The subsurface flow and hydrogeothermal simulation system SHEMAT (Bartels, J., Kuhn, M., Pape, H., Clauser, C., 2000. A new aquifer simulation tool for coupled flow, heat transfer, multi-species transport and chemical water-rock interactions. In: Proceedings World Geothermal Congress 2000, Kyushu -Tohuku, Japan, May 28 -June 10, pp. 3997-4002) is used to investigate a typical hydrothermal sandstone reservoir situated in the North German Basin. This study focuses on the prediction of long-term behavior of reservoir properties for the entire operation time with reinjection during heat exploitation for district heating. The Stralsund location in NE Germany and the Detfurth sandstone horizon (Buntsandstein) are chosen due to the combination of its already confirmed geothermal potential and the availability of a complete data set. An installation of two production wells and one well for reinjection implements heat exploitation. Reinjection is required due to high salinity of the water. In order to quantify injectivity changes and allow the separation of thermal from chemical effects, changes in the hydraulic parameters of the reservoir are at first studied without chemical reactions. Reinjection of cooled water of higher viscosity than the natural reservoir fluid leads to a continuous reduction of the injectivity. This effect is partially balanced by thermally induced mineral reactions. Dissolution of anhydrite in the vicinity of the injection well dominates Geothermics 31 (2002) 725-749 www.elsevier.com/locate/geothermics