• Eroini, Violette
• Oehler, Mike Christian
• Lønvik, Kari
• Graver, Britt Kathrine
• Skovhus, Torben Lund
• Mitchell, Anthony

Abstract

Increasing incidence of amorphous deposits in both production and water injection systems has caused considerable problems for offshore oil fields. Amorphous deposits, which are a widely recognized, but often poorly explained phenomenon, are typically comprised of both organic (biological or hydrocarbons) and inorganic material, but with compositions that vary considerably. One recurrent form of deposits, found in offshore water injection flowlines and wells, consisting mainly of magnetite as the corrosion product, was further investigated with the objectives of explaining its formation and assisting in prevention or remediation. It is proposed that the deposit formation, observed in offshore water injection systems treated with nitrate, is initiated by formation of a nitrate reducing biofilm promoting under deposit corrosion by activity of sulphate reducing and methanogenic prokaryotes, this in turn generating iron hydroxide and green rusts which are then mineralized through biotic or abiotic mechanisms to magnetite. This paper reviews current observations from the offshore oil fields and presents the potential biotic and abiotic mechanisms to magnetite formation. INTRODUCTION Fouling, composed of both organic and inorganic compounds, has caused concerns within operating assets due to the detrimental effect on production and injection, in addition to challenges with intervention and integrity. The variety of deposits and poor understanding of their nature has led to confusion and sometimes inappropriate treatment. Initial work, undertaken to classify the different substances encountered, has been previously reported.1 Systematic analysis allowed the development of a classification matrix intending to describe similar material in terms of their major components. The objective was to clarify confusion related to these deposits, variously described as “Schmoo” or “Black Sticky Stuff” and provide the industry with tools to help with identification and mitigation. In addition, knowledge of general formation mechanisms has been gained. A simplified version of the classification matrix is presented in Figure 1. From this study, attention was drawn on one particular form of amorphous deposit of type 1 A (corrosion products and micro-organisms) which has been regularly reported in seawater injection systems. The material is composed of biomass and the crystallized form of iron oxide - magnetite. Functional issues associated are mainly equipment impairment which threatens integrity, disruption of intervention and loss of injectivity. This paper focuses on understanding the potential mechanisms by which this offshore oilfield deposit forms, to assist prevention and mitigation (onepetro.org).

Topics

• impedance spectroscopy
• compound
• amorphous
• corrosion
• iron