Elastomers

Elastomers are rubber or plastic materials used as a seal. They are commonly used in packers.

Considerations in selecting an elastomer

There are many suitable elastomers on today’s market to match almost any downhole condition. Care must be taken to ensure that the elastomer selected for the packer and seal assembly meets all the downhole conditions to which it will be subjected. Things that must be considered are:

• Downhole operating temperature
• Exposure to produced or injected fluids and gases
• Exposure to completion fluids such as oil-based mud, brine, bromides, high pH completion fluids, and amine base inhibitors
• Exposure to solvents such as xylene, toluene, and methanol.

There is no single best elastomer that will perform under all conditions combined, and selection must be tailored to suit individual well requirements and application.

By far, the most common elastomer used in downhole completion packers is nitrile. Nitrile is used in low- to medium-temperature applications for packers and packer-to-tubing seal assemblies in one form or another. It shows good chemical resistance to oils, brines, and CO₂ exposure. However, its use is limited in wells that contain even small amounts of H₂S, amine inhibitors, or high-pH completion fluids. Exposure to high concentrations of H₂S and bromides generally is not recommended.

Types of elastomers

Hydrogenated nitrile (HNBR)

Hydrogenated nitrile or HNBR (chemical name: hydrogenated acrylonitrile butadiene) has a somewhat higher temperature rating and shows slightly better chemical resistance to H₂S and corrosion inhibitors than standard nitrile. HNBR is more prone to extrusion than standard nitrile and, as a result, requires a more sophisticated mechanical backup system similar to that found on most permanent and higher-end retrievable packers.

Fluoroelastomers

Two fluoroelastomers that are commonly used in the oil and gas industry are:

• Hexafluoropropylene (vinylidene fluoride, commonly known by the trade name Viton)
• Tetrafluoroethylene (propylene, trade name Aflas)

These compounds are used in medium- to high-temperature applications. Both compounds show excellent resistance to H₂S exposure in varying limits, CO₂, brines, and bromides. However, the use of Viton should be questioned when amine inhibitors are present in packer fluids and in the case of high-pH completion fluids.

Examples of Fluoroelastomers

Aflas will swell when exposed to oil-based fluids and solvents. Swelling, because of exposure of Aflas to hydrocarbons, is generally only a concern when running the tool in the well. Element swell may cause the packer to become stuck on the trip in the hole, and swelling of the seals can result in seal damage during stab-in. After the packer is set and seals are in place, the swelling generally is no longer a concern.

The use of Kalrez and Chemraz in the packer industry is by and large limited to chevron-type “vee” seals and o-rings. On the cost scale, they are by far some of the most expensive materials used in these designs. Kalrez and Chemraz show good resistance to most chemicals found in oilwell and gas-well environments. Because of their ability to maintain stability at extreme temperatures, they are normally recommended for use in HP/HT applications and in most environments in which high levels of H₂S are encountered.

Ethylene propylene (EPDM) is an elastomer commonly used in steam-injection operations. EPDM exhibits poor resistance to swelling when exposed to oil and solvents; however, EPDM can operate in pure steam environments to temperatures of 550°F.

Packing Element

The term “packing element” is used to describe the elastomeric sealing system that creates the seal between the outside diameter (OD) of the packer and the inside diameter (ID) of the casing. The ability of the packing element to hold differential pressure is a function of the elastomer pressure, or stress across the seal. To form a seal, the elastomer pressure must be greater than the differential pressure across the packer. The elastomer pressure is generated by the packoff or setting force applied to the packer.

The packing-element system consists of the seal or packing element and a packing-element backup system. When energized, the packing element expands to conform to the ID of the casing wall. The packing-element backup system contains the energized packing element and restricts the element from extruding or losing its elastomer pressure.

