Treating Bacteria Related Problems
Water can accelerate the rate at which metals corrode. The presence of bacteria in the formation or in other areas of the production system can promote corrosion as well. The problems associated with bacteria are not limited to corrosion. Bacterial infestation can affect any water-handling system even if corrosion is not readily apparent.
The corrosion of steel by bacteria is not new and has been studied since the 1940’s
- 1 What are bacteria and where do they originate?
- 2 Are all bacteria the same?
- 3 What are some indications that bacteria is present?
- 4 How does MIC differ from other types of corrosion?
- 5 Are bacteria more common in specific types of systems?
- 6 What do bacteria eat?
- 7 What are acid-producing bacteria?
- 8 How do sessile bacteria and planktonic bacteria differ?
- 9 What is a biofilm?
- 10 Where do biofilms form?
- 11 What are the SRBs mentioned earlier?
- 12 How do I determine if there are bacteria in the system?
- 13 Planktonic Sampling Techniques
- 14 What is serial dilution?
- 15 Once you know there are bugs in the system, what can be done?
- 16 What kinds of products are used to treat bacteria?
- 17 How do biocides work?
- 18 How are biocides applied?
What are bacteria and where do they originate?
Bacteria are living organisms that are naturally occurring and can be found in a very wide range of conditions. In nature, some types of bacteria can be found at the bottom of 10,000-foot deep trenches in the oceans, near volcanic areas where water would boil if it were not for the extremely high pressure (thousands of psi). More common sources of bacteria are surface water, such as river or pond water is used for injection, and in soil as well.
Are all bacteria the same?
Bacteria in the oilfield are typically classified by their interactions with oxygen, or their respiration methods, and the temperatures at which they can live:
- Aerobic – Requires oxygen in high levels to live
- Microaerophiles – Require very small amounts of oxygen to live
- Anaerobic – Absence of oxygen required for growth
- Facultative anaerobes– Can tolerate oxygen in small amounts
Aerobes are most-often found on the surface, in surface vessels, tanks, and other open systems where dust and surface water can contaminate fluids and equipment. Generally, aerobic bacteria cause the formation of slimes or other biomass that has a foul odor. Some aerobes produce acid as a by-product of growth that also causes corrosion. Anaerobes are generally found in the well. They may be in the formation, and can also be found on tubing and in production equipment at the surface. Some anaerobes, such as Sulfate-Reducing Bacteria (SRB) can be found in surface waters because they are present in anaerobic soils. They can be blown as dust into tanks and other equipment. As a by-product of their growth, anaerobes can also produce acids, which can be very corrosive. SRB also produce H2S as a by-product of their metabolism. And, as their name implies, SRB reduce sulfate.
- < 68°F – Psychrophiles (cold-loving bacteria)
- 68°F – 113°F – Mesophiles
- 113°F – Thermophiles (heat-loving bacteria)
While temperature can be a limiting factor with some bacteria, they can be found in wells with fairly high temperatures. It is not unusual to culture bacteria from wells with temperatures exceeding 200°F.
What are some indications that bacteria is present?
Just because you have bacteria in the production system doesn’t always mean you have a problem. When bacteria IS a problem, however, it can cause problems such as:
- Plugging of water injection or disposal wells
- Fouling of producing wells
- Degradation of polymer or coatings in tubular goods and other equipment
- Souring – increasing levels of H2S in a previously sweet system
- Increased or new tendencies of emulsion stabilization
There’s no question, however, that corrosion is the chief concern when bacteria are present. Corrosion caused by bacteria is called microbially-induced corrosion, or MIC for short.
How does MIC differ from other types of corrosion?
Technically, the corrosion mechanism is the same, however, MIC may exhibit a different appearance from other types of corrosion.
Bacteria corrosion usually can be found in localized areas, such as the in the pits of the tubing depicted below.
Are bacteria more common in specific types of systems?
Environment is a key factor in determining whether bacteria can exist in a specific system. These factors can determine whether a specific type of bacteria is found in a particular location.
These environmental factors must not be overlooked when evaluating a system to determine if bacteria are a problem. Suffice it to say, bacteria are everywhere.
What do bacteria eat?
Since bacteria are living organisms they do require a food source. Their nutrition usually comes from organic material such as acetate, which is found in produced or natural waters. Bacteria may require nutrients from phosphorus sources as well.
What are acid-producing bacteria?
Many bacteria produce organic acids as a byproduct of their growth cycle. These acids can include acetic (vinegar), propionic, formic, and other more complex organic acids. Remember, acid can accelerate the corrosion rate of metal; therefore, these bacteria can greatly increase the corrosivity where they are growing. Also, acid production is a direct cause of pitting that can occur underneath bacteria colonies.
How do sessile bacteria and planktonic bacteria differ?
These terms indicate whether the bacteria are free-moving or attached. They do not identify a specific variety of bacteria. Regardless of variety, Planktonic means bacteria that are free floating in the fluids (like plankton in the ocean) and Sessile means the bacteria are attached to a surface.
Planktonic bacteria spread through the system. Typically, they are not a problem until they either 1) land and become sessile; or, 2) grow to numbers large enough to cause plugging or other fouling in the system.
