Sulphate Reducing Bacteria (SRB) are one of the oldest and most widespread bacteria on the planet. SRB inhabit nearly every conceivable location where there is no oxygen (anaerobic conditions), including the depths of the ocean, the human stomach, moist soil, hot water services, and sewerage pipeworks. They also inhabit aquifer systems, coal seams, gas well drill rigs and associated infrastructure, including drill strings and mud tanks. In most cases the population of naturally occurring SRB is controlled by the restricted availability of a food source. However, if a food source is introduced via drilling fluids and concrete casings, for example, the SRB population increases and with it problems for gas wells.
Sulphate Reducing Bacteria - the driller's nemesis
SRB can work fast or slowly and are non-selective with regards to the source of their “food”, creating problems well into the future for gas well infrastructure and any other non-related underground infrastructure, such as town water bores and pipelines.
Well drillers have long known of the damage caused by SRB. These anaerobic bacteria grow on organic compounds found in water contaminated with hydrocarbons and organic material. They convert sulphate into hydrogen sulphide (rotten egg gas). The presence of hydrogen sulphide not only reduces the commercial value of natural gas, but also rapidly corrodes pipes, tanks and other iron and steel structures.
Hydrogen sulphide from SRB also plays a role in the biogenic sulphide corrosion of concrete. The bacteria feed on the sulphates in concrete and rapidly multiply While SRB can be controlled to some extent in the produced water from the coal seams by the use of biocides [ii] , they cannot be controlled in the natural aquifer system. Once in the aquifer, SRB become a very real threat to the longevity of the outer concrete casing of a gas well, as the SRB convert sulphides in the cement to food. The effectiveness of the concrete outer casing to prevent cross contamination of groundwater is thereby severely compromised. That is, low quality, eg saline, water from one aquifer could contaminate high quality water in another. The loss of this concrete seal can also result in the residual gas from the coal seam escaping to the surface and adding to climate warming.
When a gas well is drilled there is a certain amount of aquifer/drilling fluid (mud) interchange, as mentioned in all of the Eastern Star Gas Reviews of Environmental Factors and in the earlier Santos REFs for PEL 238. (Santos bought out Eastern Star Gas and its NGP in 2011). As the drilling fluid pressure is always greater than the pressure in the aquifer, drilling fluid is lost to the aquifer. The organic lignite, lignin, tannins, cellulose, starches, and fatty acids found in many mud systems are carbon food sources for SRB. These muds can also harbour SRB.
Biocides are almost ineffectual in controlling SRB in aquifers because the water/biocide dilution rate is too high. By introducing drilling muds to the aquifers, the gas industry is in fact actively encouraging SRB. While this is well known in drilling circles, it is an issue which thenCSG industry and government regulators never mention, as it one reason why rehabilitated well integrity can never be guaranteed.
In August 2018, new technology was introduced into Queensland to patch severe and localised external corrosion in well casings. According the company Saltel “it is suspected that this is caused by bacteria growth…. And that it is systemic in the region”.
What does this mean? Santos aims to seal wells at the end of their productive life by pumping full of concrete. However, the outer concrete casing will always be subject to attack by sulphate reducing bacteria, allowing cross-contamination of aquifers and possible escape of residual methane to the atmosphere, possible leakage of dangerous chemicals and the possible mixing of salty aquifers with fresh water systems.
For further information contact Graeme Sawyer Ph 0411 881 378 email@example.com