Composition Determination of Corrosion Products from Downhole Tubular of a Sour Gas Well Using new XRD and WDXRF Procedures and Configurations
Abstract
Corrosion study and control of downhole tubing and casing are critical for the economical and safe operation of oil and gas wells. Chemical composition investigation of corrosion products plays a key role in the identification of the corrosion mechanism and the determination of its root cause. In this study, the analytical techniques of X-ray diffraction (XRD) and wavelength dispersive X-ray fluorescence (WDXRF) spectrometry were used to determine the chemical composition of corrosion products formed on the pulled-out-of-hole (POOH) flow coupling and full joints of a sour gas well in Saudi Arabia. The samples were in the form of metal cuts and could not be prepared with the routine method for XRD and WDXRF analysis. These unconventional samples were handled using an adapted XRD configuration: X-ray point focus rather than line focus and an open eulerian cradle. With the new setup, XRD phase identification and quantification were successfully performed. It was found that the outer diameter surface of the pulled flow coupling and full joints consisted mainly of iron oxides whereas the inner diameter surface consisted mainly of iron sulfides. The XRD findings were further confirmed by WDXRF analysis. The findings suggested that the corrosion products were formed under different conditions.
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Introduction
Corrosion is the destructive attack of a material by reaction with its environment [1] and a natural potential hazard associated with oil and gas production and transportation facilities.[2]Almost any aqueous environment can promote corrosion, which occurs under numerous complex conditions in oil and gas production, processing, and pipelines systems.[3]Crude oil and natural gas can carry various high-impurity products which are inherently corrosive. In the case of oil and gas well, such highly corrosive media are carbon dioxide, hydrogen sulfide, and free water.[4] Continual extraction of carbon dioxide, hydrogen sulfide and free water through oil and gas components can over time make the internal surfaces of these components to suffer from corrosion effects. The lines and their component would undergo material degradations with the varying conditions of the well due to changes in fluid compositions, souring of wells over the period, and changes in operating conditions of the pressures and temperatures. This material degradation results in the loss of mechanical properties like strength, ductility, and so on. This leads to loss of materials, reduction in thickness, and at times ultimate failure.[5] Therefore, study and control of downhole tubing and casing are critical for the economic and safe operation of oil and gas wells. In this case study, a sour gas producing well in Saudi Arabia was selected for the investigation. This well was put on producing in 2009 and the production has declined over time. Four metal cuts samples were taken out from the downhole tubular (Table 1) and the corrosion products were analyzed. In order to investigate the exact composition and nature of the corrosion products, the analytical techniques of X-ray diffraction (XRD) and wavelength dispersive X-ray fluorescence (WDXRF) spectrometry were used.
Conclusion
XRD and XRF chemical composition analyses of corrosion samples in the form of metal cuttings require a special instrument configuration. In this study, a special XRD configuration and setup (x-ray point focus rather than line focus, and an open eulerian cradle) were adapted in order to handle these unconventional samples. XRD and XRF techniques have their advantages and disadvantages respectively. Therefore, the combined usage of XRD and XRF is absolutely necessary. Based on the analytical results, it can be concluded that the corrosion products of the tubular inner side were mainly iron sulfides whereas the corrosion products of the tubular outer side were mainly iron oxides, which indicated they were formed in different conditions.