Directional well design, trajectory and survey calculations, with a case study in Fiale, Asal Rift, Djibouti

Abstract

Djibouti plans to drill 4 new geothermal wells for its future production in the Lava Lake or Fialé, within the Asal Rift segment. These wells are planned to be directionally drilled and the targets are based on six previous wells (Asal 1 - 6) drilled in the late 1980s in this rift segment. These 6 wells were drilled in an area which has high temperature potential, but problems of low permeability and high salinity were encountered. This paper presents directional well planning for these new wells and calculations for: (a) the trajectory and survey, (b) the well path, (c) the vertical depth. Two case studies are presented for Fialé and the resulting well path, using the build and hold type of directional wells. A simple BHA with two-stabilizers is proposed with an optimal weight on bit. Based on the casing plan, the kick-off point should be lowered to 430 m depth rather than 350 m as proposed by the prefeasibility study, which is about 30 m below the casing shoe of the intermediate casing. Another option is drilling vertical wells, which would provide the same subsurface information as directional wells. However, a thorough cost analysis of drilling, survey tools, and equipment is needed to determine whether directional or vertical drilling is financially advantageous for the exploration wells. Directional drilling would not require new permitting or much geological and geophysical studies as such studies were done in the pre-feasibility phase in 2008. Drilling pads and targets for vertical drilling would, however, require further studies and a permit to drill inside of Fialé crater.