Slickline Surface Equipment : Stuffing Box

Hydraulic Stuffing Box

Containing wellbore pressure and fluids under dynamic conditions as the wire enters the wellbore is a critical factor in slickline operations. The selection of an appropriate sealing device is determined by the specification of the wire, the wellhead pressure and the characteristics of the wellbore fluid. Three types of sealing device are available:

• Stuffing box – suitable for all sizes of slickline
• Swabbing head – suitable for use with braided line, providing a partial seal during applications such as swabbing
• Grease injection head – designed for braided line and capable of providing an effective high-pressure seal

This section details the design and configuration of the stuffing box used for slickline applications. The principal function of a slickline stuffing box is to seal around the wire at the upper end of the lubricator during slickline operations, in both static and dynamic conditions. An adjustable packing nut can be used to increase the compression force on the packing elem ent s or r ubbers as they wear. A swivel- mounted sheave wheel and guard to guide the wire as it enters or leaves the wellbore (having 360º free movement) are fitted to the top of the stuffing box. The standard stuffing box is rated for 5000 psi service, however, stuffing boxes of 10,000 psi and higher rating are available.

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The slickline size (diameter) determines the size of the sealing components of the stuffing box and the minimum diameter of the sheave wheel. The sheave diameter for 0.092 in. line is 10 in., and for 0.108 in. line it is typically 15 in. The sealing components and sheave wheels must be replaced when the equipment is used with a different wire size.

Most stuffing box designs contain a BOP plunger, which prevents wellbore fluid escaping through the gland in the event that the slickline breaks and the free end is pulled from the stuffing box. Well pressure acting on the cross-sectional area of the wire forces the wire out when the line weight is less than the upward force. With no wire in place, the flow causes the BOP plunger to activate, with the rubber seal on the upper end deforming to pack-off in the lower gland. With the plunger in place, the wellbore is isolated until the BOP or swab valve can be closed to fully secure the well. The natural curling tendency of the wire form loops as the tension is removed when the line breaks. The sheave guard on the stuffing box is designed to trap wire, if a break occurs at surface.

Some designs of stuffing box incorporate an injection port that can be used for the following purposes:

• Injection of glycol or similar antifreeze fluid in low temperature environments or when working on high-pressure gas wells where hydrates may be a formed.
• Injection of a chemical inhibitor to protect the slickline in corrosive well conditions, e.g., H2S
• Bleed-off pressure to activate the BOP plunger if the packing has to be changed under pressure. In most cases, an adeqate seal can be regained by splitting one packing laterally and installing it with the existing packing. Re-energize the packing nut to activate the seal to enable the toolstring to be retrieved. The stuffing box can then be serviced before the next run.

The use of a remote hydraulically activated packing nut enables the packing force to be adjusted from a safe distance. This feature is particularly useful when:

• The stuffing box cannot be easily reached during operation.
• On high-pressure wells when close proximity to a pressurized stuffing box is not advisable.
• On wells containing dangerous levels of H2S.

In preparing for use, the following stuffing box checks should be carried out:

• Check the packing condition. If the packing nut is near the lower end of its movement, there may not be sufficient adjustment remaining to apply the packing compression force required to maintain a seal throughout the intended operation.
• Check the sheave diameter is the correct size for the line in use (10 in. for 0.092 in. or 15 in. for 0.108 in.).
• Check the upper and lower brass packing glands for wear. If worn or oversize, they should be replaced, since worn glands allow the wire to cut the packing faster.
• Checkthesheave wheel and bearings for free spinning and side play. The sheave should not touch the sides of the support arms. Excessive free-play also leads to a worn upper gland and subsequent reduction in packing life.
• Check the alloy side arms for damage from side play in the sheave wheel. The complete sheave staff should be replaced if cutting/wearing action has occurred on the inside of these arms.
• Check the sheave staff for freedom of swivel movement. It is essential that the sheave follow the wire direction during rig-up or the wire can jump out of the groove and be damaged.
• Check the sheave guard is tight and adjusted close to the sheave to ensure it will trap the line in the event of a line break.
• Check the BOP plunger for wear and freedom of vertical movement.

Keeping the line oiled while running into the wellbore can extend the life of the packing. The choice of packing is also important. Check the manufacturer’s recommendations for the appropriate material to suit the field conditions encountered.