ABSTRACT

Extreme sour wells presenting H2S partial pressures much higher than 10 bar are frequent in several Middle Eastern areas and such situation are not covered by ANSI NACE MR0175 / ISO 15156 environmental severity diagram. The Sulfide Stress Cracking resistance of OCTG grades is a major concern, especially for high strength material at 110 ksi SMYS. Material qualification program combining NACE TM0177 Method A (Uniaxial Tensile) and NACE TM0316 (Four-Point Bend) was performed to cover risk of Sulfide Stress Cracking (SSC). Additionally, hydrogen electrochemical permeation tests. Most of the tests were performed at 24°C and 15 bar H2S, which is approximately the maximum accessible pressure at that temperature before forming dense H2S phase. Work performed highlights that material qualifications according to ANSI NACE MR0175 / ISO 15156 may be not enough when material is used at much higher H2S partial pressures than 1 bar H2S. Indeed, electrochemical hydrogen permeation tests performed up to 15 bar H2S show the increase of hydrogen content and permeation fluxes with H2S partial pressure, which illustrates that 1 bar H2S does not correspond to a saturation situation for hydrogen uptake inside steels. It is recommended for such qualifications to use Four-Point Bend test method to limit the evolution of the applied stress due to the corrosion rate while allowing an efficient mitigation of this issue by an increase of the specimen thickness.

INTRODUCTION

Oil and gas operations are pushing the limits of Sour Service always further with the need to combine high Sulfide Stress Cracking resistance (SSC) in H2S partial pressure while keeping high levels for Specified Minimum Yield Strength (SMYS). Operators are indeed entering into new drilling challenges when targeting complex well formations where production casings could be exposed to Sour Service environment and tubing completions to increased differential pressure. Having high levels for Specified Minimum Yield Strength (SMYS) paves the way for addressing Multi-Stage Fracturing (MSF) developments that reach pressure differentials as high as 15,000psi during fracturing operations in challenging environments with minimum intervention possibilities. The application limit of low alloy steels regarding the risk of Sulfide Stress Cracking occurrence is defined by NACE MR0175 / ISO 15156 standard.1 For instance, API T95 grade is acceptable to be used in all regions of the NACE environmental severity diagram (pH-PH2S) and for all temperatures. According to Annex B of the same standard, a material can be qualified as SSC resistant in all regions, meaning up to 10 bar of H2S, if it presents a threshold stress higher than 80% Actual Yield Strength (AYS) in the reference environment NACE TM01772 Solution A saturated by 1 bar H2S, as evaluated by uniaxial tensile, C-ring or Four-Point Bend (FPB) test.

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