ABSTRACT

High strength steels with yield strength greater than 965 MPa (140 ksi) typically used as oil country tubular goods (OCTG) have very limited use in marginally sour wells. Currently, NACE(1) MR0175 / ISO(2)15156-21 provides caution on the use of these materials even in the non-sour Region 0. However, high pressure high temperature (HPHT) wells are requiring these grades to meet design requirements. Consequently, there is an increasing need to define the operating envelope for these materials.

To establish the operating envelope, sulfide stress cracking (SSC) tests were performed at various temperatures lower than 107 °C (225 °F) in mild sour environment. This paper reports SSC test results at low temperatures and the issues experienced at high temperatures. It was found that high strength material has very limited sour resistance in Region 0 when the material yield strength reaches 965 MPa (140 ksi). At low temperatures, testing in Region 0 using NACE TM01772 Standard Method A (30 days) may not be adequate to detect cracking susceptibility. In addition, SSC data suggests that 4 °C (40 °F) may be worse than ambient temperature.

For SSC testing at high temperatures, significant general corrosion and large pH drift proved to be challenging issues. Several Method A samples exhibited significant corrosion after 720 hours to an extent that rendered the evaluation of cracking impossible and the higher corrosion rates resulted in a significant drift of pH during test. The effect of different buffer solutions on the pH and corrosion was investigated and recommendations on high temperature testing are discussed.

INTRODUCTION

In the oil and gas industry, especially for high pressure deep wells, there is a strong need of high strength casing with some sulfide stress cracking (SSC) resistance. However, B. Craig reported that when low alloy steel yield strength reaches about 827 MPa (120 ksi), the SSC threshold stress for crack initiation will start to decrease quickly with strength.3 Further, when yield strength reaches 965 MPa (140 ksi), NACE MR01751 also warns that material could suffer SSC or HSC in traditional nonsour Region 0 with H2S partial pressure less than 0.3 kPa (0.05 psi).

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