Abstract

Many operators have encountered failures due to MIC in seawater injection systems contaminated with sulphate-reducing bacteria (SRB). In some cases, severe pitting and grooves have resulted in flowlines being abandoned or replaced. On the other hand, there are reports of little or no significant MIC in some systems, despite significant contamination with SRB being confirmed. As all the systems are of similar design, transport seawater and employ organic biocide treatments, it may sometimes seem that incidents of MIC are a case of luck rather than judgement.

A qualitative model is presented which predicts when conditions within the system will allow MIC to proceed. The model involves two phases. Firstly a biotic phase which predicts the time required for a sulphide film to develop. Secondly, an abiotic phase which considers three parameters that may stimulate the initiation of MIC pits. The model can be made semi-quantitative by introducing a constant for pitting rate. The ability to predict (even if only qualitatively) the probability of MIC under a given set of circumstances (e.g. different biocide applications, pigging programmes, etc.) during the planning stage of a seawater flood project could significantly improve the development of control strategies.

It is anticipated that the development of such models could allow a corrosion management system to predict and monitor the mitigation of MIC risk. Currently, the use of MIC risk assessment models is not widespread and most MIC monitoring, at best, estimates biocide efficacy only. Whilst biocide efficacy monitoring is an important control, it is not corrosion management. The model allows for ongoing review of the MIC risk once monitoring data is available.

INTRODUCTION

It has been an almost universal standard practice to apply biocide treatments to the seawater injected into oil reservoirs for pressure support. There are multiple requirements for such biocide additions. These include the control of marine macrofouling (hard and soft bodied flora and fauna); the control of microbiofouling (biomass) in deaerators and associated residence vessels; and, perhaps most widely reported, the control of growth and activity of sulphate-reducing bacteria (SRB). The sulphide produced as the result of SRB activity is implicated in the economically negative problems of Microbially Influenced Corrosion (MIC) and reservoir souring.

In 1997, a Joint Industry Project (JIP) investigating groove corrosion in North Sea water injection pipelines included analyses of 23 sub-sea flowlines. At that time 9 (39%) had already failed due to grooving along the 6 o'clock position resulting in at least one (if not multiple) perforations in each flowline. All the failures occurred within 4 to 15 years of operation, well within the original design life (typically 20 - 25 years). Corrosion rates were typically 2 mm per year. Only one of the 23 flowlines had lasted more than 15 years [1]. It was concluded that the mechanism which resulted in the grooves was MIC mediated, the location of the corrosion being related to the presence of deposited material in the bottom of the flowline [2]. In all cases, regardless of the biocide treatments applied at the time, significant SRB activity in biofilms was confirmed on the internal surfaces of the flowlines.

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