High-Strength Heavy-Wall Casing for Deep, Sour Gas Wells
Authors: Greer J. Brison , Holland Warren E. 
Published: December 2007

Summary
A completion program for a high-pressure deep, sour gas well (DSGW) requires exceptionally heavy-wall large-diameter casing, particularly where an additional tubing string is required to provide for continuous chemical injection and tubing set on a packer. Production and protection casings of unprecedented weight and thickness were obtained recently for such a well. This paper describes the qualification program for these casings and the specifications and controls used to ensure acceptable sulfide stress cracking performance. The specifications are the result of prototype casing and connection testing in H2S to 25,000-psi (172000-kPa) internal pressure and 1 million-lbf (4450-kN) axial load. New mill specifications including microstructural control and sulfide stress cracking tests are discussed along with the justification for such controls. Connection design including a new non-galling connection used on the protective string is discussed as well as the triaxial stress analysis used for high-pressure casing design. Manufacturing process control, nondestructive testing (NOT), and intensive follow-up inspections for these strings also are discussed.

Introduction
Exxon Co. U.S.A. has drilled a deep, abnormal-pressure sour gas wildcat in central Mississippi. The completion concept1 for this well includes the use of a 4-in. (I0.2-cm) special alloy tubing and packer2 set at about 12,000 ft (3660 m) and a 2 3/8-in. (6.03-cm) concentric tubing string for continuous chemical injection and production.
An 18,000-psi (124000-kPa) shut-in design pressure mandated use of 7 3/8-in. (19.4-cm) 90 grade [90,000-psi (621 000-kPa) minimum yield strength] production casing having a wall thickness of 1.2 in. (3.05 cm). For this pressure level, a triaxial stress design that incorporates hydrostatic pressure as an independent radial stress variable along with hoop (burst) arid axial stresses was used. DSGW specifications were written to ensure that the required sulfide stress cracking resistance would be obtained and that the tubes would be mechanically sound. New metallurgical features of the DSGW-90 specification and laboratory and prototype testing that validated the heavy wall casing and connection design are presented along with discussion of the intensive quality assurance (QA) program that was followed. A torque-turn procedure was developed to aid in running the special clearance connection.
The 10 1/4-×9 5/8-in. (27.3-×24.4-cm) protective casing string experienced a maximum indicated hook load of 1.48 million lbf (6580 kN) during the hangersetting operation making this a record-weight string for an oil or gas well. Modified API connections were designed to handle the anticipated 1.5 million-lbf (6670-kN) tensile and 14,000-psi (96 500-kPa) maximum burst pressure requirements. The eight-round threaded portion incorporated a recently developed 3 entrance and exit thread profile for improved makeup and anti-galling characteristics. As with the production casing, DSGW specifications were used to ensure that metallurgical and QA characteristics were consistent with critical service requirements.

Production Casing

Von Mises or Triaxial Stress Tubular Design
For low-pressure (design pressure is small compared with usable metal strength) and shallow-well situations, relatively simple and conservative casing design is possible using the API Barlow4 formula for burst and a load/area formula for tensile stresses.

Von Mises or Triaxial Stress Tubular Design
For low-pressure (design pressure is small compared with usable metal strength) and shallow-well situations, relatively simple and conservative casing design is possible using the API Barlow4 formula for burst and a load/area formula for tensile stresses.