Bactericides, also known as biocides, are additives added to drilling fluids to kill bacteria. Natural starches and gums present in water-based muds are prone to bacterial attack and require bactericides. Bactericide choices are limited due to its toxicity, and only those can be used that are approved by the government.
The addition of bactericides prevents the microbial degradation of organic additives in drilling muds and completion fluids. It also suppresses the formation of hydrogen sulfide and works by destroying, preventing, or controlling the effect of microorganisms or bacteria present in the drilling fluid. They are biodegradable and effective against aerobic and anaerobic bacteria.
Bactericides are added to drilling fluid to prevent the microbial degradation of organic additives and suppress the activity of sulfate-reducing bacteria that produce hydrogen sulfide.
Bactericides are also used to control slime-forming bacteria, iron-oxidizing bacteria, and bacteria that attack polymers. The purpose of bactericides is to kill living organisms present in the drilling fluid and it can be assumed that it may be harmful to other living organisms living in the ecosystem. Since bactericides can be harmful to the environment, regulation of their use and disposal is stringent.
The Environmental Protection Agency (EPA) requires bactericides to be registered with the agency and their effects on the environment, treating dosages, and efficacy to be verified for registration. Drilling fluid is an important part of trenchless drilling, and different additives, including bactericides, are added to make the right mix suitable for the successful completion of a particular project.
Trenchlesspedia Explains Bactericides
Drilling mud is a mixture of natural and synthetic chemical compounds that function as lubricant and carrier. Drilling mud is mixed with appropriate additives such as bactericides,corrosion inhibitors, fluid loss, and viscosity control agents, etc. in a water base or oil base depending on the formation through which the bore will pass.
The addition of additives provides essential characteristics to the drilling fluid such as filtration and suspension. A good drilling fluid mix will also prevent inadvertent returns, heaving, or settlement of ground surface and failure during pullback. The EPA has strict laws for environmentally sensitive locations for drilling fluid disposal, and carelessness can incur heavy penalties for contractors and owners.
Need for Bactericides
Contamination of drilling fluid due to the presence of bacteria can cause many problems. Drilling muds contain sugar-based polymers in their formulation that is a good food source for bacteria. Some problems that can be caused by mud degradation due to the presence of bacteria are:
- Generation of deleterious products such as hydrogen sulfide.
- Formation of problematic solids like iron sulfide.
- Corrosion of drilling hardware.
- Increase in fluid loss due to degradation of polymers used for fluid loss and viscosity control.
In water-based and low-solid muds, organic material is subject to attack by microorganisms introduced with the water. Aerobic bacteria, if present, converts organic molecules to cell structure, carbon dioxide, and water. This results in the reduction or destruction of additive properties and upsets the mud's rheological properties.
Microbial organisms such as sulfate-reducing bacteria grow in anaerobic conditions and derive energy by reducing sulfates and consequently producing hydrogen sulfide. This type of bacteria is widely found in nature and can be unknowingly added to the drilling fluid. When water contaminated with this bacteria enters the producing formation, the bacteria can grow and produce corrosive amounts of hydrogen sulfide.
Preventing Bacterial Attack
A fast-acting biocide is required to treat water against bacterial contamination. Usually non-cationic bactericides – aldehydes and phenols are used in drilling muds which are effective against aerobic, anaerobic sulfate-reducing bacteria and fungi. pH control or control of ionic strength can also be done to control bacterial growth, however, the addition of bactericides is the most effective and common method to achieve the desired result.