Predicting output curves for deep wells in the Asal Rift, Djibouti

Abstract

Decade’s long combined effort of various surface exploration campaigns and deep drilling has confirmed the existence of a high-temperature geothermal anomaly within the Asal Rift in Djibouti. Several km2 of at least 1 km thick 240-350°C saline resource hosted in a basaltic extensional rift environment is seen. The high scaling potential of the very salty geothermal fluid encountered at depth is however a potential show stopper for long term operations. This is seen as rapid wellbore scaling deposition on the order of 3 cm/year. By doing a numerical wellbore model, calibrated with field data gathered earlier in well A-3 in the Asal Rift, a large 13⅜” casing well design appears amongst the straightest forward scaling mitigation options. Coupled with a decision to operate future Asal Rift wells at wellhead pressures of 20 bars or more. The wellbore model infers a more beneficial wellhead output curve if a reservoir volume hotter than the current 260°C of well A-3 can be intersected. One interesting option in continuing the Asal Rift steam field development is therefore to drill a large diameter, ~2000 m deep directional well from A-3 towards the ~300°C temperature measured in well A-4. Thereby intersecting the presumable near-vertical fracture-dominated permeability of the rift zone, tapping the proven permeability in wells A-3 and A-6 right under the casing shoe and, ideally, finding more permeability at deeper and hotter levels in the bedrock near well A-4. As the reservoir near A-3 was suffering long term pressure drawdown due to the presumably tight outer reservoir boundaries, reinjection should be part of the steam field management strategy. Ideally, one or two successful wells drilled from A-3 towards A-4 can be used for early 5-10 MW production and then some of the older existing wells near A-3 for injection.