Analysis of geothermal well test data from the Asal Rift area, Republic of Djibouti

Abstract

The Asal area has been described as a segment of the world oceanic rift system by earlier investigators. A total of six deep wells have been drilled in the area, the first two in 1975 and the last four in 1987 and 1988. Well Asal 2 was damaged, wells Asal 4 and Asal 5 were impermeable although very hot, but wells Asal 1, 3 and 6 have produced extremely saline fluids from 1000-1300 m depth where the aquifer temperature is about 260°C. Well test data from wells Asal 3, 4 and 6, including injection test data, draw-down test data, pressure build-up data and pressure interference data have been analyzed in order to estimate the reservoir properties of the Asal geothermal system. The permeability thickness of the deep geothermal reservoir is estimated to be about 4-8 Dm. A low storativity value of about 2×10-9 m/Pa reflects a reservoir thickness of about 250 m. During long-term exploitation a large pressure draw-down is observed in the reservoir. Wells Asal 3 and Asal 6 produce highly saline (120 g/l) reservoir fluid and the scaling of galena at high pressure reduces the discharge rate. Extensive exploration and field tests need to be performed to accurately estimate the actual size and capacity of the Asal reservoir. Laboratory studies should be conducted in order to find chemical inhibitors that may solve the scaling problem. If the outcome of these tests is positive, new production wells should be drilled. It is recommended that the suitability of a 130°C resource found between 400 and 600 m depth in the Asal area be studied for binary power production.