All water-bearing systems have the potential for microbial
contamination. In general, biocides are applied to oilfield
systems to control microbially influenced corrosion (MIC)
and alleviate the problems associated with by-products from
bacterial respiration e.g. poisonous gases, inorganic and
organic acids, slime and scales such as iron sulphide. Bacteria
can survive in one of two phases, planktonic phase (suspended)
or in biofilms (attached to a surface).
Frequently, oil-producing systems experience
severe problems due to the growth and proliferation of bacterial
populations. The three most important genera of bacteria in
oilfield systems are listed below with an outline of the main
problems associated with their growth and proliferation:
Sulphate Reducing Bacteria
- Microbially Influenced Corrosion (MIC)
by cathodic depolarisation of metal surfaces resulting
in removal of the atomic hydrogen layer and the formation
of localised pitting.
- Produce hydrogen sulphide as a by product
of their metabolism which poses a significant risk to
health and safety and can result in the formation of iron
sulphide deposits.
- Due to their metabolic requirement for
sulphate, SRB are commonly found in water injection systems
(downstream of deaeration), production systems with seawater
breakthrough and crude oil / water storage vessels.
Acid Producing Bacteria
- APB are also implicated in the corrosion
process as they metabolically secrete organic and inorganic
acids which can become trapped under bacterial biofilms
and promote corrosion by the removal of the passivating
oxide film.
Slime Forming Bacteria
- Potential to produce large volumes of exopolymer
ideal for biofilm formation.
- Slime production can result in fouling
and blocking of filters, lines, injection pores etc.
- Establishment of oxygen concentration cells
that can lead to under deposit corrosion and promote an
ideal environment for SRB’s.
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Bacterial
Control
