Plug design and applications
Michael Tunstall,James VickTechnological improvements to slickline-retrievable wellhead plugs have expanded their use from a temporary lock mandrel plug to a cost-efficient permanent barrier between the well and the environment. This barrier can provide full access to the well. Designs have been developed for numerous applications, and with the cost consideration needs in today's economic climate, proper design for each application is important.
Since the key to providing the sought-after cost saving opportunities in the well completion menu lies in proper selection, this article will define the function of each plug design and will discuss the general application(s) for which it is intended. The discussion will cover scenarios from surface to subsurface trees for both conventional and horizontal trees.
Conventional and horizontal trees, those without wellhead plugs, employ a valve arrangement consisting of two redundant, swab valves positioned in vertical alignment with the wellbore. These valves are opened to allow vertical access to the well. Wellhead plugs can be used to replace one or both of these valves.
On conventional trees, one wellhead plug acts as a direct replacement for a surface valve. An additional wellhead plug, located below the production port, functions either as a temporary positive well-control device or as a backpressure valve with a pump-through design. On horizontal trees, wellhead plugs can be installed in the tubing hanger above the production port, thus replacing one or both valves.
Moving subsurface with a conventional subsea well, a wellhead test plug is set below the control valves in the vertical run of the subsea tree. This allows pressure testing of the riser and equipment above the tubing hanger. An additional wellhead test plug, usually smaller, can also be set in the annular side of the conventional subsea tree. These wellhead test plugs are used for testing only and are removed during production. This plug employs two redundant packing stacks.
Subsea treesWith subsurface horizontal trees, a wellhead plug can be installed in the tubing hanger above the production port, thus replacing one of the valves. A second wellhead plug with a larger outer diameter (OD) can be installed in the tree cap, replacing the second valve. By designing a profile for the wellhead plug in the tree cap, the lower wellhead plug, and the well in general, can be accessed without removing the tree cap.
Wellhead plugs range in sizes compatible with 2 3/8-in. to 9 5/8-in. production tubing. The size of wellhead plugs must be evaluated and specified for a given application by the critical interface dimensions. The plugs must be large enough to permit full access to the completion tubing through the tree since completion accessories (such as downhole electronic gauges, sliding sleeves or chemical injection mandrels) require service tools that must pass through the bore and profile of the wellhead plug.
The requirements of the subsurface tubing-retrievable safety valve service tools as well as the lockout contingency equipment must also be considered. While satisfying the through-bore needs, the wellhead plug OD must still be small enough to pass through the lubricator, blowout preventor (BOP), riser, and installation and intervention equipment. Other requirements that can influence plug diameter are test pressure from above and below and minimum yield of the profile and wellhead-plug materials. Once these critical parameters have been addressed, the next step is to determine the appropriate wellhead-plug lengths. A short wellhead plug design results in a short tree design. The cost of adding a few inches to the tree to accommodate a long wellhead plug substantially increases the cost of the overall completion.
Well conditions will normally determine the materials selected since operating conditions, reservoir production fluids, injected fluids, and gas will impact the opportunity for corrosion. Providing metallurgical compatibility between wellhead plug and hanger materials will minimize corrosion and promote plug retrieval when needed.
MaterialsMetals commonly used are Nickel-Chromium-Molybdenum alloy Inconel 718 (UNS-NO7718), Nickel-Iron-Chromium-Moly bdenum alloy Incoloy 925 (UNS-NO9925), 17-4 PH, F6NM, 13% chrome, 410 Stainless Steel, 9-chrome/1 moly, and 4140 Alloy steel.
Metal-to-metal seals, elastomer seals, nonelastomeric Vee-ring packing, and crimp-type seals are typically used with a redundancy of seal elements in all cases. An example of a redundancy situation would be a wellhead plug that has an elastomeric wiper ring, a metal-to metal-seal, a non-elastomeric packing stack with an elastomeric O-ring, and a dirt seal. In addition to the redundant seals, potential leak paths are also considered and kept to a minimum. Most often, designs will have only one possible leak path though the redundant seals.
The wellhead plugs are set and retrieved using slickline services. The surface slickline equipment will require interface verification. Items to consider are the connection from the surface tree or riser to the slickline BOP, and sizing of the lubricator. The wellhead plugs employ different setting methods, based on the primary sealing element type. Wellhead plugs with primary metal-to-metal seals are set using a hydrostatic running tool. The pressure rating of the riser must allow for the setting pressures required to properly set wellhead plugs that use the hydrostatic running tool.
Wellhead plugs that use elastomeric packing as the primary sealing mechanisms are set using a slickline mechanical running tool and slickline jars. Wellhead plugs are assisted into the proper landing place in the profile with pressure. Controlling the pressure above and below the wellhead plug also assists in proper wellhead plug retrieval. In a two-wellhead plug system, the top plug is often removed by pumping into a vent-line located between the two wellhead plugs. The wellhead plugs are retrieved using mechanical and/or hydraulic means.
