Small Hardness and Yield Strength Deviation from Heat Treatment Process Opens Susceptibility to Sulfide Stress Cracking in Oil & Gas Casing Tubulars – Case Study.

Casing Tubulars are an essential component in the oil and gas production process. Their integrity is essential to prevent contamination of the adjacent water sources while being able to maintain stable hydrocarbon production flow.

Industry organizations such as the American Petroleum Institute (API) set up minimum requirements for the hardness, mechanical, toughness, and chemistry requirements for the adequate performance of these tubulars. Environmental conditions affect the operational efficiency of the tubulars. Hardness and tensile strength requirements are necessary to avoid susceptibility to Sulfide Stress Cracking (SSC) as stated by NACE MR0175/ISO15156.

Small deviation from those hardness and tensile requirements can drastically and rapidly affect the material’s resistance to SSC. This in turn diffuses hydrogen in the base material producing intergranular, brittle fractures.

Three casing tubulars sequentially positioned in a producing well were exposed to typical hydrocarbon reservoir conditions. Minute levels of hydrogen sulfide, most likely from sulfate producing bacteria, affected one of two tubulars with above maximum recommended tensile strength properties in a two-week period. The tubular with the highest yield strength experienced heavy cracking that resulted in a washout and a longitudinal fracture that compromised the well’s integrity.

A small deviation in the heat treatment process altered the hardness and yield strength to the point where it compromised the tubular’s resistance to SSC. The work shows methodical analysis and how minute changes in the mechanical properties drastically and catastrophically affect the tubulars in an extremely short time compared to materials within the established normative requirements.