Biocides, also called “bactericides” or “antimicrobials,” are used in oil and gas production. Their aim is to kill microorganisms, especially bacteria, or interfere with their activity. Microorganisms in oilfields or in injection water are generally classified by their effect. Sulfate-reducing bacteria (SRBs), denitrifying bacteria (hNRB), slime-forming bacteria, iron-oxidizing bacteria, and miscellaneous organisms such as algae, sulfide-oxidizing bacteria (NR-SOB), yeast and molds, and protozoa can be encountered in bodies of water of oilfields to be treated.1 Even carbonate-scaleforming bacteria have been observed in a Middle East field.2 Bacteria can be found in solution (planktonic), as dispersed colonies or immobile deposits (sessile bacteria plus their waste products). Bacteria can utilize a wide variety of nitrogen, phosphorus, and carbon compounds (such as organic acids) to sustain growth. Nitrogen and phosphorus are usually sufficiently present in the formation water to sustain bacterial growth but injection of organic nitrogen-and phosphorus-containing chemicals can increase the growth potential.