New Insights into Hydrothermal Fluid Circulation Affected by Regional Groundwater Flow in the Asal Rift, Republic of Djibouti

Abstract

The Asal Rift hosts a lake located in a depression at 150 m below sea level, where recharge is influenced by regional groundwater flow interacting with the Ghoubbet Sea along the coast of Djibouti. This regional groundwater flow is believed to influence hydrothermal fluid circulation, which we aim to better understand in this study, having the objective of developing concepts for geothermal exploration in the area. To this end, magnetotelluric data acquired in the Asal Rift were processed and analyzed. 1D inversion models of electrical conductivity were interpolated for interpretation. These data were then used to build a 2D hydrogeological model, allowing multiphase flow and heat transfer simulations to be performed, considering the regional groundwater flow near the surface and the site topography, in order to confirm the preferred path of fluid flow. Geophysical data analysis indicates the presence of normal faults, notably the H fault, which may act as a conduit for the circulation of hydrothermal fluids and where the hanging wall can be a hydrogeological barrier within the hydrothermal system of the Asal Rift. The results from the 2D numerical flow and heat transfer modelling show the importance of groundwater flow responsible for thermal springs located at the periphery of Asal Lake. Reservoir temperature inferred by means of geothermometry ranging from 200 to 270 °C was shown to correspond to simulated temperature at potential reservoir depth. Moreover, simulated temperature between 600 and 1700 m depth is close to the temperature profile measured in the geothermal well Asal 6 of the area, with less than 20 °C difference. Simulations indicate that hydrothermal fluid circulation is likely influenced by the regional groundwater flow controlled by the topography and the major water bodies, the Ghoubbet Sea and Asal Lake, feeding buoyant fluids interacting with a deep magmatic source and where tectonic activity created normal faults offering a preferred path for fluid circulation.

Numerical Modeling of Hydrothermal System Circulation Beneath Asal Rift, Republic of Djibouti

“…A 2D numerical model parallel to the rift axis and to the shallow groundwater flow direction believed to be from the Ghoubbet sea to Asal Lake has been developed for the first 3 km of the upper crust [13]. This previous work showed the importance of the topographydriven groundwater flow interacting with the hydrothermal circulation in such a regional 2D model.…”

“…This code allows the modeling of flow and heat transport where the differential equations describing the conservation of mass and energy are coupled to be solved numerically with the finite difference method using the Newton-Raphson algorithm [21,22]. The description of the governing equations was presented in [13] and are expressions of mass and energy conservation with respect to enthalpy and pressure. Pressure is chosen as the dependent variable for fluid flow because density fields can be formulated as a function of pressure but not as a function of the potentiometric head.…”

“…The simulation of scenario 1 was performed for comparative purposes to later evaluate how anisotropic permeability can influence the deep hydrothermal circulation and allow lateral fluid circulation. The thermal conductivity and specific heat capacity varying with temperature were calculated with the methods presented by [13].…”

“…The lower boundaries of R2 and R3 on the edges of the model were assigned a basal heat flow of 0.13 W m −2 , which corresponds to the brittle-ductile transition temperatures. Thus, a conductive heat flux (Fourier) was used and the assigned value was an average inferred from the temperature log of geothermal wells (see Figure 2 in [13]). Lateral boundaries were adiabatic.…”

“…The Asal rift is an eroded and aerial rift where numerous northwestsoutheast normal faults intersect the surface of the rift. Both the Asal rift and Ghoubbet Bay have main northwest-southeast normal faults, which may act as a preferred path for hydrothermal fluid circulation [8][9][10][11][12][13]. Volcanic and tectonic activity affecting the region since the late quaternary controls the topography and current structural features of the rift.…”