3D Numerical Modeling of the High-Temperature Geothermal System in Asal-Ghoubbet Rift, Djibouti

The Asal-Ghoubbet Rift is a less-than-one-million-year-old area characterized by both magmatic and tectonic activity. It is underlain by very thin crust and exhibits exceptionally high temperatures at reachable depths, making it highly favorable for geothermal energy development. However, despite the high temperatures, geothermal exploitation in this region faces significant challenges, mostly related to sustainable access to fluids, including generally low permeability, low flow rates, and highly saline fluids. Some of the identified sites have proven unproductive, which can be attributed to a limited understanding of the subsurface geothermal system.

This resource is challenging to evaluate, and a reliable 3D geological model is therefore essential. This paper presents the first static model of the geothermal site in the Asal-Ghoubbet region, which serves as a foundation for numerical simulations. To build the geological model, a combination of well data, surface geological observations, geophysical studies, and structural knowledge was employed. As the data are not equally spread across the considered area, four geological cross-sections, representative of the subsurface structures, were created and used as input data for the modeling. The geometry and properties of the subsurface structures were interpolated using the Discrete Smooth Interpolation (DSI) method, implemented through the SKUA/GOCAD software. The resulting 3D model consists of four main stratigraphic units and 17 normal fault planes aligned parallel to the rift.

Numerical modeling of temperature and fluid circulation was performed using the TemisFlow basin modeling software, which integrated the geometry of the geological model built in SKUA, the lithological fill, and a representation of the lithosphere in the study area. Several scenarios were tested to address uncertainties regarding the depth and extent of the magmatic chamber, which influence heat transfer by conduction. Additionally, uncertainties in the hydraulic properties of faults and lithofacies, which control fluid infiltration and consequently heat transfer by convection and advection, were considered. The results were calibrated against measured temperatures in wells.

These findings highlight the main controlling factors of the geothermal system in the Asal Rift, provide a 3D visualization of heat transfer and fluid circulation, and will enable future quantification of the geothermal potential of the area.