Packing element system design

There are many different packing-element-system designs. Each element-system design is suited to a specific application and covers a myriad of well environments. The most basic packing-element system consists of a single packing element with fixed metal backup rings located above and below the element. More sophisticated designs may consist of multidurometer elastomers using a lower durometer element between two elements of a higher durometer. In this design, the lower durometer, or softer-center element, creates the working seal while the higher durometer, or harder-end, elements expand to the casing ID to restrict extrusion. Fixed metal backup rings also may be replaced with flexible or expandable backup rings to further restrict the extrusion of the elastomer.

Packer-to-Tubing Seal Stacks

Permanent and retrievable sealbore packers contain a honed sealbore to accept packer-to-tubing seals or seal assembly to connect the tubing string to the packer. This seal assembly, or stinger, consists of a seal sub with multiple packing units or seal stacks fixed on its OD. The packing units come in a variety of configurations and elastomeric compounds to suit a wide range of downhole conditions. There are two basic types of packing units:

• Bonded
• Chevron

Bonded packing unit

The bonded packing unit is composed of one or more metal rings, with a specific elastomer compound bonded or molded to the ring. The bonded seal by design is slightly larger than the ID of the sealbore, and a predetermined amount of stress on the elastomer is created when the seals are inserted into the honed packer bore. The elastomer pressure generated by this stress creates a seal between the seal assembly and the honed packer bore.

Because the bonded seals are self-energized, they are particularly useful in low pressure/low temperature (LP/LT) gas-injection operations such as CO₂ flood projects. The bonded seals are also less susceptible to dynamic unloading damage and should be selected any time that the seals must leave the honed bore under pressure.

Only a few elastomer compounds are suitable for use in bonded seal designs. The three most common compounds found on bonded seal stacks are:

• Nitrile
• Viton
• Aflas

Because the bonding tends to fail at higher temperatures, most bonded seals are generally not recommended for service above 300°F.

Chevron packing unit

Chevron seal stacks come in a wide variety of designs and elastomeric compounds. They consist of a number of “vee”-shaped chevron seal rings supported by metal (or a combination of metal and nonelastomeric) backup rings such as Ryton or Teflon. Each individual chevron seal ring holds pressure in one direction only, so each seal stack must contain a number of seal rings facing in either direction.

The chevron seal stacks are the most versatile and widely used. They are available with various elastomers and designs. Common materials used for the vee-type seal rings include:

• Nitrile (the most common)
• Viton
• Aflas
• Kalrez

Some specialized premium seal stacks can handle pressures up to 15,000 psi (and beyond) at temperatures approaching 550°F. Each has its own environmental application, as well as temperature and pressure rating. Matching the proper elastomer to the environment is a key to long-term sealing success.

The Chevron seal stacks do not lend themselves well to differential unloading conditions that might be experienced during fracturing or treating operations in which locator-type seal assemblies are used in sealbore packers. The temperature and piston effects will cause the tubing to shorten, and the seal assembly will move upward out of the packer bore. Any Chevron seal that is allowed to leave the polished sealbore will be subject to severe damage, because of the sudden change in differential pressure. Because of this, locator-type seal-assembly designs should be such that the working seals are never allowed to leave the polished packer bore under differential pressure.

Effects of seal movement

To reduce the possibility of seal failure and greatly extend the life of the seal assembly, it is recommended that seal movement be restricted whenever possible. While both chevron and bonded seals are designed to hold pressure under dynamic conditions, completion designs that allow continuous seal movement over the life of the well can significantly shorten the life of the seal. Seal movement should be eliminated altogether if possible by anchoring the seals in the packer bore. Locator seal assemblies should be landed so that the locator sub will be in constant compression when the well is producing, thus limiting movement to those cases in which the well is either treated or killed.

General references

Allen, T. and Roberts, A.P. 1993. Production Operations, fourth edition, I and II.

Factors and Conditions Which Cause Seal Assemblies Used in Downhole Enviornments to Get Stuck. Baker Oil Tools—Engineering Tech Data Paper No. CS007.

Patton, L.D. and Abbott, W.A. 1985. Well Completions and Workovers: The Systems Approach, second edition, 57–67. Dallas: Energy Publications.

See also

Packers

Completion systems

PEH:Completion_Systems