SIMPLIFIED MODEL OF REGIONAL FLOWS IN THE LOWER AND MIDDLE OUEME VALLEY IN BENIN

Abstract

Models of geothermal reservoirs, essential tools for estimating resources, simulating subsurface behavior, and optimizing heat exploitation, have been developed by several researchers. Pressure and temperature data have been determined for the Hêtin-Sota geothermal reservoir, but regional flow data are still lacking. This article has enabled the development of a simplified model of the Hêtin-Sota geothermal reservoir. The objective of this study is to understand the regional flow dynamics in the large-scale underground reservoir (aquifer), i.e. to map the flow directions, in order to improve the management of groundwater resources in the Lower and Middle Ouémé Valley, to which the locality of Hêtin-Sota belongs. The natural regional flow of the Hêtin-Sota geothermal reservoir aquifer was assessed using piezometric, groundwater depth, and borehole depth maps of the Benin coastal sedimentary basin. This map, extracted from the aquifers of the Benin coastal sedimentary basin, was produced in 2024 by Benin Water Directorate (DGEau). The average depth of the static level, that is, the depth of the water measured from the ground surface, is estimated to be less than 10 m. The hydraulic gradient, which is the decrease in hydraulic head (total energy) per unit length in the direction of water flow, was estimated at approximately 0.2%, graphically derived from the piezometric map extract. Mapping the depth of the reservoir in this basin, based on iso-depth curves, indicates that the Cretaceous sand aquifer of the hydrogeological unit to which Hêtin-Sôta belongs is located at a depth of approximately 400 m. This limit value confirms the depth of the Hêtin-Sôta artesian thermal well, which is 417 m. The piezometric map, which presents a reserve of water contained in the pores of saturated aquifer rocks, located below the surface of the Hêtin-Sota aquifer allowed for the representation of aquifer flow at the regional scale but not at the local scale of the study area. Thus, four (04) radiating directions (A, B, C and D) were identified, confirming the phenomenon of radial flow towards the study area. These directions indicate a radial flow from the recharge zone (high loads: 70 m) towards the depressions (low loads: 5 m), favoring high flow rates at the boreholes, confirmed by the artesian and thermal borehole of Hêtin-Sota delivering a flow rate of 60.16 m³/h before heading towards the bed of the Ouémé river.

The hypothesis of relatively homogeneous regional flows of low amplitude, characteristic of the Hêtin-Sota reservoir alone, appears too restrictive. These results, which are currently partial, serve both as support for and as a constraint on the flow model because of the variation in water temperature which induces convection flows, which do not follow the pressure gradient in a simple way, given that it is a geothermal reservoir (water temperature at the top of the borehole of 53°C and above 30°C).