Failure Investigation of Capillary Tubing (ASTM 316L) Used for Calcite Inhibition

Abstract

A failed capillary tubing (ASTM 316L) used for injection of calcite anti-scalant chemical was investigated. Results of microscopic, petrographic and SEM EDAX analysis suggest that the capillary tubing was subjected to stress corrosion cracking (SCC). The cracking is preceded by localized pitting corrosion. The local corrosive environment leading to cracking is rich in chloride. Sulfur compounds are also present and there may be a contribution of sulfide to the SCC. The presence of chloride, sulfur and other elements (Ca, Al, Si, K) suggest that the pitting and cracking occurred in contact with geothermal fluids. The results of the investigation do not clearly define the time of cracking. However, there is strong evidence that the pitting occurred in well SP-4D. A moving area of flash zone, which crossed the area of final failure, induced this. Periods of failed inhibition (when the flash point was below the injection point) in SP-4D caused the deposition of calcite in the tubing. The calcite deposits provided crevice areas for the concentration of chloride on the tubing surface where pitting occurred. Final failure of tubing occurred during pull-out of the used tubing from well MO-2. Several mechanisms for SCC in geothermal environment are outlined. Detailed analytical work, history and postulated sequence of events leading to final failure are discussed. Action plans to avoid similar failures in capillary tubings in service to calcite inhibition systems are forwarded.