C125 Casing-A Viable Solution for HPHT in Mild, Sour Service Applications
Authors: Urband Bruce E. , Morey Stephen C. 
Published: December 2007

In today's environment with deep, high pressure, high temperature reservoirs, the need for high strength tubulars with a modicum of some sulfide stress cracking (SSC) resistance is ever increasing. The growing popularity of high strength, SSC resistant materials such as Grades T95, C100 and C110 is paving the way for a new generation of even higher strength SSC resistant casing products. Although careful consideration and utilization of these higher strength carbon alloy steels is critical, their use can provide safe and adequate SSC resistance if utilized in relative weaker H2S environments

This paper details the results of extensive testing, undertaken to explore the possibilities of using 125 ksi specified minimum yield strength (SMYS) casing in sour or mildly sour environments. The SSC resistance of the 125 ksi material was evaluated utilizing the NACE TM0177–96 1 test Method-A and test Method-D. Method-A and Method-D specimens were tested in solutions with H2S concentrations of 3%, 7%, 10%, and 100%. A short discussion is also presented on the work API is doing in standardizing these higher strength steels for sour environments. This paper also details the key manufacturing steps necessary to produce a high strength SSC resistant tubular product and the relationship between SSC resistance.

INTRODUCTION

The need for higher yield strength casing with SSC resistance continues to increase as the petroleum industry develops deeper, higher pressure, and higher temperature formations that may contain in situ H2S or be susceptible to reservoir souring from water injection. The desired combination of strength and SSC resistance has in the past led to the development of grades such as T95, C100, and C110. These grades were seemingly impossible just 20 years ago, but with advancements in metallurgy and manufacturing technology, these high strength, SSC resistant grades, are today routinely used by well designers, with confidence, in deep sour service applications. Grade C110 is currently the highest minimum yield strength material commercially available that is specifically intended for sour service.

This paper documents the initial research and development of a 125 ksi minimum yield strength SSC resistant casing. This experimental casing is designated as Grade C125 and has the following attributes:

Yield Strength Window…….125,000 - 140,000 psi (862 - 965 MPa)

Minimum Tensile Strength…135,000 psi (931 MPa)

Steel Chemistry……………..Modified AISI 4130

Hardness Maximum…….…..36 HRC

Impact Toughness……..……API minimum absorbed energy requirements for Grade Q125

EXPERIMENTAL PROCEDURE

Test Material

The material utilized in this study is a modified American Iron and Steel Institute (AISI) 4130 carbon steel. This steel chemistry has a low-sulfur content and is produced using ultra clean steel practices with vacuum degassing and alloy additions for inclusion shape control. The chemical composition of the modified AISI 4130 steel used in this study is shown in Table 1. The chemical composition for standard AISI 4130 steel is shown in Table 2.2

Testing was performed on five heats of 13.625″ × 86.86 lb/ft (0.625″), and one heat of 7.625″ × 38.08 lb/ft (0.500″) seamless casing that were heat-treated to Grade C125 attributes. Table 3 is a comprehensive listing of all heats used in this study.