Corrosion resistant alloys containing 13% Cr or more have frequently been used for completion components in severe carbon dioxide (CO2) environments due to their corrosion resistance, whereas carbon steel is not used in these conditions due to potentially elevated corrosion rates. However, in some cases, the conditions may not be severe enough that 13% Cr tubulars are required. For these conditions, 3% Cr alloys may be sufficient and allow for reduced capital expenditures without risking downhole system integrity during normal production. There are, however, challenges with protection of these alloys during acidic scale dissolver treatments at elevated temperatures.
Calcium carbonate (CaCO3) scale dissolves readily in several organic and mineral acids, however, the application of acids in wells completed in both 3% and 13% Cr tubulars at elevated temperatures may, in many cases, not be viable due to material integrity concerns. Another challenge at elevated temperatures is ensuring compatibility of the acidic fluids toward the non-metallic materials used downhole.
Discussed herein is the development of a high efficiency acid for scale removal that is compatible with 3% and 13% Cr alloys at elevated temperatures, with low general corrosion rates and no pitting. The stability and the potential for re-passivation of the protective CrxOy layer was achieved through inclusion of various corrosion inhibitors, additives, and synergists. The inhibited acidic scale dissolver is thermally stable at 100 °C, compatible with various downhole elastomers and thermoplastics up to 100 °C, and environmentally acceptable. The product has been used successfully at several offshore installations on the Norwegian Continental Shelf.
Key words: Scale dissolver, Corrosion inhibitor, Cr alloys.