Microbiology
Bacteria represents a major cost factor for infrastructure by causing corrosion, sludge accumulation, and undesirable off-gassing.
Categorizing the Ecology
Sulfate‐Reducing Bacteria (SRB) – Anaerobic
Produces hydrogen sulfide, which is corrosive to steel. SRBs form a part of a microbial community (sometimes referred to as a "consortium"). Within these consortia, SRB can function deep within biofilms under both anaerobic (no oxygen) and aerobic (contains oxygen) conditions. Generally, the biofilms are formed within tubercles, encrustations, and slimes. Since SRB are deeper down in these growths, they may not be recovered in water samples taken from flow over the growths.
Sulfate-reducing bacteria fall into over 20 separate genera which can be classified by the types of substrates utilized.
In anaerobic wastewater treatment systems, sulfate reduction accounts for up to 50% of the mineralization of organic matter.
Iron‐Oxidizing Bacteria (IOB) – Aerobic
Convert iron from the ferrous [Fe+2] to the ferric [Fe+3] state and produce ferric hydroxide [Fe(OH)3]. Ferric hydroxide is a highly insoluble by‐ product that will damage formations and plug pipes and equipment. IOBs also produce corrosion, but they are considered harmful mainly because they cover SRB colonies and protect those colonies from mitigation using bactericides.
They often colonize transition zones where de-oxygenated water from an anaerobic environment flows into an aerobic environment. These bacteria use ferrous iron or manganese in their metabolic processes, weakening the metal. Iron-related bacteria can be either metal-reducing or metal-oxidizing.
Acid‐Producing Bacteria (APB) – Aerobic
Recognized as a possible major cause of corrosion, mainly through fermentative activities, which cause the pH (particularly in the biofilms) to drop into the acid range. This is due to production of fatty acids and acetic acid: sulfuric acid, acetic acid, nitric acid and carbonic acid. They are found in a wide range of environments and often co-exist with other bacterial species. APBs can sometime be “first colonizers” that make the environment around pipelines and metal components more inviting for other bacterial species.
BACTERIA GROWTH CONDITIONS
It is important to understand the conditions in which bacteria grow and reproduce to determine how to deal with them in oilfield systems.
• Temperature: Estimated from 14°F to 210°F. Optimum temperature range for SRBs is 77°F to 120°F.
• pH: Estimated from 0 to 10.5. However, the optimum pH range is 5 to 9.
• Total Dissolved Solids (TDS): Bacteria prefer fresh water but can grow in brines. Sea water has a TDS of over 35,000 mgs/L and easily supports bacteria growth. SRB have been reported in brines with a TDS >100,000 mgs/L, but the growth rate is reduced at higher TDS.
• Dissolved Oxygen: Aerobic bacteria require oxygen to grow. Anaerobic bacteria grow best in the absence of oxygen. Facultative bacteria grow in either the presence or absence of oxygen.