The wellhead plugs have internal fishnecks that allow pulling with standard slickline service pulling tools.
Landing profilesLanding profiles are created through a variety of methods. The tree, tubing hanger, or tree cap can be machined with a bore to allow for the placing of a nipple profile insert. The profile can also be machined directly into the tubing hanger or the tree cap. A profile interface is coordinated with the tree manufacturer, and tolerances are inspected according to print dimensions. The profile internal diameter (ID) steps down, based on the determining factors previously discussed.
Wellhead plugs are designed so the pressure ratings are equal to or exceed the ratings of the tree itself. The no-go interface shoulder is a determining factor on the test pressure from above for wellhead plugs that use a metal-to-metal sealing as the primary seal. For wellhead plugs that use elastomeric packing as the primary seal, the no-go shoulder size is not a determining factor with the key/profile interface capable of supporting the pressure from above. For all types of wellhead plugs, the pressure below the wellhead plug is supported by the keys/profile interface.
Complimentary accessories designed to locate and set in the same profiles employed by the wellhead plugs are available during slickline or coiled tubing intervention. The nipple protectors are installed and retrieved using the same tool applied for running and pulling. A short nipple protector will locate in the lower profile in the hanger and is designed to permit unrestricted access into the production tubing. This accessory is used to protect the polished bore of the tubing hanger. A long nipple protector will be located and set in the same lower profile and provides an upward extended fishneck to protect the upper tree cap profile and seal bore.
AccessoriesOther accessories such as an isolation sleeve have been designed to locate and set in the same profiles as the wellhead plug. The isolation sleeve is installed and retrieved using the same tool for running and setting. An internal-latching, short isolation sleeve locates on the no-go of the lower profile and is designed to seal above and below the production outlet. The pressure rating will match or exceed the test pressure of the tree. This pressure testing capability is the cause for a reduction in ID as compared to the nipple protectors. A long isolation sleeve will affect the same set location and seals but is extended upward into the tree cap to prevent communication between the upper nipple profile bore and to seal above the production outlet.
Should the need arise to pump fluids down the flow line outlet, there is also an isolation sleeve with a plug and equalizing device option. Again, the lower profile is used, but there is a port between each seal of the isolation sleeve to allow fluid access into the production outlet. Once fluid or testing is completed, the isolation sleeve-equalizing device is shifted, allowing the isolation sleeve to be equalized and pulled from the set profile. Standard slickline tools that latch internal fish necks perform primary retrieval of all complimentary equipment.
Since the wellhead plug acts as a primary barrier between the well and the environment, an extensive qualification test program is needed. Neither API nor any other regulatory agency covers the testing and qualification of wellhead plugs. However, API does provide guidelines for testing and the qualification of trees and safety-valve lock mandrels. Using these guidelines as a reference, wellhead plugs are tested and qualified with performance verification testing meant to emulate the requirements of the specific area. The reference guidelines are "API Specification 6A (6) and 17D," "PR-2, Appendix F (7)" and "SI-913." Additional testing can also be specified by the end user to address the needs of the particular application.
Wellhead plugs are created to meet specific functions, and specialization and optimization have resulted in a number of design options tailored to individual applications. To take advantage of these specialized wellhead plug capabilities, careful preliminary analysis of interfacing needs of the completion and tree design is required. With the state-of-the-art specialization features now available, the cost savings possible with wellhead plug applications extend beyond simple valve substitutions to the riser, tree, intervention, and well control equipment interfaces.
Brighenti, G, Mancini, P, "Economical and Technical Evaluation of Horizontal Subsea Trees Compared To Conventional Trees" Offshore Mediterranean Conference and Exhibition, March 1995.
Skeels, H., Hopkins, B., Cunningham, C., "The Horizontal Subsea Tree: A Unique Configuration Evolution", Paper No. 7244, Offshore Technology Conference, Houston, May 1993.
Vick, J., Miller, J., "A Wireline-Retrievable Wellhead Plug System for Use With Horizontal Trees, Paper No. 7917, Offshore Technology Conference, Houston, May, 1995.
Vick, J., Robb, E., Thomson, D., Collie, G., "A Static Slickline-Retrievable Wellhead Plug System for Use with Horizontal Trees in Offshore Systems." Paper No. 50591, European Petroleum Conference, October 1998.
Ray, T., "High Pressure/High Temperature (HP/HT) Seals for Oil and Gas Production, Paper No. 39573, SPE India Oil and Gas Conference and Exhibition, New Delhi, India, April, 1998.
API 6A "Specification for Valves and Wellhead Equipment," 17th Edition, (1996).
API 17D "Subsea Wellhead and Christmas Tree Equipment," First Edition, (October,1992).
Wellhead plugs are created to meet specific functions, and specialization and optimization have resulted in a number of design options tailored to individual applications.
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