Abstract
During well clean-up of four subsea wells in a new gas field, elevated concentrations of H2S were unexpectedly measured. The concentrations left the tubing (13CrS) and casing material (Q-125) outside of the pre-qualified areas of ISO 15156. Therefore, it was required to demonstrate the fitness of the materials against relevant environmental cracking corrosion. This publication describes the methods and processes followed to qualify the materials and well barriers for service.
Testing was performed conservatively to NACE TM0177 to establish the stability of the materials against Sulphide Stress Cracking (SSC) and Stress Corrosion Cracking (SCC). The testing atmosphere designs included both partial pressure as well as fugacity approaches. As part of the Fitness for Service analysis, the mechanical stress as realistically expected in the wells was used in those experiments. However, for the casing and due to expected deviations from the ideal circular hollow cylinder geometry of the tubulars and the uncertainty on the casing wear evaluations, a 90% Actual Yield Stress (AYS) experiment was conducted on the casing material as well, and a statistical approach was selected to evaluate the risk of SSC, using Monte Carlo simulations including Finite Element Analysis. The laboratory testing for the tubing was passed with an adjusted stress level in an environment exhibiting the shut-in H2S partial pressure. For the casing, the simulations including casing wear indicated potential stresses above the yield strength in the deterministic scenario. For those, the Monte Carlo simulations demonstrated that significantly lower stresses are realistic and thus, the risk for local stresses exceeding the established crack-safe stress level was very low. Further layers of well integrity were implemented, such as H2S scavenger injection or short-notice readiness of a rig for the unlikely event of communication to annulus A. Subsequently, all four wells could be successfully started up.
