| Abstract |
In geothermal installations exploiting the Dogger aquifer in the Paris Basin, field esperience has shown that corrosion in production and injection well casing? of carbon steel occurs mainly because of the compounds which confer a high potential for corrosion to the well casings. The relatively high tluid temperature total dissolved salts ( 5 to 35 and the presence of and make them one of the most corrosive naturally occurring lluids The corrosivity of various geothermal fluids was assessed with respect to different materials, including carbon steel, under different operating conditions (flow rate, fluid temperature, treatment by corrosion inhibitor). Both weight loss and electrochemical methods were used to measure corrosion rates. The scale produced under these conditions was characterized by classical chemical analysis, Xray diffractometry (XRD), scanning electron microscopy and electron dispersion spectroscopy These experiments show that carbon steel samples suffer fairly general corrosion. The corrosion rate and scale thickness are decreasing functions of the flowate of the fluid. Mineralogical analysis shows that the scale is composed mainly of iron sulphides (mackinawite, pyrrhotite, pyrite, greigite, marcasite), calcite, siderite, and iron hydroxide and hydroxychloride and that the ratio in the scale increases as a function of exposure time. The nature, distribution and abundance of these phases is a function of the nature of the metal substrate upon which they have formed, of certain physicochemical parameters of the fluid, of the origin of the scale (depth) and of the operating conditions. These results, together with those from the study of the formation conditions of the various phases, has made it possible to explain the origin, the on site formation conditions and the relative abundance of the different phases. Mackinawite, pyrrhotite, greigite, siderite, and iron hydroxides and hydroxychlorides are products of casing corrosion, whereas pyrite, silica, quartz and marcasite come mainly from the reservoir in the form of euhedral micrometer-sized particles. origin of the iron (from both the reservoir and corrosion) and of the dissolved sulphides (from both the reservoir and current bacterial activity), and the influence of these species on scaling processes are discussed. A model of scaling regulation (control) is proposed, suggesting that the formation of scale is controlled by the diffusion of anions (sulphide, hydrocarbonate, chloride) through the scale. |