Once bacteria are sessile and attached to the surface of pipes or vessels, they become potential bacterial corrosion problems. Sessile bacteria can grow and produce acids. If they are SRBs they produce H2S, which can be very corrosive to the surface upon which they’re attached.
What is a biofilm?
A biofilm is a colony of bacteria residing in a porous organic material that transports nutrients into the colony and waste products out from the colony. A biofilm is formed in stages as depicted in the graphic below.
Where do biofilms form?
Biofilms will form wherever conditions are suitable, and that could be anywhere. Some examples are:
- In a closed valve
- Shut-in well
- Dead leg in a pipeline or flowline
- Bottom of vessels and tanks
- Underneath solids, such a corrosion products, sand, or other debris
- At the oil/water interface in separation vessels
What are the SRBs mentioned earlier?
SRBs are sulfate-reducing bacteria. These bacteria produce H2S that reacts with iron to form iron sulfide (FeS). As the bacteria remove sulfate from the water, they produce H2S. Through a complex reaction with the metal surface, the metal is corroded and generates iron sulfide. Whenever there are signs of iron sulfide, you should attempt to either confirm or rule out the possibility of SRBs. SRBs in a system under your care should always get your attention!
How do I determine if there are bacteria in the system?
Methods for detecting bacteria in a system vary according to the type of bacteria. All methods require a sample of some type from the system.
Planktonic Sampling Techniques
Planktonic bacteria are typically the easiest bacteria to find in a system because they are moving and, consequently, are distributed throughout the water. The process for detecting planktonic bacteria requires a sample of water from the system. This can be done with part of the water sample drawn for other analyses. When collecting the sample, make sure you use good sampling techniques. In the case of bacteria detection, those techniques include:
- Make sure the sample area closely matches conditions in most of the system
- If the area does not have flow, it may be nutrient-depleted or may not have received bacteria from the bulk fluid in the system. Such locations are not good sample points.
- Sample should be free of contamination such as solids or other debris
- Use clean, sterile container. Contaminated sampling equipment could result in a false positive reading.
- Label properly so you will know where the sample came from, the date collected, and notable conditions at the time of sampling, etc.
Sessile Sampling Techniques
Sampling for sessile bacteria is more involved than collecting a water sample. There is a variety of equipment available for obtaining samples of sessile bacteria.
You also may be able to culture sessile bacteria directly from solids in the system, from contaminated pipe walls, or equipment. Other methods include biofilm probes and screens.
What is serial dilution?
Bacteria samples are taken to determine if bacteria are in the production system. In addition to wanting to know if bacteria are present, however, you also will want to know their strength in numbers. Whereas bacteria in smaller concentrations may signal a problem ”down the road,” concentrations that grow large within a span of hours likely will require immediate action to avert damage to the system.
Serial dilution is the most common method used to determine the bacteria count, or the numbers of bacteria in the production system. The serial dilution process is performed using a series of small bottles that contain a specific medium (sterile liquid containing nutrients specific for growing a particular type of bacteria).
One milliliter of fluid that may contain bacteria is injected into the first bottle in a series. The fluid is shaken, then a new syringe is used to transfer one milliliter of the diluted bacteria to the next bottle. This process is repeated with every bottle in the series. Each “injection step” dilutes the bacteria from the previous bottle by a factor of 10. Using the chart below, you can determine the total number of the specific bacteria in the original water sample by the number of bottles that show growth.
Procedures for conducting serial dilution is included in the appendices of this manual.
Once you know there are bugs in the system, what can be done?
Bacteria can be treated in two ways:
1. Inhibit the development of the biofilm 2. Eradicate the biofilm
Inhibition is accomplished through the use of chemical “biocides,” which kill the bacteria and prevent a biofilm from forming. Eradication involves the removal of the biofilm from the vessel either by pigging or some other physical means.
What kinds of products are used to treat bacteria?
Champion’s biocides are marketed under the trade name BACTRON®. There is a wide variety of organic as well as a few inorganic chemicals used in the oilfield to control the growth of bacteria. These chemicals are generally from one of two categories, “oxidizing” or “non-oxidizing” biocides.
- Oxidizing biocides
– Cl2 (chlorine), Br2 (bromine), chlorinated compounds
- Non-Oxidizing biocides
– Aldehydes – Quaternary amines – Phosphonium quat
How do biocides work?
Different chemicals control bacteria by working against, or attacking, different parts of the bacterial cell. Some products, such as glutaraldehyde, attack the cell wall, while other materials affect the cytoplasm, the material inside the bacterial cell containing many of the microstructures and components required to grow and reproduce. The routes of attack for other chemistries is depicted in the graphic below.
How are biocides applied?
The two types of treatment applications used most are:
Typically, slug-type applications are the preferred method to control bacteria problems. They are often more cost-effective than continuous-type treatments and provide a better kill. A basic slug treatment could consist of dumping a specific quantity of biocide directly into the water holding tank. Another example might consist of a continuous injection over a 6 hour period of a high ppm-level of biocide into the system once a week to control the bacteria problem.
In this method, a selected biocide is injected into the system continuously at an effective ppm range. Continuous injection treatments are often enhanced by switching products periodically to prevent the bacteria from developing resistance to the biocide. There are also other factors to consider when designing or contemplating a biocide treatment. For example, you must have a good understanding of conditions in the system, and you should also be familiar with the outcome you are seeking.
Other considerations include: