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.