Injection of spent (cooled) thermal fluids began in the Tianjin geothermal district, north-eastern China. at the end of the 1990s. Well injectivities declined after 3-4 years because of self-sealing processes that reduced reservoir permeability. The study focuses on the factors that may have caused the observed decrease in permeability. using chemical and isotopic data oil fluids (water and gas) and mineral phases collected from production and injection wells. The results of data processing and interpretation indicate that (I) it is very unlikely that calcite and silica precipitation is taking place in the reservoir: (2) the Fe- and Zn-rich mineral phases (e.g. sulfides, hydroxides and silicates) show positive saturation indexes: (3) SEM and XRD analyses of filtered material reveal that the latter mineral phases are common; (4) visual observation of casinos and surface installations, and of corrosion products, suggests that it poor quality steel Was used in their Manufacture: (5) significant quantities of solids (e.g. quartz and feldspar crystals) are carried by the geothermal fluid; (6) seasonal changes in fluid composition lead to a reduction in casing corrosion during the summer. It was concluded that the decrease in injectivity in the Tianjin wells is caused only in part by the oxidation of casings, downhole pumps, and surface installations, triggered by free oxygen in the injected fluids; the utilization of better quality steels should drastically reduce this type of corrosion. Self-sealing of pores and fractures by reservoir formation solids and by the Fe-corrosion products suspended in the injected fluids seems to be a more important phenomenon, whose effect could be greatly reduced by installing filtering devices at all sites. (C) 2008 Elsevier Ltd. All rights reserved.