Base Material Considerations and the Selection of NACE Approved High-Strength Alloy Overlay for Use on HP/HT Exploration and Production Equipment
Authors: Huff Phil , Khandoker Shafiqul Islam , Landthrip Greg , Newbury Melissa 
Published: October 2008

Petroleum exploration and production operations have been and are expanding into areas that demand resistance to operating temperatures and pressures well above those that are presently considered " normal?? by the existing API oil tool equipment specifications. Consequently, the common low alloy steel materials from which the present oil tool equipment is manufactured have been and are now being reexamined to determine their suitability for use in these high pressure high temperature, HP/HT, operating environment applications.

Furthermore, with these HP/HT applications, consideration must be given not only to the elevated temperature tensile strengths of the low alloy steel materials but, also to their resistance to the hostile wellbore fluid constituents to which they may be exposed. Principally the effect of any Hydrogen Sulfide, H2S, which may be contained in the wellbore fluid on the materials of construction, must be thoroughly considered. In addition, other hostile wellbore fluid constituents, such as, Carbon Dioxide, CO2 and Chlorides must be considered as well.

Development work has been ongoing to explore the advancement of the common low alloy steel materials of construction for this equipment with greater section thickness, higher tensile strengths and continued qualification of the material to the requirements of NACE MR0175 / ISO 15156. This development has recently begun to include the incorporation of other materials to augment the mechanical properties of these common low alloy steel materials.

This materials augmentation includes the use of high strength alloy overlays. These overlays not only can provide higher strength and concurrent resistance to Sulfide Stress Corrosion Cracking, SSCC, they can also provide improved corrosion resistance in service compared with equipment manufactured from the common low alloy steel materials alone. The development of the use of these materials and combination of materials will help advance the technology for the exploration and production of petroleum for enhancing of the entire industry. As wellbore fluid temperatures and pressures continue to increase, the materials of construction used for the manufacture of wellbore fluid containing equipment will have to continue to improve to maintain pace with the needs of exploration and production.

INTRODUCTION

Exploration for petroleum and natural gas has proceeded to greater offshore water and wellbore depths below the sea floor. Exploration in these far reaching depths beneath the sea floor has begun to encounter reservoir temperatures and pressures that far exceed the standard pressures and temperatures common for equipment manufactured to API 16A. This specification, API 16A, is the current drill through equipment specification, and other API specifications, such as, API 6A and API 17D control the manufacture of oil tool equipment for production on land and subsea respectively. The maximum operating pressure to which equipment is commonly designed and manufactured to and complies with API 16A requirements is 15,000 psi. (103 MPa). Drilling to these greater depths is expected to result in operating temperatures and pressures that are well above those presently allowed in the current API specifications.