ABSTRACT

Drilling high pressure, high temperature (HP-HT) wells over the last several years has brought forth and accentuated the need for higher strength OCTG (Oil Country Tubular Goods) grades which are suitable for use in sour gas environments.^{1, 2} Drilling engineers are designing casing strings at depths where higher strength is needed. The presence of hydrogen sulfide has traditionally restricted the use of casing grades with minimum yield strengths from 90 to 95 KSI (API Grades C90 and T95). Recent advances in metallurgy and pipe making now enable the drilling engineer to utilize high strength, sour service (HSSS) tubular grades with minimum yield strengths from 100-110 KSI (C100, C105 and C110).

This paper discusses the metallurgical attributes of high strength, sour service (HSSS), tubular goods and provides information about specific cases where without HSSS grades successful, economic completions would not be possible. The paper also discusses the manufacturing techniques and material properties necessary to assure that casing products are suitable for use in sour environments at high pressures. A brief history and description of many of the API casing and tubing grades are discussed relative to their properties and intent for sour service applications. Details of the testing necessary to assure proper serviceability when exposed to hydrogen sulfide (H₂S) are provided. Specialized testing, which the National Association of Corrosion Engineers (NACE) has standardized for oil and gas production equipment will also be discussed. Information is provided on both casing and coupling stock with thickness' that have been traditionally difficult to assure the metallurgical uniformity and resistance to hydrogen sulfide stress cracking (SSC).

INTRODUCTION

As the search for oil and gas reservoirs is expanded, it is becoming a more common practice to drill and produce from formations at depths in excess of 15,000 feet. At these depths, high formation pressures and gas compositions are encountered which require tubulars to not only have high yield strengths, but satisfactory resistance to environmental cracking by hydrogen SSC. It is generally recognized that this embrittlement phenomenon is a result of the exposure of high strength steels to aqueous hydrogen sulfide gas, which induces a brittle failure. As the depths of drilling, completing and producing increase, even higher yield strength pipe with adequate hydrogen SSC resistance is needed to sustain the higher stresses encountered¹. The casing design envelope is greatly affected by the inverse relationship between pressure and H₂S (ppm) concentration.

HYDROGEN SULFIDE STRESS CRACKING (SSC)

Considerable literature exists detailing the theories of the hydrogen SSC mechanism.^4,5,6,7 Hydrogen sulfide stress cracking (SSC) is defined as the spontaneous fracturing of steel subjected simultaneously to a aqueous corrosive hydrogen sulfide medium and a static stress less than the tensile stress of the material. It usually occurs in a brittle manner, resulting in catastrophic failures at stresses less than the yield strength of the material. Hydrogen SSC is basically a hydrogen embrittlement mechanism resulting from the formation of hydrogen ions (H ⁺) in the presence of aqueous hydrogen sulfide (H ₂S).