Enhanced Steel Alloys Outperform Typical Bottom Hole Assembly Chemistries in Sour Service Applications
Authors: Chan Alvaro , Brown Glenn Robert , Chandler Raymond Brett , Sanclemente Leianne W. , Hinger Danijel 
Published: December 2007

Many of today's aggressive drilling programs require well construction through sour formations to reach remote reservoirs. This has exposed limitations of traditional steel chemistries used for bottom hole assembly (BHA) components. For many years, the industry has used traditional API materials in environments with low concentrations of hydrogen sulfide (H2S) and carbon dioxide (CO2). Higher temperature and low stress conditions in the deeper portion of the well have allowed for these BHA materials to operate adequately with few issues. However, the drive to drill in more corrosive and sour environments has increased the need to develop materials more aptly suited for harsh sour service conditions.

To date, standardization bodies such as API and ISO do not provide specifications for the manufacture of sour service drill stem components. Consequently, organizational bodies such as the Industry Recommended Practices (IRP) in Canada are developing their own requirements for sour applications. The stringent requirements and improved mechanical properties required for sour applications prompted the industry to move away from API materials.

This paper will present test results comparing standard API material and several alloyed variations. Results indicate that alloyed variations show a slight increase in resistance to hydrogen sulfide under low level concentrations while under higher concentrations; the materials show little to zero improvement. This paper will also present emerging alloying technology and enhanced heat treatment processes that have yielded materials exceptionally suited for aggressive sour applications. These materials exhibit excellent resistance to sulfide stress cracking (SSC) and have been successfully used in some of the most severe corrosive environments in the world (also presented). These enhanced BHA materials will significantly improve drilling efficiency by eliminating the risk of failure due to H2S and CO2 exposure.

Introduction

More aggressive drilling programs have pushed the limits of existing downhole components especially for sour service applications (H2S is present). For many years, the drilling industry has been using traditional material chemistries for BHA components that were developed decades ago. API or ISO does not have specifications regarding sour service applications. Regulating bodies for sour service applications such as The National Association for Corrosion Engineers 1 (NACE) focus on casing and tubing components and do not address specifically drill pipe or BHA components.

The industry has responded by developing its own set of recommendations for sour service specifications individually. In order to satisfy the industry's need for sour service BHA products, the industry has had to develop and test new material chemistries and improved heat treatment operations to satisfy the new more aggressive drilling programs.

Background

The use of HWDP is a result of modern drilling practices involving the need to taper the drill string. HWDP is a transition component that not only applies weight to the bit but also reduces the high bending stresses between the connections from the heavy, stiff drill collars to the light, flexible drill pipe. It offers an ideal location for the drill string's neutral point, the location where tension and compression stresses meet and cancel out.

Currently there is no API or ISO specification for sour service drill pipe. In response, the industry has established its own set of requirements. The IRP were developed in 1999 by a committee in Canada comprised of regulatory boards, operators, drilling contractors, manufacturers and rental tool companies to address the need for drilling component requirements to be utilized in applications where H2S is present. These requirements are a result of extensive field experience under severe H2S conditions.

The IRP2 have established specifications for hardness, material strength, and heat treatment processes along with material's chemistry recommendations for sour service applications. The mechanical specifications and chemical recommendations for sour service grades are listed in Table 1 and Table 2, respectively.