Completion operations, for oil and gas wells, are usually performed in casing sizes not exceeding 13-3/8" (34.0 cm) with the majority of completions being performed in 9-5/8" (24.5 cm) and smaller. As such, the availability of completion equipment, for large diameter casings, becomes extremely limited or nonexistent, Occasionally, conditions may exist which require the completion to be designed in a casing size larger than 13-3/8" (34.0 cm). In order to complete in these casing sizes, special equipment must be designed.
This paper discusses the design criteria, development, and subsequent use of perforating and gravel pack equipment generated for use in 20" (50.8 cm) casing. The theoretical approach to the evolution of this equipment is applicable to the development of additional systems.
While completion equipment is largely designed for 9-5/8" (24.5 cm) casing sizes and smaller, with some equipment available up to 13-3/8" (34.0 cm), perforating and gravel pack equipment is virtually nonexistent in sizes greater than 9-5/8" (24.5 cm). Due to specific well completion requirements it may be advantageous to be able to complete in the larger diameter casing sizes. With equipment being unavailable for these larger sizes, special equipment must be developed.
The herein discussed completion system has been specifically developed for 20" (50.8 cm), 133 lb/ft (198 kg/m) casing. In this application a very shallow, low pressure, gas zone exists beneath a platform. It is the operator's intent to set 20" (50.8 cm) casing across the zone and attempt to blow down the gas. Following depletion of the zone, the 20" (50.8 cm) wellbores can be cleaned out and deepened to the primary objective zones. Further development drilling could then commence with less risk and lover costs.
With the primary objective of the project being the depletion of the reservoir, the resultant completion had to be installed in the most efficient manner possible. Owing to the less than 500 psi (3447.5 kPa) static reservoir pressure and the desire to deplete as quickly as possible, the perforating system had to deliver the highest shot density possible with the greatest amount of charge performance. This would provide for the maximum flow rate by minimizing pressure loss through the completion. A systems analysis plot, Fig. 1, supports this and, thus, fostered the need of a specialized perforating gun.
Existing perforating equipment would have required multiple trips to achieve the desired perforation shot density, and would not have been able to offer positive, shot orientation. Performance would also suffer as clearance between the gun outside diameter and the casing inside diameter would have been difficult to control.
Consequently, a perforating system was needed that would offer a very high shot density with maximum performance, while affording positive shot orientation.
Due to the zone being gas, positive well fluid isolation and pressure containment became a criteria. This precluded the use of existing large diameter equipment, which typically have lead seals that are not capable of effecting a suitable gas seal. Additionally, the original design parameters called for the use of a retrievable seal bore packer as the perforating packer.
With the expressed desire to underbalance perforate, the requirement for positive gas pressure containment, and the need for secure casing anchorage, a special packer and associated gravel pack assembly was essential.