Corrosion failure and SCC mechanism of super 13Cr tubings in high pH completion fluid

Key words:  super 13Cr tubing  high pH completion fluid  corrosion failure  corrosion mechanism  SCC mechanism

Abstract:

Based on the cracking mechanism, corrosion morphology, corrosion products and finite element analysis of super 13Cr tubings in a failed high-pressure gas well, the corrosion failure and stress corrosion cracking (SCC) mechanisms of super 13Cr tubings in the high pH completion fluid were probed. The result shows that in the high pH completion fluid, the super 13Cr tubing has suffered from SCC, and most cracks originate from the local corrosion pits on the outer wall of the tubing. The crack growth transited from the transgranular cleavage-like cracking to the intergranular cracking.The outer wall of the super 13Cr tubing was significantly corroded, and the thickness of the corrosion product layer increases greatly with the increase of the well depth. The three layers are the Ca₃(PO₄)₂ deposition product film, FeCr₂O₄ high-temperature oxidation product film and Fe₃O₄ main corrosion product film from the outside to the inside of the tube, where the Fe₃O₄ corrosion product film takes the phenomenon of bidirectional growth. The corrosion mechanism of super 13Cr tubings in the working environment is the reaction-diffusion process of metal ions and oxygen ions in the oxidation film and corrosion product film. The SCC mechanism is dominated by the anodic-dissolution, accompanied by the film-induced cleavage, and the existence of fatigue load promotes the initiation and growth of SCC cracks.