Commonly used oilfield biocides

The following is a summary of the nonoxidizing biocides that are most commonly used in all aspects of oilfield operations. For each biocide, the structures, mechanism of action, and compatibility concerns will be discussed. It should be noted that all oilfield biocides are reactive chemicals that should always be handled with extreme care and in accordance to the manufacturer s instructions on the material safety data sheets (MSDS). 

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Compatibility with other system additives. This is another important requirement for the injection water biocide. The most common concern about chemical incompatibilities is an interaction between the biocide and the oxygen scavenger. Most commercially available biocides for oilfield use will react with the bisulfite based oxygen scavengers and become deactivated. This interaction will be discussed in greater detail in the discussion of the commonly used oilfield biocides. 

General information. Aqueous solutions of formaldehyde are generally used both as biocides and as hydrogen sulfide scavengers in the oilfield. It is very common for formaldehyde to be used in combination with quaternary ammonium compounds, glutaraldehyde and THPS (see 5.4.7.). Solutions of formaldehyde tend to be inexpensive (relative to the other commonly used biocides), so it continues to be widely used. 

Compatibility concerns. THPS is not compatible with oxidizing biocides, but is compatible with most other commonly used nonoxidizing oilfield microbicides such as QACs or glutaraldehyde. THPS is more stable under anaerobic conditions than aerobic conditions. This is due to its tendency of the THP molecule to be oxidized to the phosphine oxide in the presence of oxygen. It works faster in alkaline systems than acidic systems, since the active THP is formed faster under alkaline pH s. In anaerobic oilfields, THPS is extremely stable and has been observed to travel through a formation and reemerge in the produced water (Bryan et al, 1995). Also, being the sulfate salt, solutions of THPS have been observed to react with divalent ions such as calcium and barium in oilfield produced waters to form insoluble calcium sulfate and barium sulfate precipitates. 

Preservation of hydrotest fluids. The use of biocides in hydrotests presents some unique challenges to the oilfield service companies. They have to balance the need to protect the pipeline from microbial attack, while also being concerned about the potential of negative environmental effects that may arise for a spill or discharge of biocide-containing water. The choice of a hydrotest biocide depends mainly upon the length of time the water will be kept in the pipe, the environmental fate profile of the biocide, and also whether the biocide can be easily deactivated. The most commonly used biocides in hydrotests are glutaraldehyde, THPS, and quaternary ammonium compounds. 

Biocides are used in oilfields for primarily two reasons to preserve water-based fluids that are used in various oilfield operations, and to control biofouling. The fluids that are used in oilfield operations are varied in composition and properties, but require the use of a biocide to prevent the degradation of important components of the fluids. Biofouling control, in this case, refers to controlling the growth or proliferation of microbes that can potentially be harmful to the operation of the oilfield, or to the value of the hydrocarbon product that is produced. Both applications for biocides have in common the goal to limit or control the introduction of bacteria into the oil-bearing formation. 

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