| Abstract |
North German Basin aquifers exploited for geothermal energy uses are Mesozoic sandstones, more rarely Permo-Carboniferous volcanic rocks and sandstones. The thermal waters are highly saline Na(Ca)Cl solutions. Their salinity and Ca/Na ratio increase with depth. Reducing conditions and pH values ranging from 5.5 to 6.5 are typical. Secondary components are mainly heavy metals (particularly iron and lead). Scaling may form a massive problem in the operation of geothermal systems. In Germany, mainly sites with aquifer temperatures more than 90 °C and highly saline thermal waters are affected. Since 1994, a geothermal plant is operated successfully in Neustadt-Glewe. However, solid precipitations were found at several locations in the thermal water loop during maintenance work and well inspections. They consist of calcite, aragonite as well as heavy metal compounds such as iron hydroxides or oxides, and lead-bearing compounds (galenite). Poorly soluble coelestine-barite solid solutions ((Ba, Sr)SO4) are observed, mainly in heat exchangers. Although the cooled fluid passes a fine filtration unit prior to injection, a deterioration of the injectivity has been registered in recent years, which could be partly improved by periodically soft HCl acidizing. In order to prevent further clogging of the near-well zone by poorly soluble compounds such as barite and galenite, adequate inhibitors were selected adapted to the rocks, fluids, and temperatures occurring in the North German Basin. These investigations are an integral part of the research project “ContraPart” (FKZ: 0325408 A), which is funded by the Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety of Germany. The efficiency of these inhibitors was determined in laboratory tests by means of turbidity measurements with oversaturated BaSO4 model brines. Potential water-rock-inhibitor interactions were studied in batch tests with representative rock materials and site-typical Na-Ca-Sr-Ba-Cl-SO4 model water at different temperatures. All tested inhibitor mixtures resulted in a reduction of the reaction rate of barium sulphate precipitation compared to tests without inhibitor. Furthermore, no negative rock-inhibitor interactions were observed. For the very first time, adequate inhibitors are applied to avoid barite scaling in the Neustadt-Glewe geothermal plant in the North German Basin after giving due consideration to the cost, benefits, and risks involved. |