Assessment of heat mining and hydrogeochemical reactions with data from a former geothermal injection well in the Malm Aquifer, Bavarian Molasse Basin, Germany

With high temperatures, high transmissivities, and low salinities the Malm aquifer in the Bavarian Molasse Basin, Germany, offers ideal conditions for the exploration of geothermal energy in Germany. In 2011, the Pullach geothermal doublet system was extended with a third geothermal well (Th3) to account for increasing heat demand. The third well was then used as injection well and the former injection well (Th2) was converted to a production well. Hence, for the first time in the history of geothermal exploration of the Malm aquifer, data became accessible from the formation surrounding an injection well which had been in operation for more than 5 years. This data provides access to the processes at the injection well and sets the baseline for an assessment of the long term behaviour of geothermal heat and power plants in the Molasse Basin. The increase of the production temperatures at the former injection well after conversion from injection to production went faster than expected; after 3 years of production the initial temperatures have almost been reached. This can be explained with a vertically heterogeneous distribution of the aquifer transmissivity with one thin, highly permeable layer. In this setting, the cold water forms a thin disc which extends much further from the injection well compared to a vertically homogeneous setting, with a cylindrical propagation of the injected water. Hence, in a heterogeneous setting the effective interface for conductive heat exchange between injected cool water and hot aquifer matrix with low permeability is larger. The hydrochemical data of the water from the former injection well showed a significant increase of the concentrations of calcium, magnesium, and bicarbonate, indicating dissolution of dolomite with about 0.6 mmol/L of produced water. Together with thermoelastic effects and hydrostatic stress, the dissolution significantly contributes to an increasing injectivity. The effects are overcompensating the effects of the increased viscosity of the injected cold water. Results from the hydrogeochemical model show that lower temperatures and different lithostratigraphic origin of the injected water are responsible for the dissolution. The kinetic model suggests that the dissolution takes place close to the bore hole. The data collected from the Pullach site provides experimental proof for the processes in the surrounding of injection wells, i.e., the assumed aquifer heterogeneity, the connection to overlying strata, and for the hydrogeochemical model of the Malm aquifer. The data serves as a benchmark and reference for future exploration.