The US oil and gas supply has been increased significantly due in large part to the recent increases in production from shale fields. However, this development has also presented unique challenges with respect to materials integrity and corrosion issues. In this paper, two recent case studies of failed pipe fittings recovered from an onshore shale field are described. A perforated tee and pipe nipple retrieved from shale production systems were analyzed to determine the precise mechanism(s) that contributed to their corrosion and subsequent failure. It was determined that both fittings experienced localized corrosion due to exposure to carbonic acid and dissolved hydrogen sulfide. Mitigation strategies were developed to minimize the potential for similar failures in the future.

INTRODUCTION

Carbon dioxide (CO2) is routinely encountered in oil and gas production. Much research has been done and compiled on this damage mechanism since the 1940s with the nomenclature changing over the years: condensate well corrosion, organic acid corrosion, carbonic acid corrosion, sweet corrosion, and mesa attack.1 It depends primarily on three factors: pH (CO2 partial pressure), temperature, and flow rates. At low pH values (<4), high values of uniform corrosion are observed since the FeCO3 scales are loosely adhered to the surface and not protective. At higher pH values (>5), corrosion rates were found to be flow dependent and more or less independent of pH. At high pH and high temperatures, the metal surfaces are protected by stable FeCO3 scales. In the intermediate pH range, the FeCO3 scale is semi-protective, which means localized corrosion is possible. The presence of acetates and acetic acid dissolves these protective scales, causing higher corrosion rates. The presence of sulfides, on the other hand, tends to lower corrosion rates because FeS scales are more stable than FeCO3. Addition of 0.5–1% chromium to the carbon steel can lead to reduction in mesa corrosion rates. Proper metallurgy and microstructures also play a role; with normalized steels having a ferritic-pearlitic microstructure providing a better anchor pattern for the protective films compared to quenched & tempered steels that have a martensitic microstructure. Preferential dissolution of ferrite lamellae within pearlite colonies and the anchoring effect of the Fe3C layers, lead to ease of formation of protective carbonate scales.

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