Reports of corrosion failures implicating bacterial activity - and in particular the activity of Sulphate-reducing Bacteria (SRB) - continue to appear in the literature and documented case histories.In many cases, the Microbially Influenced Corrosion (MIC) has occurred despite the application of biocide chemicals into the affected system.It is not uncommon to hear of ‘ineffective biocides’, the implication being that the efficacy of the chemical to kill bacteria is questionable.It is clear, however, that the most commonly applied generic formulations of oilfield biocides (such as Glutaraldehyde, THPS, etc.) are effective killers of bacteria.If the field application of these chemicals does not result in acceptable microbiological control then the inference should be, not that the chemical is an ineffective biocide, but that the application of the chemical is such that it is not contacting the target bacteria with sufficient concentration, retention time and/or frequency to achieve a kill.
This paper presents an overview of how oilfield biocides can be applied such that those bacteria with a role in corrosion are controlled to sufficiently low levels to inhibit MIC.The emphasis is on seawater injection systems, where the majority of the total tonnage of oilfield biocide is applied.However, the key parameters are also relevant to oil production systems.
In order to be presented as an oilfield biocide, proposed chemical formulations require the results of laboratory testing to clearly demonstrate that they can affect a significant kill of a target population of bacteria.A significant kill is often considered to be a 5 Log10 reduction in viable planktonic bacteria numbers by a single treatment.The lower the concentration and/or exposure time required to achieve this reduction in numbers might be considered to reflect the potency of the biocide.However, despite favourable laboratory data, the application of the biocide in the field often does not achieve the expected, and required, results.
Historically, oilfield biocide efficacy tests concentrated on planktonic (free floating), marine bacteria in suspension.These tests indicated that moderate concentrations of active ingredient (in the order of 50 ppm w/v) in contact with a bacterial suspension for a finite period (say 4–6 hours) were capable of achieving a complete kill of the target bacteria (Figure 1).During the 1970's and 1980's, therefore, a commonly applied biocide treatment for seawater injection systems in the North Sea was typically 200 ppm v/v of 25% glutaraldehyde solution dosed once per week for six hours .As the systems were of similar design, it was not thought unreasonable that a common treatment strategy could be applied.Strangely, however, similar concentrations and residence times were also thought appropriate as treatments for production systems.
It took several years before it became apparent that such treatments were incapable of controlling the corrosion activities of SRB in these systems.The inability to determine the poor efficacy of biocide treatments before corrosion failures occurred was due to a number of factors, including:
Lack of adequate bacterial monitoring
Improper interpretation of bacterial results
Emphasis on laboratory results rather than field data
A lack of knowledge of the role of biofilms in MIC
Low priority for MIC issues
Scarcity of microbiological field expertise