Algorithm of the protectiveness of corrosion layers : 2 - Protectiveness mechanisms and H2S corrosion prediction

Part 2 follows the same route as Part 1, by using the large field experience recently gathered on H₂S corrosion. The 50 years of production of the deep sour gas field of Lacq (France) are also analyzed, and this emphasizes the never publicly addressed interaction between the isothermal decompression of formation water and the inevitable trapping of water slugs at the bottom of HP wells.
A new type of profuse and non protective layer is also defined, and called "Ca" because it is basically made of CaCO₃ and FeS. It forms in any concentrated CaCl₂ brine saturated in CaCO₃, and despite enormous P_H2S in wells, the perfect solubility of Fe⁺⁺ within such layers is due to the immediate exhaustion of any cathodic alkalinity and sulfide ion by CaCO₃ precipitation.
This new layer joins the former non protective anionic insoluble layers (AI), in which the local iron solubility is due to a shortage in H₂S only (AI₁), or both H₂S and CO₂ (AI₂). With these three non protective layers specific to H₂S corrosion, and the previous protective cathodic insoluble layers (CI), Part 2 gives a complete algorithm of the corrosivity of sour oil or gas wells, as well as lines.