Mechanisms and kinetics of carbon steel corrosion/scaling and inhibition in actual or reconstituted and treated geothermal waters
Résumé
Most of the geothermal installations in France are exploiting low-enthalpy geothermal waters of the Dogger aquifer in the Paris Basin. The geothermal operations are made up of two wells (production-injection, both are about 1,800 m deep), and of surface installations. The operating principle is simple: the waters flow assisted through the carbon steel XC38 (CS-XC38) casings to the surface without releasing their gases since the water production pressure, managed by an immersed pump, is always higher than those of bubble point pressure and, after flowing through heat-exchangers, are re-injected into the same aquifer, a few hundred meters farther away. Initially, field experience has shown that, in both production and injection well casings, there is corrosion, scaling and, in some cases, partial clogging. The geothermal waters, exploited by 50 doublets, are highly mineralized and contain dissolved gases, including H 2 S and CO 2. The main water characteristics at the production wellheads are: Anaerobic conditions in which temperatures range from 47-85 °C, pHs from 6.1-6.4, total dissolved solids (TDS) from 6-35 g/L (ionic strength, 0.1-0.6 M) much of which is Cl-, [H 2 S/HS-] from 5-100 mg/L, [SO 4 2-] from 300-1200 mg/L, [CO 2 /HCO 3-] from 250-600 mg/L. The concentrations of TDS, [H 2 S/HS-] and [CO 2 /HCO 3-] vary from place to place. In some production wellheads [H 2 S/HS-] varies versus time and exploitation flowrate. These water characteristics classify them among the foremost corrosive natural waters for carbon steel.
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