Chemistry of Concentrated Salts Formed by Evaporation of Formation Water and the Impact on Stress Corrosion Cracking of Duplex Stainless Steel

To simulate fault conditions, a method has been developed to characterize the solution composition formed by the evaporation of formation water associated with a simulated offshore gas production system. This involved sampling from the concentrated solution through a filter in an autoclave at 136°C with 1.8 bar carbon dioxide (CO₂) superimposed on the water vapor pressure. Following analysis of the solution chemistry, the data were used to test predictions of solution composition at different levels of water loss based on thermodynamic modeling. The model predictions exhibited the broad trends of the experimental data but deviated markedly in absolute terms at the higher levels of water loss, above about 80%, the regime of most relevance to stress corrosion cracking. Four-point bend stress corrosion tests were conducted on 22 Cr duplex stainless steel (DSS) and 25 Cr super duplex stainless steel (SDSS) at 136°C and 1.8 bar CO₂ in simulated evaporated solutions, including testing at 93% water loss, with a chloride concentration of 272,100 ppm by mass in the remaining water phase. These tests, conducted on both parent and welded steels, gave no indication of cracking, in contrast to previously reported stress corrosion tests for similar duplex stainless steels in evaporated formation water. Oxygen contamination is considered a possible explanation for stress corrosion cracking in this system.