The production of gas and oil is often accompanied by water, either from the formation, from condensation, or from water injected as lift assist. Acid gases, such as hydrogen sulfide (H2S) and carbon dioxide (CO2) are often present in produced fluids, and oxygen is sometimes a contaminant in the water used for injection. These acid gases increase the corrosivity of the waters to steel, and can significantly reduce the safe operating life of production tubulars and equipment, production vessels, and transportation systems.
The presence or absence of multiple phases (gas, water, and oil or condensate) in the same system can complicate the problem of controlling corrosion. The flow regime or pattern of fluids in a tubing string, vessel, or pipeline can have a significant impact on corrosivity. If a well or pipeline experiences "slug" or intermittent flow, highly corrosive conditions may exist where the pipe is in slug flow.
Pipelines can experience top-of-line corrosion when conditions promote the rapid condensation of water in a cooler section of the line, causing a film of water to form at the top of the line. This water at the top of the pipe becomes saturated with acid gases and corrodes the pipe. A further complication is a change in conditions, such as flow rate, temperature, and pressure over the life of a well, production or processing system, or pipeline, which can result in changing corrosivity or even a change in the potential corrosion mechanisms.