The Development Of High Strength Casing Steels With Improved Hydrogen Sulfide Cracking Resistance For Sour Service
Authors: Waid G.M. , Stiglitz R.J. 
Published: October 2008

The need for higher strength steels with good resistance to sulfide cracking has been more apparent with the increase in energy demands and the decrease of easily obtained sweet oil reserves. Oil fields now being explored often require drilling to depths beyond 20,000 ft where hydrogen sulfide is often found. The development of two new high strength casing steels with improved hydrogen sulfide cracking resistance is discussed. The effects of both compositional and processing variables on sulfide stress cracking resistance of these two steels are presented. In addition, the advantages of a unique internal-external quenching system for heat treatment of these casing steels are presented.

INTRODUCTION

Sulfide stress cracking (SSC) failures were initially experienced in downhole tubular goods and other well equipment in the early 1950's, mainly in tubular grades N-80 and higher. At that time, NACE and individual companies established research programs to study the problems involved.

Past research and field experience have shown that, in general, the hardness, strength, microstructure, chemical analysis, cold working and stress are factors to be considered for casing and other well equipment. PH level, H2S concentration, presence of moisture, temperature and corrosion inhibitors in the well are other important factors to be considered for a sour environment.

The first development of an H2S resistant steel resulted in the recommendation of a 22 HRC maximum and 90,000 psi maximum yield strength material. In 1963, NACE published "Tentative NACE Specification 60, Tubular Goods," and the API published "Tentative API Specification 5AC Casing and Tubing." These specifications required a restricted yield strength of 75,000 to 90,000 psi; a 95,000 psi minimum tensile strength; restricted chemical composition; a specific heat treatment of normalize and temper with minimum time and temperature; no cold working other than normal straightening; and no cold die stamping. However, there was still a great demand at this time for an 80,000 psi minimum yield strength grade that would offer at least the same degree of SSC resistance for use in deeper sour wells.

Also, in the early 1960's, a quenching and tempering heat treatment of seamless pipe was begun to be used to achieve high physical properties. At the time the above-mentioned NACE and API specifications were written, it was general practice to produce pipe at the 80,000 psi yield strength level by a normalizing heat treatment, which these specifications required. When quenching and tempering became widely accepted, the API included it as an acceptable heat treatment for grade C-75 in Std. 5AC in 1965.

An example of an external quench unit is shown in Fig. 1. A ring-type external water spray quench is used, containing two separately controlled sets of water spray nozzles, both inclined at a 30 degree angle. One set is inclined in the same direction as the travel of the pipe. The other set is inclined in the opposite direction and quenches the back end of the pipe. This eliminates the possibility of water entering the inside of the pipe and possibly causing quench cracks.