High Pressure and High Temperature (HPHT) applications demand OCTG with increasing yield strengths, at least up to 140ksi. Due to the high total pressures, even trace amounts of H2S lead to significant partial pressures of H2S in wells. Consequently, the risk of SSC needs to be adressed. Typical sour conditions in such wells are pH 3.9-4.5 and 0.0035-0.03 bar of H2S. A single steel grade has been designed so as to be suitable for a wide range of tubular products, from casing to thick accessories (up to 2.2 inch). The present paper describes the qualification approach applied to field projects using NACE TM0177 method A and D.
More and more High Pressure High Temperature (HPHT) sour wells are operated worldwide. Challenging material selection is required for such severe operating conditions.1,2 Very high strength materials, presenting yield strength above 896 MPa (130 ksi), are required for sustaining the pressure. Consequently, even a low amount of H2S in the gas phase may lead to a H2S partial pressure beyond the limit of 3.5 mbar (0.05 psi) established in NACE MR0175 / ISO 15156 standard.3 Indeed, both high yield strengths and partial pressures of H2S contribute to a situation where the risk of Sulfide Stress Cracking (SSC) is high. The present paper is focusing on the SSC resistance of 130 ksi minimum yield strength material developed for covering such HPHT applications.
A single steel grade composition has been selected (Table 1). It is a AISI 4130 modified steel grade enriched in Mo, element acknowledged for enhancing the SSC resistance of low alloyed steels,4-6 microalloyed with Al, V and Nb. The steel composition has been selected for producing pipes hardenable up to 60 mm wall thickness and includes the latest developments on the design of sour service materials.7 Consequently, a single steel grade can be used for a large range of products, from casing to coupling stock or accessories. The list of pipes with dimensions variation and their respective mechanical properties is given in Table 2. The yield strength properties as determined by ASTM E88 are in the range of 131 to 142 ksi.