| Abstract |
At the geothermal power plant in Soultz-sous-Forêts, France, geothermal brine (100 g/l TDS) is produced from a fractured granite reservoir, which is a natural radioactive rock. The formation of scalings affects negatively the efficiency of the heat exchangers and the injection well performance. Those scalings are formed mainly at the cold part of the plant and consist of solid solutions of strontium rich barite (Ba1- xSrxSO4) and minor amounts of sulfide minerals like galena (PbS). Within the Organic Rankine Cycle (ORC) heat exchanger act those scalings as isolation material and decreases the efficiency of the heat transfer between geothermal and organic fluid. As a consequence, cleaning operations for scale removal are required in regular time intervals to keep up the efficiency of the energy production. During cleaning and disposal operations, strict regulations for safety at work have to be followed due to radioprotection rules, related to the presence of 226Ra and 210Pb in the scalings. In the injection wells, the inner diameter of the casing decreases slowly but continuously by deposition of scales. Moreover, a progressing precipitation front exists in the injection wells versus depth which can reach the open-hole section in the future as a function of the injected volume of geothermal brine. For reasons of safety at work and long term operation of the injection wells in Soultz, the formation of barite needs to be inhibited continuously. Therefore, 4 different products based on phosphonic acid were tested in laboratory experiments. These study included tests for calcium tolerance, effectiveness, dose rate adjustment, thermal stability and the interaction with reservoir rocks. Artificial solutions and original Soultz brine was utilized. All four inhibitors showed a high calcium tolerance. Three of them inhibit successfully the formation of barite with both, artificial solutions and original brine. One of the products triggered barite precipitation instead of blocking it. Only two products were able to suppress co-precipitation of iron-silica compounds. Finally the DTPMP based product was selected for injection at the geothermal power plant in Soultz and an injection system was installed. Concentrated phosphonic acid is added dropwise to the production stream at 433 K (160°C) and 2 MPa (20 bars) before entering the heat exchanger. The on-site injection test worked out very well and the Ba2+ concentration in the brine kept a constant level from the production to the injection well. Besides the successful blocking of barite precipitation, a slight dissolution of the galena was observed. |