| Abstract |
Scale formation in the surface and subsurface installations of the Soultz geothermal power plant affect negatively operational performance of the geothermal cycle. The Soultz EGS site operates a naturally fractured granitic reservoir percolated by Na-Ca-Cl brine with Total Dissolved Solids (TDS) up to 100 g/l. Geothermal brine is produced at 160°C/20 bars and re-injected at 70°C/18 bars. For power production, an Organic Rankine Cycle (ORC) is installed which includes three heat exchanger units, one evaporator and two preheaters. The heat transfer in this binary cycle is limited continuously by formation of scales in the tubular heat exchangers. Due to the temperature decrease of 90 K in the ORC heat exchanger system, strontium rich barite (Ba0.6Sr0.4SO4) becomes oversaturated and forms a homogenous scaling layer. Additionally, minor amounts of sulfide minerals like galena (PbS) are present. Those scales form an insulation layer inside of the heat exchanger tubes and decreases thereby efficiency of the heat transfer between geothermal and organic fluid. In consequence, cleaning operations for scale removal are required in regular time intervals to keep up efficiency of energy production. During cleaning and disposal operations, strict regulations for safety at work have to be followed due to radiation-protection regulations. The scales have to be classified as NORM (Naturally Occurring Radioactive Material) related to the presence of 226Ra and 210Pb, incorporated by chemical substitution in the scales. Moreover, the inner diameter of the reinjection wells decrease slowly but continuously by deposition of scales. Recently, well loggings proofed the existence of a progressing precipitation front inside of the injection wells versus depth which can reach the open-hole section as a function of injected brine volume and brine temperature. For reasons of safety at work and long-term power plant operation, the formation of barite needs to be inhibited continuously. Therefore, several inhibitors, based on phosphonic-acid, were tested in laboratory experiments. These studies included tests for calcium tolerance, effectiveness and dose rate adjustment by tube blocking tests. Out of these products, Diethylenetriaminepentakis (methylenephosphonic acid) (DTPMP), showed the best results for barite inhibition. Therefore it was selected for additional tests which investigated long-term efficiency, thermal stability and reservoir rock–inhibitor interactions. The inhibitor suppressed successfully barite formation for 24 h at 90°C in contact with reservoir rock. Geothermal fluids trapped within deep-seated fractured crystalline rocks from the Upper Rhine Valley must be carefully investigated in order to minimize scaling (sulfate, sulfide) in geothermal power plants and seriously limit the natural radioactivity associated with. At the Soultz site an inhibitor injection system was designed, installed and tested for continuously inhibitor injection and on-site efficiency tests showed very good results during short time tests. |