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Abstract:
A Triassic, siliciclastic aquifer was hydraulically tested for its suitability as aquifer thermal energy storage in the city of Berlin (Germany). This study investigates the effects of groundwater mixing, change of temperature, and water-rock interactions on the chemical and microbial water composition. The water chemistry was monitored during a step rate test and two subsequent tracer push pull tests (one conducted with aquifer water and the second with heated aquifer water). Apart of the main ions and physicochemical parameters, the microbial community were determined. In addition, molecular size distribution of dissolved organic carbon as well as the stable isotope compositions of strontium and water were measured in selected samples. This comprehensive monitoring resulted in the following main findings:
The chemical water composition of all analysed samples was similar to seawater with main cations and anions correlating well with each other. Although the total element content varied over time, the produced water was homogenously mixed as evidenced by constant ratios of stable water isotopes. The most remarkable observation was an extremely high aluminium concentration (up to 4.7 mM) measured in the flowback after cold water injection. This can be explained by a hydraulic connection between the Triassic Formation and an overlaying Tertiary sand. Water-rock interactions between oxygen dissolved in the injected water and the pyrite-containing Tertiary sand eventually mobilized the aluminum.
The measured effect of injecting hot water into the aquifer on the chemical and microbial composition was relatively small. However, temperature-dependent reactions might still be of relevance due to (I) the generally high microbial cell numbers that could trigger microbial enhanced reactions and (II) a calculated oversaturation of calcite that might result in mineral precipitation.
