| Title | The Asal Geothermal Site, Djibouti Republic (Model Update, 2012) |
|---|---|
| Authors | Abdou Mohamed Houmed, Abrourahman Omar Haga, Saida Abdilahi and Jacques Varet |
| Year | 2012 |
| Conference | ARGeo |
| Keywords | Afar, Djibouti, Asal, rift, geothermal, drilling, |
| Abstract | The aim of this paper is to provide an account of the present knowledge and a vision of future perspectives, based on a geothermal model that we can ascertain on the eve of an important deep drilling exploration program led by the Ministry of Energy, Water Resources and Natural Resources of Djibouti Republic with the financial support of an international banking consortium led by the World Bank. The proposed model is based on the results gained from several successive phases of geothermal exploration undertaken on the site since 1970, with 3 major steps : the initial studies (1970-1975) run by BRGM, including 3 deep wells (Asal 1 to 3), the successive works by Aquater (1986- 1988), and the present stage from which we draw new data and a model established by ISOR for REI (2008), as well as recent geophysical studies (Doubre et al. 2007, Manighetti et al. 2007) and successive interpretations (Doubre et al. 2009). Whereas a pretty high geothermal gradient (18°C/Km) is provided by the active spreading of the Asal-Ghoubbet ridge itself, with a hot (1300°C) anomalous mantle of a low velocity zone at 7km depth and above along the rift axis, a supplementary heat source is provided by the shallow magma chamber located underneath Fiale, that is a former lava lake located right on the rift axis which generated the 1978 tectonic event (a few meter wide opening of the rift) and the Ardoukoba basaltic eruption. The infilling and development of the Fiale magma chamber which resulted from the 1978 event, was achieved 10 years ago, and locally induced an even higher geothermal gradient (up to 60°C/Km) consistent with the presence of a 2km deep magma chamber at Fiale (Vergne et al. 2012) The difficulty encountered in the previous steps did arise from two major problems: - The lack of permeability of the volcanic formations crossed by the (vertical) deep exploration wells; and - The salinity of the fluid encountered at Asal1, the most productive well which was plugged by metallic sulfides and silica deposits at the flash level of the 240g/l brine. We believe that these difficulties will be solved in the coming months with the following steps: - Drilling deviated wells with an orientation perpendicular to the rift axis in order to cross the normal faults and open fissures observed in the axial part of the rift and benefit from a high permeability and productivity of the wells. - Locating the wells in the vicinity of the shallow magma chamber, in order to cross a superficial reservoir displaying suitable temperatures for industrial productions (200-300°C). The major challenge – to be solved by further MT-EM geophysical works - will be finding suitable deep deviated wells location to cross through a fractured reservoir benefiting from the vicinity of this shallow heat source without touching the magma itself! This is going to be achieved during the next phase of the program, financed by a public banking consortium led by the World Bank (with the OPEP fund, ADB, GEF and AFD). Evidence of 4 deep deviated wells and long lasting tests of the reservoir(s) will be necessary in convincing investors to further develop the geothermal power production there up to 50MW, in response to the present needs of the base load of Djibouti town. |