttings recovered from the well Asal-S.

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2 REVIEW OF PREVIOUS WORK

2.1 Geological outline of the Asal Geothermal Pield

The Asal Geothermal Field (fig . 1) lies within the Asal-Ghoubbet rift zone between Ghoubbet e1 Karab in the south-east and Lake Asal in the north-west. This part of the rift is characterized by active tectonism, periodic eruptions (the last volcanic eruption was Ardoukoba eruption in 1978) and continuous seismicity. The zone has also been uplifted due to magma intrusion at depth (BRGM report 1972-1973). The tectonic features of the Asal rift are normal faults along two principal trends i . e. NW-SE and NNW-SSE. A third tectonic trend (E-W) has been recognized in some parts of the rift zone. The NNW-SSE and E-W tectonic features are older than the current NW-SE tectonic feature of the rift and they can be associated with an older geological formation (stratoide series) that outcrop at the margins of the active part of the rift. The geological formations that outcrop within the Asal Geothermal Field are lava flows, pyroclastics (mostly hyaloclastites) and lake sediments (diatomite and limestone). The lava flows and pyroclastics have been produced by fissures trending NW-SE. Several geological units were distinguished on surface by stieljes et al. (1976) . These are in decreasing age towards the centre of the rift: -Hyaloclastites at the southern external rift -Lava flows (Asal series) -Phreatic tuffs of the lIinner rift ll -Lake sediments -Recent lava flows All these formations are less than 1 my.

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2.2 The Exploration History of Asa1 and Han1e Geothermal Fields The geothermal surface exploration in the Asal rift zone (fig.l) was carried out by BRGM since 1970 . Among the various res each techniques and methods applied were : -Geological mapping -Geochemical analyses of hot springs and fumaroles -Geophysical methods such as resistivity, magnetic and gravimetric surveys -Shallow gradient wells The results of the abov e methods led to the siting of two deep exploratory wells , Asal-l and Asal-2 (fig . 2) at the external edge of the rift . Asal-l and Asal-2 were drilled to 1146 m and 1554 m depth respectively . Asal-2 was dry, but hotter(280 °C) tha n Asal-I (258 °C), while Asa1-1 was productiv e. Asal-1 confirmed, during well testing in 1975, the existence of a water dominated high-enthalpy geothermal reservoir. Unfortunately the salinity of the geothermal fluids was very high , which might create problems during the exploitation of the resource . Due mainly to the high sa l inity of the geothermal fluids of Asal-1, the strategy of the geothermal development shifted to Hanle area (fig . 1) , which was assumed to contain geothermal fluids of lower salinity . After sev eral years of surface exploration and the drill i ng of three gradient wells in the Hanle area, two deep exploratory wells were sunk in 1987; Hanle-1 (1600 m deep) a nd Hanle-2 (2000 m deep) . Both wells were dry and the measured temperatures were only 70°C and 120 °C respectivel y . The results of these two exploratory wells confirmed that the Hanle Geothermal Field is a low temperature field and not suitable in terms of economic production of electric power. Further e xploration of the Hanle Field was thus abandoned and the attention redirected to the Asal Field in 1987.

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In June 1987, Asal-3 was sited 4 m east of Asal-1 and drilled to a depth of 1316 m in order to verify the potential and the extention of the reservior already intersected by Asal-1 in 1975. The well was productive and the geothermal fluids had the same chemical characteristics as those produced by Asal-1. However, during three months of well discharge, a decline in mass flow occurred due to scaling with in the casing. It became therefore evident that scaling, as observed in Asal-1 and 3 would impose serious problems in the utilization of the field. The exploration emphasis was put on locating a less saline part of the reservoir. The managers of the Asal Geothermal Project believed that the densely fissured "inner rift" would be more permeable than the "external riftll and would thus increase the probability of finding those lower salinity geothermal fluids. According to the above hypothesis, Asal-4 was sited 1250 m north of Asal-3 and drilled to a depth of 2013 m. Asal-5 on the other hand was s ited 4750 m north of Asal-3 at the edge of the axis of the rift and drilled to a depth of 2105 m. Unfortunately, both well s were dry and did not give any information about the chemical characteristics of the geothermal fluids. They, however, indicated the high temperature characteristics of the area (T < >

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Fig. 3: Lithological log of well Asal-5

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