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Zonal isolation tools, including bridge plugs and straddle systems, using metal-to-metal sealing technology in place of conventional elastomers may radically challenge existing engineering philosophies by providing a pressure sealing system capable of exceeding the limitations of other existing sealing technologies and eliminating many traditional elastomeric failure modes. Metal-to-metal sealing technology is an innovative type of high expansion seal that uses expanding metal to form a high integrity pressure seal and incorporates an extraordinary seal performance envelope that cannot be achieved by conventional elastomeric seals. The seal design utilizes the controlled application of load to expand the metal seal to achieve a fully formed pressure barrier which allows for the metal-to-metal seal in bores of up to 160% of the original seal diameter. The metal-to-metal seal has the potential to change the way downhole equipment provides pressure integrity during pressure pumping operations. In the well servicing market, recent applications of the metal-to-metal seal and the latest lab testing have suggested the prospective of the technology for products for not only the offshore market but also in the hostile HPHT arena. Metal-to-metal sealing technology could make well suspensions and intervention possible in hostile HPHT wells that were previously deemed inoperable by the complete removal of elastomers from the design. Ideal for offshore and deepwater applications, metal-to-metal sealing devices can be easily deployed on slickline, electric wireline, coiled tubing, or drill pipe to save operator rig time in areas where cost savings are critical. Case studies of the high performance characteristics of the metal-to-metal sealing technology in terms of design, testing, and implementation will recognize the alignment to the increased demands of offshore applications.
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The required adaptability of the production systems to field evolution presents the flexible flowlines as a suitable solution, with advantages to rigid pipelines. The usage of flexible flowlines associated to the challenges of deep and ultra-deep water explorations and increment of Working Pressure promotes the metal-to-metal swivel connections to required components for subsea equipment, leading installation reliability and assuring operational confidence.
In order to reduce operational risks during pipelines and in-lines installations, different designs of Swivel connections had been used on the last 10 years. All designs had the same concept of a Metal-to-Metal Seal with an energizing system that add some risk to the system and requires ROV operations.
The present work shows a new Swivel concept for 2500 meter of water depth and 10,000psi of working pressure, that uses a Metal-to-Metal Seal on seat and not require a ROV intervention. On this concept, the Metal-to-Metal Seal run during the installation on energized position and allow relative movement between the inlet and outlet Swivel connection. Also, this kind of concept permit to work with one Metal-to-Metal Seal design for different Swivel sizes, which means that the deliverable time for the Swivel could be reduced and make easier the maintainability of the Swivel connection.
This paper describes the application of a new bi-directional metal-to-metal seal design for use in oilfield wellhead equipment.
A metal seal has been developed for working pressures up to 30,000 psi (207 MPa) on mandrel style pressures up to 30,000 psi (207 MPa) on mandrel style tubing or casing hangers.
Also discussed is a metal-to-metal seal for use directly on field machined casing which has been made practical with the development of a portable field practical with the development of a portable field casing lathe.
The "state of the art" of well drilling technology is almost routinely exceeding 25,000 foot (7620 m) depths into formations with pressure gradients approaching 1 psi per foot. These bottom hole pressures are in turn producing wellhead pressures in gas wells in excess of 20,000 psi (138 pressures in gas wells in excess of 20,000 psi (138 MPa. In other areas of hydrocarbon exploration, bottom hole temperatures are being encountered of 425 degrees Fahrenheit (218 C) and greater. Still other areas are producing effluents which are highly corrosive and require the injection of corrosion inhibitors, which are incompatible with existing elastomer seal technology. These factors, combined with the improved manufacturing capabilities of wellhead equipment manufacturers, have led to the development of a series of equipment which utilizes metal-to-metal sealing for all well fluid isolation seals. This paper describes one such wellhead seal design and the advancements it offers,
TYPICAL WELLHEAD ARRANGEMENT
Figure 1 represents a typical wellhead arrangement having a mandrel style tubing hanger. Identical primary and secondary metal-to-metal seals isolate the inside of the flange joint from well pressure. This flange annulus provides a chamber for pressure. This flange annulus provides a chamber for field testing of the tubing hanger seals, the flange gasket and all lockscrew and test port seals.
Also shown is a metal-to-metal primary seal for the production casing string, located in the bottom of the tubing head.
METAL-TO-METAL SEAL TUBING HANGER
The tubing hanger in figure 1 incorporates both primary and secondary metal seals. The secondary primary and secondary metal seals. The secondary seam is mechanically initiated by tubing weight, Lockscrews located in the flange are positioned to lock the hanger in place. A back pressure valve is installed in the hanger to ensure well control during removal of the blowout preventers and installation of the christmas tree. The primary metal seal is installed prior to the installation of the tubing head adapter flange. The extra long neck on the tubing hanger serves to guide the adapter and christmas tree into position to insure alignment, protecting both the metal seal and the flange gasket protecting both the metal seal and the flange gasket from damage during installation. Figure 8 shows a casing hanger metal seal during installation.
MAKE-UP OF METAL-TO-METAL SEALS
Figure 2 depicts the primary tubing hanger metal seal prior to make up of the tubing head adapter flange bolting. An initial standoff of the flange face is caused by the flange gasket and the tubing hanger metal seal.
Figure 3 depicts this same metal seal after the flange bolting has been fully made up. The flange faces have been drawn together and both the flange gasket and the tubing hanger metal seal have been mechanically energized.
EFFECTIVE METAL-TO-METAL SEALING
For metal-to-metal sealing to be effected, the seal interface must create contact stresses sufficient to cause plastic deformation of the seal element, thereby fully conforming it to the contour of the mating component and effectively closing off all leakage paths. These contact stresses must be kept under that level which would cause deformation to the mating surfaces.