Well design for the Asal geothermal field, Djibouti : A case study for well GLC-2

Abstract

The purpose of this report is to prepare a well design for a future well, namely GLC-2 located in the Asal geothermal Field, Djibouti. Existing well lithology data was referred to in order to prepare the well design. The wells Asal 1, Asal 3 and Asal 6 are among the producing wells in the Asal geothermal field and they have a close lithological relationship. The wells Asal 3 and Asal 6 produce highly saline (about 120 g/l) reservoir fluid. Consequently, the BPD curve for 12 wt% NaCl salt water was used and it influences also the effective containment pressure. The curve was then used for the determination of minimum casing setting depths. The planned depth of the well is 2000 m and the resulting minimum depth of the production casing is set at 1100 m with an 8-1/2” production section down to 2000 m, and the anchor and the surface casings have minimum setting depths of 670 m and 400 m, respectively. Casing stresses during the drilling operation and production phase were evaluated according to the method indicated in the African Union Code of Practice for Geothermal Drilling. The sizes of the casings are 20”, 13 3/8” and 9 5/8”, and based on the calculations the weight was selected as 94 lb/ft, 68 lbf, 43.5 lb/ft, respectively. The selected casings are of material grade K-55, which is a grade recommended for geothermal wells. The density of the water and cement is 0.988 kg/l and 1.617 kg/l respectively. The maximum pressure (effective containment pressure) in the well is 14 bars. To ensure the casing design of the well, the data was used to assess the axial, radial and hoop stresses generated by the temperature, to the applied pressure or static pressure. The casings were selected based on design factors that were higher than the minimum design factors indicated in the African Code of Practice for Geothermal Drilling.