The injection of fluids into formation has a potential to introduce bacteria to the reservoir. This concerns any injected fluid, which might be contaminated with bacteria, and in particular, fracturing gel, which are commonly a good source of nutrition for bacteria growth. Bacteria in the reservoir can create severe problems. Slime forming bacteria can multiply in such numbers that they reduce permeability by blocking the pore throats and, consequently, damage the formation. Even more damaging are certain types of anaerobic bacteria which produce hydrogen sulphide H2S, creating a safety hazard. Hydrogen Sulphide also corrodes tubulars and production equipment. Bacteria can also form in surface equipment given the correct conditions, and therefore if batch mixing or storing fracturing fluids, the addition of biocides should be considered and tested to ensure fluid characteristics remain unchanged.