Implications of Residual Chlorine in Seawater Injection Systems
Low Concentrations of chlorine (less than 0.5 ppm) are effective in maintaining microbiological control in seawater injection systems by rapidly killing planktonic bacteria. Chlorine is, however, a poor penetrant and control of established biofilms is best achieved by the additional use of batch organic biocides when the microbiological monitoring programme indicates that this is necessary. Chlorine is also rapidly inactivated by the oxygen scavengers which are frequently added during deaeration of the seawater. It is possible to install, optimise or uprate deaerator tower performance to produce water with less than 10 ppb oxygen without the need for oxygen scavengers. This allows the chlorine to pass to the injection wellheads to maintain microbiological control throughout the system. It also reduces the requirement for regular, frequent, batch organic biocide treatment. Evidence for the direct corrosive effect of chlorine on mild steel in such systems is somewhat contradictory; whilst chlorine should be as corrosive as oxygen, in field situations, low corrosion rates are frequently found, even with concentrations as high as 1.5ppm. It is likely therefore, that any corrosive effects of chlorine will be significantly outweighed by the reduction in microbial corrosion caused by its biocidal activity.