Deepwater high pressure, high temperature (HPHT) drilling environments present difficult challenges to well engineers. Typical deepwater pore pressure and fracture gradient profiles result in a narrow drilling window that can lead to seven to nine casing points. The high cost of these wells demands a high rate completion for economic payback, which defines the size of the production casing and liners. Drilling casings are restricted by the standardized 18-3/4" through bore diameter dictated by high pressure wellhead housings, blowout preventers, and riser systems. Furthermore, high pressures require thick wall casing, especially if sour service materials are specified. Satisfying all of these pressure and geometrical constraints requires some unconventional practices.
Current and emerging technologies offer several ways to address this design dilemma. Riser-less drilling can be an effective way to delay running the high pressure wellhead, allowing for additional large diameter casing seats. Dual gradient drilling is a concept that can decrease the number of required casing points. Solid expandable liners provide a way to add or push casing points without the geometry impact of a conventional string. Managed pressure drilling may also show promise for eliminating a seat. However, heavy reliance on these new technologies may run counter to the guiding principle of keeping HPHT wells as simple and reliable as possible.
This paper presents the concept of using conventional oil country tubular goods (OCTG) in unconventional sizes to increase the number of available casing points in deepwater wells. The method has several advantages in the areas of performance and reliability compared with the previously listed technologies. The decades of industry experience with conventional OCTG make the technology especially appropriate for containing high pressures and sealing off trouble formations.
Related issues such as manufacturing lead time, costs, connections, and hangers are discussed. Several HPHT examples are included to illustrate the trade-offs with other design options.