Sour gas constitutes 60% of the Middle East natural gas reserves (Nersesian 2015). The term "sour" is used to refer to environments containing hydrogen sulfide (H2S). In sour wells, H2S causes sulfide stress cracking (SSC) in carbon low-alloy steels under tensile stress. A form of hydrogen embrittlement, SSC causes sudden, brittle fracture in drillpipe. To combat the effects of H2S exposure, sour service drillpipe has been manufactured to meet IRP Volume 1 - Critical Sour Drilling specifications. IRP requires SS75 ksi, SS95 ksi, or SS105 ksi specified minimum yield strength (SMYS) tubes with all three grades using 110 ksi SMYS tool joints for use in sour operations. However, these lower yield strength IRP grades have limited load capacity which restricts drilling parameters such as total depth (TD) and margin of overpull.
The next higher yield stress grade available is the API S135 grade. This grade is a non-sour service grade with SSC-resistance limitations due to its higher hardness and higher yield strength values. The SSC-resistance of S135 in an H2S drilling environment is dependent on using drilling muds with pH control (above 10.0), H2S inhibitors, and H2S scavengers. Therefore, a grade of SSC-resistant drill pipe is needed to bridge the gap between the fully SSC-resistant SS105 grade and limited SSC-resistant S135 grade.
Using advanced steel heat treatment methods, a 125 ksi SMYS grade sour service drillpipe has been developed. The 125 ksi tube is mechanically incompatible with the standard 110 ksi IRP sour service grade tool joint because of the large difference in yield strength between the two components. Therefore, a new class of sour service tool joint with an SMYS of 120 ksi was required for the 125 ksi sour service tube. This new higher yield strength tool joint needs to meet the same level of SSC-resistance as the 110 ksi IRP tool joint, i.e., pass testing in solution A method A of NACE TM0177 (100% H2S @pH = 2.7, 720 hour test) at a testing stress of 65% of SMYS. Like the high yield strength 125 ksi tube, advanced heat treatment methods were required to achieve SSC-resistance in the higher yield strength tool joint.
This paper introduces an SSC-resistant 125 ksi drillpipe with SSC-resistant 120 ksi tool joints and presents the results of NACE TM0177 testing of this new technology. The new 125 ksi drillpipe will allow for increased load capacity over IRP SS105 grade drillpipe in drilling environments with moderate to low concentrations of H2S. In addition, it can serve as an improved substitute over S135, a non-sour service grade drillpipe.