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Geochemical indicators of hydrogeologically isolated structures in the Danube Basin; implications for potential gas storage objects

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Author(s): Tomáš Lánczos | Ján Milička | Miroslav Pereszlényi

Journal: Acta Geologica Slovaca
ISSN 1338-0044

Volume: 3;
Issue: 1;
Start page: 43;
Date: 2011;
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Keywords: Danube Basin | isolation of hydrogeological structures | underground gas storage objects | geochemical indicators | aquifers | carbon dioxide

ABSTRACT
One of the key conditions for underground gas storage (UGS) objects establishment is the presence of hydrogeologically isolated structures. These are mainly exhausted hydrocarbon deposits and aquifers in the Cenozoic fill and the basement of the Danube Basin. The geochemical indicators used to determine the aquifer isolation level of potential underground gas storage objects are based on the ratio of stagnant and dynamic water components. The most widely used indicator is the rHCO3/rCl molar ratio, assuming that the HCO3 - concentration represents the dynamic origin of dissolved rock material by meteoric water and that the Cl- concentration is the stagnant marine component. To distinguish these two components of the water in the aquifers the rHCO3/rCl indicator was combined with other parameters, such as rCl/rSO4 and rCl/rBr molar ratios. As for more aquifers of the Danube Basin contain higher amounts of carbon-dioxide we attempted to quantify the influence of the carbon dioxide on the water chemistry. The numerical model simulating rock-marine water interactions under different PCO2 temperature and ion-exchange conditions proved the rHCO3/rCl molar ratio increase caused higher carbon dioxide partial pressure for this purpose. Consequently under the conditions of higher partial carbon dioxide pressure the rHCO3/rCl ratio is indicating a lower isolation level of the structure. Taking in account signs of the higher carbon dioxide pressure based on the numerical model combined with the abovementioned geochemical indicators the most convenient aquifers for UGS establishment were identified within the Slovak part of the Danube Basin. These are located within Badenian sediments in the Trakovice and Sereď areas, due to the presence of hydrogeologically isolated structures buried at depth intervals of 800 to 1150 m below ground level. Other potential convertible aquifers are present in the Lower and Middle Badenian sediments in the Komjatice and Želiezovce Depressions, especially in the following localities: Modrany at 1145 to 1670 m below the surface, Vráble approximately up to 2110 m and Zlaté Moravce at 1364 to 1374 m below surface.
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