In-Situ Oil Shale Retort;Thermocouple Well Failure Analysis.

摘要

A plain-carbon steel pipe, 3/4-in. diam.,was used as a thermocouple well during field tests of in-situ oil-shale retorts near Vernal,Utah. The pipe had nearly completely corroded during service stated to be for long periods of time in the temperature range 1000to 2000exp 0 F. One leg of the chromel--alumel thermocouple,the chromel branch,had corroded,and the Inconel 600sheath was partially corroded. X-ray energy dispersive analysis in a scanning electron microscope revealed that the steel had oxidized to a mixture of iron oxides and/or iron--calcium silicates. A minor amount of iron sulfide was detected. The insulation of the thermocouple had been infiltrated by either liquid nickel sulfide or a eutectic of Ni sub 3 S sub 2and Ni. The corroded chromel wire had been converted to Cr sub 2 O sub 3and a nickel sulfide. Intergranular formation of Cr sub 3 S sub 4in the exterior of the Inconel 600sheath was noted also. The observations showed that oxidation was the major cause of failure and was selectively assisted by sulfidation, e.g.,the thermocouple. The results are consistent with the known products of in-situ retorting of oil shale. Plain-carbon steel is not an oxidation-resistant material and is not suited for high-temperature use. The extensive oxidation clearly showed that the steel was used at much too high a temperature (generally not suited for temperatures above about 900exp 0 F) for excessive time periods. The highly corrosive atmosphere of the gases produced by the in-situ combustion of shale oil requires a chromium-rich alloy such as ferritic or austentic stainless steels,both of which are considerably more expensive than plain-carbon steel. The only alternative to upgrading the material would be to use an aluminum coating. Various coatings are suggested. (ERA citation 04:045936)