There has been renewed interest in the corrosivity of high-density well completion and packer fluids in recent years following a series of failures of Corrosion Resistant Alloy (CRA) tubing strings from the annular side as a result of stress corrosion cracking (SCC). It is now realised that the original corrosion testing carried out on traditional halide-based completion and packer brines may have been too cursory, and missed some important environmental factors that make CRA susceptible to SCC in these fluids. New studies have shown that some CRA become susceptible to SCC in halide brines charged with CO2 and H2S, but there remain some uncertainties about the boundary conditions that trigger the SCC.
The objective of the laboratory experiments described in this paper was to examine how the contamination of high-density brines with oxygen, chlorides and CO2 might influence the susceptibility of CRA to SCC under high temperature-high pressure (HTHP) conditions. Our results highlight the increased risk of SCC occurring in CRA tubular materials when bromide-based completion brines become contaminated with oxygen or CO2 during field use in HTHP wells. Our tests also indicate that similar contamination of formate-based completion brines is less likely to create SCC problems in CRA tubular materials in HTHP wells. We conclude that the current North Sea trend for using formate-based completion fluids in place of bromides may provide better insurance against SCC-related tubing failures.