La véritable nature de la corrosion par CO₂ des aciers dans les puits de pétrole et de gaz. Les mécanismes correspondants

The very nature of the CO₂ corrosion of steels in oil and gas wells, and the corresponding mechanisms

Société Nationale Elf Aquitaine (Production)

Abstract

The laboratory studies which have been published so far on the CO₂ corrosion of steels where always focused on processes of uniform corrosion in solutions of NaCl and carbonic acid. A careful analysis of the corrosion damage in the field shows on the contrary that the CO₂ corrosion is basically a type of localized corrosion, and that, most often, it cannot be ascribed to flow effects.

The need for prediction of the CO₂ corrosion is also analysed in the context of the present industrial situation. Thus, it is shown that the key point is to predict why and when a severe localized corrosion will occur in wells.

Mechanisms of localized corrosion are then proposed. They are first checked in the laboratory. In particular, some galvanic couplings between areas with a different coverage of carbonate layers (FeCO₃/CaCO₃) have been shown to be strong, autocatalytic and stable.

These mechanisms are also shown to be consistent with all that which has been reported about CO₂ corrosion in the fields. They especially emphasize the role of the water chemistry, and they explain why CO₂ corrosion is not systematic in CO₂ wells, and why it is suddenly so severe when it occurs.