Selecting Representative Laboratory Test Conditions for Fit-For-Purpose OCTG Material Evaluations

Abstract

Abstract The sulfide stress cracking resistance of steel oil country tubular goods (OCTG) is commonly correlated to the hydrogen sulfide (H2S) partial pressure, calculated by multiplying the mole fraction H2S by the total system pressure. However, sour gas mixtures exhibit non-ideal behaviors at high system pressure. Neglecting these non-ideal effects causes large over-conservatism at total system pressures above 5000 psi. It is important to compensate for non-ideal gas and solution behaviors in order to select representative laboratory test environments for sour service fit-for-purpose materials evaluations. Introduction High-pressure gas wells containing hydrogen sulfide (H2S) present material selection challenges. Casing steels with yield strengths of 110 ksi and greater can be required for high-pressure wells, but the material must also be resistant to sulfide stress cracking (SSC) under the applied loads in the presence of production fluids. High strength carbon and low alloy steel oil country tubular goods (OCTG) are typically qualified for sour service using one of the test methods described in NACE Standard TM-0177–96.1 The standard test environment described in NACE Standard TM-0177 is acidified brine saturated by 1 atm H2S at 75°F and ambient pressure. Although the standard test environment is useful for quality assurance testing, it not necessarily representative of production conditions. Therefore, fit-for-purpose (FFP) material testing is frequently employed to qualify materials for specific well environments that are perceived to be less (or more) severe than the standard test environment. FFP tests are typically conducted in anticipated worst-case conditions. For SSC, worst-case test conditions are typically the highest H2S partial pressure, coldest temperature, and lowest solution pH expected to exist concurrently in the well.