| Abstract |
Reinjection of thermal waters is a key parameter for sustainability of geothermal systems. However, injection can face difficulties due to several factors, e.g. mineral precipitation, fines migration or biological processes, especially in case of clastic reservoirs. These processes can clog the well screens and the pores of the reservoir, which can result in the decrease of injectivity. Mineral precipitation, which is the focus of this paper, is controlled by hydrochemical and physical changes. In this study, we focus on injection problems at the geothermal site in Mezőberény, SE Hungary. A geochemical model was set up to simulate the geothermal reinjection processes and to model the saturation of the minerals during injection, using changing parameters: fluid composition, rock composition, air contact, injection temperature and injection pressure. For hydrochemical modelling we use PHREEQC Version 3 with the phreeqc.dat database. Based on the results general and site-specific conclusions could be drawn: In general, high concentrations of Fe and Mn content play a role at the injection problems at the Mezőberény site through precipitation of Goethite, Hematite and Mn-oxides. Furthermore, Calcite is also oversaturated, therefore able to form carbonate-scaling. The geochemical model was validated with XRD analysis performed on scale samples from the Mezőberény site: precipitation contains Goethite, Calcite and Magnesioferrite, which confirms our model. Our findings include that air contact radically increases the saturation of Mn minerals, slightly enhance saturation of Fe minerals and decreases the saturation indices of carbonates. Lower injection temperature enhances the saturation of Fe-Mn mineral, in contrast to carbonates. Injection pressure has a negligible effect on the saturation of minerals. The reinjection of heat depleted thermal water into sandstones has a relatively short history in Hungary. With two decades of experience and several successful projects in SE Hungary (Orosháza, Hódmezővásárhely, Szeged), reinjection is successful. Compared with these geothermal systems, saturation indices and the amount of possible precipitation are higher at Mezőberény. By preventing contact with air, iron and manganese minerals would be less saturated and the possibility of precipitation would decrease. |