Scalings in Hydrogeothermal Systems

Abstract

The majority of the scalings observed at geothermal facilities exploring the Malm Aquifer in the Bavarian Molasse Basin are carbonates. They are formed due to a disruption of the lime-carbonic-acid equilibrium during production caused by degassing of carbon dioxide. These scalings are found in the production pipes, at the pumps, and at filters and can nicely be described using existing hydrogeochemical models. In order to mitigate those scalings, the process of scaling formation and the influencing factors have to be better understood. Therefore, scalings of all sections of geothermal facilities have been taken. The database consists of scaling samples from 13 geothermal pumps, 6,000 m production pipe (sample interval 10-12 m), 11 evaporator revisions, 2 injection pipes and numerous filter elements. The samples were analyzed by SEM-EDX, XRD, Raman spectroscopy and acid digestion to assess their chemical and mineralogical composition. The development of scalings directly above the pumps correlates with the number of startup cycles. Precipitation closer to ground level is rate limited. Scalings in the production pipes become critical if they are detached by hydraulic or thermal stress. Scalings observed in ground level facilities were traced back to residues of the cleaning procedure and caused additional downtime. At the injection wells degassing causes high scaling rates. The developed hydrogeochemical models are able to predict the occurrence of scalings and to assess mitigation measures.