Speaker
Description
The exploration of the North Alpine Foreland Basin (NAFB) for geothermal heat and power production is a cornerstone of the energy transition in Bavaria. So far more than 25 facilities are exploring the Upper Jurassic reservoir in a doublet or multilateral setting. To increase productivity the operators are interested in a higher thermal spread by reducing temperature of the injected water.
The hydrochemical conditions in the Upper Jurassic carbonates in the NAFB are characterized by low salinity and a sodium-calcium-bicarbonate and calcium-magnesium-bicarbonate type depending on the extent of ion-exchange. Geothermal waters west of Munich are known to contain higher concentrations of methane. While these waters are in equilibrium with the host matrix under reservoir conditions the waters are undersaturated at injection temperatures.
From a fluid mechanics point of view, injecting at lower temperatures should lead to an increase of the injection pressure due to increased viscosity. However, it has been shown that the dissolution of the rock matrix in the vicinity of the injection well overcompensates this effect by opening up the flow paths [1]. On the other hand a dissolution of the rock matrix along preferred flow paths can lead to a heterogeneous heat extraction and early thermal break-through, thus incomplete exploitation of the heat-in-place.
The reactions can be predicted quantitatively with hydrogeochemical models [2] While the models have been proven to be robust, the analysis data itself has to be questioned, especially for deep geothermal operations. During production the pressure is decreasing sharply and temperatures are decreasing slightly. Degassing can significantly change the hydrochemical composition. As a result the hydrogeochemical predictions can be off by an order of magnitude. Another unknown in the prediction is the change of the reactive surface during dissolution which affects the reaction rates and possibly the fluid dynamics and the mechanical stability.
In this contribution we present autoclave experiments to visualize and quantify the dissolution effects and a standardized workflow for backcalculation of hydrochemical analyses to reservoir conditions. The workflow was tested at geothermal facilities in the North Alpine Foreland Basin which are characterized by a limestone setting. The autoclave experiments [3] indicate that dissolution along the fractures is increasing the surface roughness and thus the reactive surface. Together the results enable a more accurate assessement of potential adverse effects of decreased injection temperatures on the long-term performance of geothermal reservoirs.
| References | [1] T. Baumann, J. Bartels, M. Lafogler & F. Wenderoth (2017): Assessment of heat mining and hydrogeochemical reactions with data from a former geothermal injection well in the malm aquifer, bavarian molasse basin, germany, Geothermics 66, doi://10.1016/j.geothermics.2016.11.008. [2] M. Ueckert & T. Baumann (2019): Hydrochemical aspects of high temperature aquifer storage in carbonaceous aquifers - evaluation of a field study, Geotherm. Energy 7, doi://10.1186/s40517-019-0120-0. [3] A. Dietmaier, J. Mattheis, D. Weller, I. Stober & T. Baumann (2025): Visualization and semi-quantitative analysis of dissolution processes at artificial structures in carbonate rocks using optical, 3d micro-scanning and confocal laser scanning microscopy, Geothermal Energy doi://10.1186/s40517-025-00355-4. |
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| Country | Germany |
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