Estimating the lithium content in a hydrothermal reservoir using a modelling-based sensitivity analysis

Abstract

The North German basin holds high potential for geothermal lithium exploration. Yet, so far, the assessment of reservoirs concentrates on locations in the Upper Rhine Graben. This thesis gives an example for the evaluation of a potential geothermal lithium reservoir of the Middle Buntsandstein at the location Horstberg in Lower Saxony. The targeted layers are part of the Solling and the Detfurth formation.

To determine the porosities and volumes of the two potential reservoir layers, including their uncertainties, well log data and a structural model are evaluated. The model is created using \textit{Gempy}, an open-source library to create 3-D models in Python. Based on the estimated parameters and data from the literature, the lithium and heat quantities stored in place are estimated. Furthermore, calculations are carried out that target the design and production volume of a geothermal doublet with a theoretical production rate of 24\,L/s.

For the Solling, the resulting mean quantities are 13,836\,t of lithium and a stored heat of 2.59 - 3.04\,GJ\,m$^{-2}$. The values of the Detfurth are much lower. The amount of stored lithium amounts to 3,773\,t and the heat-in-place is determined to be 1.85 - 2.19\,GJ\,m$^{-2}$. Regarding the calculations of the doublet, depending on the input parameters, the mean required distance between injection and production well is 1,037 - 2,021\,m. Based on these results, 8-28 doublets would fit in the study area. One doublet system could produce 540.6 - 660.5\,t of LCE per year.

Overall, the conditions for geothermal lithium exploration at Horstberg are rather poor. The reservoir is mainly limited by its low thickness and low production rate. Nevertheless, this study gave a good example of how to assess a potential geothermal lithium reservoir in the North German Basin.