Oilfield Corrosion: Sweet Corrosion Scales

Abstract

Internal corrosion of oilfield pipelines, fabricated from carbon steel, due to dissolved CO2 remains one of the major problems encountered in the oil and gas industry. Under specific conditions, CO2 corrosion, more commonly known as sweet corrosion, can result in the growth of corrosion scale on the inner pipeline walls. Such scale can lead to a considerable reduction in the corrosion rates, if it is densely packed and well adhered to the surface. The work in this thesis aims to improve understanding of the development of CO2 corrosion products and how different factors can influence the formation and evolution of the formed scale. Motivated by the recent identification of an unexpected cylindrical habit for the primary sweet corrosion scale component, siderite (FeCO3), effort has focused on examining habit variation as a function of environment. Initially, scales formed on both corroding Fe substrates, and non corroding PTFE substrates, after immersion in CO2-saturated salt solution were explored. The siderite crystal habit was found to change as a function of Fe2+(aq), varying from rhombohedral to cylindrical as the the amount of Fe2+(aq) is increased in solution. These results are consistent with theoretical modelling of the FeCO3 crystal habit by collaborators from Imperial College, and may be the basis of alternative approaches to corrosion control (e.g. habit engineering). Besides, the addition of Ca2+ resulted in formation of FexCayCO3 solid solution, while addition of the Mg2+ resulted in a formation of rhombohedral siderite with the appearance of un-identified elongated rods. The siderite crystal habit on Fe substrates was further explored as a function of increasing CO2 partial pressure and temperature by developing and implementing an autoclave facility for high pressures and high temperatures (HPHT) testing. An increase in CO2 partial pressure from 0.5 to 4 bar was found to change the siderite crystal habit from cylindrical to rhombohedral. This observation is also found to be consistent with the theoretical modelling of siderite crystal habit, and demonstrates that crystal habits are likely to vary as fluids progress through an oilfield facility. Finally, an improved design of a custom built cell for in situ synchrotron grazing incidence X-ray diffraction was employed to gain insight into sweet corrosion scale evolution as a function of time and temperature. Electrochemical data and in situ GIXRD diffractograms reveal that siderite is mainly responsible for the reduction in corrosion rate by forming a protective scale on Fe substrate immersed in CO2-saturated water (buffered to pH = 6.8, T = 80 C, PCO2 = 0.5 bar). Furthermore, a temperature excursion results in scale dissolution as the temperature decreases to 25 C, and scale regrowth with the increase in temperature back to 80 C, suggesting that local variations in temperature may be detrimental to scale properties.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Dirk Engelberg (Supervisor) & Rob Lindsay (Supervisor)