Oilfield Corrosion Scales: Composition, Structure and Morphology

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Abstract

Within the oil and gas industry, internal corrosion of oilfield equipment fabricated from carbon steel due to acidification of production fluids caused by the presence of dissolved CO2 (sweet corrosion) and H2S (sour corrosion) is a major concern. Such corrosion can be severe although in some instances solid corrosion product (scale) formed may be protective. The formation, composition and morphology of this scale are contingent upon a variety of parameters such as temperature, pressure and solution chemistry. Scale is often identified simply as iron carbonate (sweet) or iron sulfide (sour), yet in reality can be more complex. The aim of this project is to further understanding of these scales via a fundamental approach building upon previous work from this research group. For the bulk of this thesis high purity iron substrates and 100% CO2 gas or 1% H2S/N2 gas have been used (Ptotal = 1 atm) and scale has been characterised by a number of in and ex situ methods. The initial scale formed by both sweet and sour systems in a variety of temperatures (30°C/55°C/80°C) and pH’s (4.5 (CO2) and ~ 4.9 (H2S/N2)) after 24 h immersion has been characterised. Appreciable amounts of siderite (FeCO3) and chukanovite (Fe2(OH)2CO3) have only been detected for CO2 systems at 80°C/pH 6.8, whilst a thin layer of porous interconnecting mackinawite (FeSm) scale has been observed under all conditions in H2S systems. Subsequently, the effect of immersion duration on sour scale has been investigated. Upon high purity iron substrates immersed in H2S/N2-saturated solutions at 30°C/pH ~ 4.9 and 80°C/pH 6.8 for 24, 48, 72 and 144 h, a porous, interconnecting layer of FeSm scale is observed in all situations. Cross-sections show at 30°C/pH ~ 4.9 scale depth is consistently thin, whilst at 80°C/pH 6.8 depth increases with immersion time. Analysis of grazing incidence x-ray diffraction (GIXRD) data shows whilst at 30°C/pH ~ 4.9 FeSm is consistent with reference data, at 80°C/pH = 6.8 as immersion time increases a second form of FeSm develops as well, exhibiting variation in the c-axis. Elongation of FeCO3 crystals by an organic additive (malic acid) has been observed upon high purity iron immersed in CO2-saturated solutions (80°C/pH 6.8), with the effect increasing with rising additive concentration (0.075, 0.75 and 7.5 mM) up to a point at which solution chemistry provides a scale inhibition effect (15 mM). In addition, a currently unidentified dendritic phase is observed. The habit of the modified FeCO3 crystals has been determined to be a micro-facetted cylindrical form with trigonal-pyramidal caps. Finally, an in situ synchrotron radiation GIXRD (SR-GIXRD) study upon a 1% Ni Weld-joint immersed in CO2-saturated de-H2O (pH ~4) using an electrochemical cell (E-Cell) has characterised the main scaling component to be Fe2(OH)2CO3, with FeCO3 and magnetite (Fe3O4) also observed after 14.5 h immersion. Given the first appearance of each scale varies between distinct weld-joint regions a galvanic effect is suggested, with initial corrosion of the fusion zone creating a local chemistry conducive to chukanovite formation. Laboratory work utilising the E-Cell is consistent but also highlights the sensitivity of SR techniques in comparison to lab data, whilst glass cell work suggests E-Cell usage alters the kinetics of scale formation favourably.

Keyword(s)

Carbon Dioxide Corrosion; Corrosion Scaling; Electrochemistry; FIB-SEM; GIXRD; Hydrogen Sulfide Corrosion; In Situ Synchrotron GIXRD; Iron Carbonate (Siderite); Iron Sulfide (Mackinawite); SEM; Scale Modification; Sour Corrosion; Sweet Corrosion

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