Justification and Recommendation for a High Sour Region beyond NACE MR0175 / ISO15156 Region 3

NACE MR0175 / ISO15156 is the global standard for metallic materials suitability for H2S-containing (sour) oil and gas production environments. For carbon and low alloy steels, it currently advises suitability based on four (4) regions of environmental severity with respect to sulfide stress cracking (SSC): Region 0 (least aggressive) through Region 3 (most aggressive). Suitability for materials in Region 3 requires hardness and manufacturing restrictions or qualification in 0.1 MPa H2S environment. This test condition is arguably in the “middle” of the plotted area of Region 3 with significantly more sour exposures, i.e. >>0.1 MPa H2S, permitted through 0.1 MPa H2S qualification testing. Implications of extreme sour environments including phase behavior and response of carbon and low alloy steels in test environments are reviewed. Known field failures of linepipe and oil country tubular goods (OCTG) are also presented in this paper. Improvements in material grades and mill practices have generally resulted in widespread success of Region 3-rated materials in highly aggressive sour environments but certain types of materials and service combinations have proven to be more prone to failure. This paper proposes caution and recommended qualification practices for these susceptible materials with the intent to add a region beyond Sour Region 3 to NACE MR0175 / ISO15156 with appropriate advisory clauses for materials selection and qualification in the new region.