The Kristin Field is a high pressure high temperature gas condensate field located in the Haltenbanken area on the Norwegian Continental Shelf.
The well design and operation challenges during the drilling and completion phases, as well as the long term production phase, of the Kristin wells required the need for a "mild sour service" production casing grade capable of handling large differential pressures and high axial loads. To meet the requirements, the initial selection was a large OD, heavy weight C-110 grade casing. However, the overall casing weight, the annular flow area during cementing and the compression capabilities of the available connections called for a material grade with specified minimum yield strength of 125 ksi (862MPa). This grade would allow for a down scaling of the casing dimensions and the use of qualified premium connections suitable for the Kristin conditions.
The recent development of a 125ksi (862MPa) high strength low alloy (HSLA) steel casing grade for "mild sour service" applications has permitted this specific design requirement to the Kristin production casing OD to be met.
This steels metallurgical design included reducing the dislocation density and controlling carbide morphology to enhance the sulphide stress cracking (SSC) resistance. It was found that vanadium containing steels, produced lower dislocation densities than vanadium-free steels, at same strength levels, because vanadium containing steels were tempered at a higher temperature than vanadium-free steels. The higher temperature tempering enhanced the SSC resistance, as a result of the uniform and spheroidized M3C distribution and decrease in dislocation density. A decrease in chromium (Cr) content, which had little effect on temper softening resistance, was beneficial for preventing M23C6 (M=Fe, Cr, Mo) formation. The 0.5Cr-0.7Mo-0.1V steel without M23C6 carbide had higher SSC resistance than the 1Cr-0.7Mo-0.1V steel.
This paper will describe this specific well design challenge provided by the Kristin field production wells and how this was resolved by the introduction of this new HSLA steel grade. Then explain the detailed metallurgical design development needed to provide improved SSC resistance that was required for the "mild" sour service conditions found in the Kristin reservoir fluids.