The general conclusion is that the removal of oxygen from injection water is usually worthwhile, but the removal of hydrogen sulfide is much more expensive and more difficult to justify. Only in isolated instances has it been found necessary to remove carbon dioxide where there is no hydrogen sulfide.
During the past 15 years the oil industry has become more and more aware of the importance of corrosive gases in injection water in the economics of secondary recovery of oil.
Experience indicates that removal of hydrogen sulfide to a level of 1 to 2 ppm in the absence of oxygen reduces corrosion to acceptable rates at a neutral pH and that reduction below 1 ppm is possible with existing equipment. If oxygen is reduced to 0.1 ppm in sweet water or brine at a neutral pH, corrosion will be reduced to a negligible rate, but the oxygen must be reduced still more in a system containing small amounts of sulfide. The principal effect of carbon dioxide in water is the reduction in pH, with resultant solubilization of iron. Until recently, however, removal of such gases has not gained the interest of the industry at large. Several successful operations followed by published papers have aroused interest to the extent that a review of the means available appears to be in order.
For many years corrosion due to oxygen in water has been accepted as a necessary evil. It was thought that the water itself was the corrosive medium; therefore, subsurface water was permitted to come in contact with air as it was pumped to the surface and stored in open tanks.
With present knowledge, the prudent operator cements his water-supply well to the surface, seals the wellhead and tests to determine whether dissolved oxygen is native to the water. If it is not, and if oxygen-free natural gas is available, a gas blanket is placed on supply wells and tanks. Often this is the only action necessary.
If there is dissolved oxygen native to the water, however, some other action must be taken for the following reasons.
Pitting-type corrosion, typical of an aerated system, can cause failures even in a fresh-water system and much more rapidly in a brine system.
Iron oxides and hydroxides are serious plugging agents that will increase injection pressures and decrease injection rates. These compounds may result from corrosion or from a mixing of one water containing oxygen with another containing dissolved iron.
Aerobic bacteria, algae, and slime form in the system, create plugging, and enhance pitting.
If an oxygen-bearing water is mixed with a hydrogen-sulfide-bearing produced water, free sulfur will be precipitated.