ABSTRACT
Corrosion products could have a significant impact on the corrosion process of carbon steel in H2S environment. It is widely believed that the different iron sulfides have different effects due to their distinct physicochemical properties. Therefore, an in-depth knowledge on the formation of these polymorphous phases is of virtual importance for the understanding of sour corrosion process.
Many studies have been performed to determine the key factors, such as temperature, solution pH, H2S partial pressure, and the duration of exposure, in the formation of polymorphous iron sulfides; yet their formation conditions, stability, and phase relations are still not fully understood. This is partially due to the complex nature of sulfide and iron chemistry and their sensitivity to oxygen. Seemingly minor changes in test conditions can often lead to dramatically different results. Besides, laboratory tests were often performed under conditions less complicated than actual field systems.
In this paper we present a study on the inorganic deposits formed on downhole tubulars of some high H2S gas wells in Saudi Arabia. Results show a wide range of mineral phases and significant variation among the samples analyzed. The polymorphous iron sulfides include pyrrhotite, pyrite, marcasite, troilite, mackinawite and greigite, in the order of abundance. Ferric iron compounds, such as akaganeite, goethite, lepidocrocite, hematite and magnetite are also identified in some samples analyzed. In addition, ferrous iron products, especially siderite, and calcium carbonate are often detected. Based on the deposit structure analysis, the formation mechanisms of these different types of minerals are discussed. It is hoped that the results from this work will contribute to further understanding of the sour corrosion process, and provide value for corrosion and scale mitigation in oil and gas fields.
INTRODUCTION
Iron sulfide (FeS) deposits are ubiquitous in sour oil and gas production systems. Corrosion of carbon steel in waters containing H2S almost always leads to the formation of FeS precipitates on equipment surface. Such layers play an important role on corrosion processes by controlling both general and localized corrosion rates. It is generally believed that a dense and continuous FeS layer can be very protective for long period of time by acting as a barrier for mass transport and thus reducing the corrosion rates. A porous and cracked FeS layer may result in localized corrosion attack by establishing galvanic cells between the steel and the iron sulfide deposits.
