This article describe the categories and types of seals used for completion equipment.

2 categories of seals:

• static seals where the sealing surfaces do not move relative to one another,
• dynamic seals where the sealing surfaces do move relative to one another.

3 types of seals:

• polymeric or resilient seals. Which are either elastomeric (natural or synthetic rubbers) or plastomeric e.g. Teflon.
• metal-to-metal seals.
• metal encapsulated polymeric seals (combination of the first two types).

1. Polymeric/ Resilient seals

• O-ring: The workhorse, self energised, static conditions
• T-seal: Self energised, with anti-extrusion rings for HTHP and for dynamic applications.
• V- seal: Chevron ring with self energising lip-seals, used in stacks and supported by back-up elements.

Polymeric seals: Nitrile, Viton, Aflas, Chemraz, Peek, Teflon.Functional requirements should include the well conditions (temperature, pressure, well liquids & gases), the seal seat design, the mechanical requirements, the required life and the compatibility with chemicals to be used used.

Nitrile

The first material to be considered is the nitrile compound. This has been a workhorse for the oil and gas industry for a long time. Nitrile rubber is a copolymer of a diene and an unsaturated nitrile.

Nitrile elastomers can be used over a temperature range of -20°F (-28.9°C) to 450°F (232°C) depending on the application. Special compounding must be done for low temperature service. For use as O-rings or other seals that might have movement, the upper temperature limit is 275°F (135°C).

Nitrile elastomers are subject to swelling if used in the presence of aromatic fluids such as toluene or xylene. The swell is usually in excess of 25%. These elastomers are also affected by heavy fluids such as zinc bromide and calcium bromide. Also, nitrile cannot be used as an active seal where H2S is present.

Viton

The fluorocarbon elastomer, better known as Viton, isused extensively in downhole equipment. This elastomer is made up of vinylidene fluoride and hexafluropropylene. Fluorine-containing polymers have long been known for their outstanding resistance to hostile environments. Of the many fluoropolymers available, the fluorocarbon elastomer has played an important role in the oil and gas industry.

Fluorocarbon elastomers perform adequately in sour environments. The sour fluids or gases could contain such materials as carbon dioxide and methane. When dealing with this type of fluid or gas, the elastomer must be selected on the basis of how all the compounding ingredients will affect the seal. Nitrile would have a somewhat lower interaction with CO2, however, nitrile cannot be used because of the H2S present.

If organic amine corrosion inhibitors are going to be used in the well, then Viton is not recommended for seals such as O-rings, Vee-rings or other type seals where there is a possibility of seal movement. Amines were one of the first curing systems used, therefore, the inhibitor continues to cure the material until it becomes hard and brittle. The rate of reaction is dependent on concentration of the inhibitor, the pH of the solution and temperature. Actual field data indicates damage to this elastomer can occur when the temperature is as low as 190°F (88°C) and the concentration of inhibitor is 0.5%.

·Viton is the registered trademark of DuPont Company.

Aflas

Aflas is a copolymer of propylene and tetrafluoroethylene. Aflas elastomers can be used in sour environments as well as those conditions where organic amine corrosion inhibitors are used. This compound has been tested in organic-amine-corrosion inhibitors at 330°F (165.6°C) in a 10% solution of both water-soluble and oil-soluble inhibitors. No cracks were evident in this compound, however, cracks were observed in the Viton compound when tested under the same conditions.

Company does not recommend use of Aflas where temperatures are expected to be below 100°F (37.8°C). Tests conducted at low temperatures along with field experience have shown Aflas is subject to sealing problems.

·Aflas is the registered trademark of Asahi Glass Company, Inc.

Chemraz

Chemraz is a member of the perfluoroelastomer polymer family of which Kalrez® is in this same family. Chemraz is molded of an elastomer that has the broadest chemical resistance of any elastomeric material. Chemraz combines the resilience and sealing force of an elastomer with chemical resistance approaching that of Teflon.

Chemraz resists attack by nearly all chemical regents, including inorganic and organic acids, alkalines, ketones, esters, aldehydes, alcohols and fuels. As a result they provide long-term service in virtually any chemical and petrochemical process streams, including many where additives or impurities cause other elastomers to degrade or swell.

Tests have indicated that at low temperatures, below 40°F this particular material is not recommended.

·Chemraz is the registered trademark of Green-Tweed & Co., Inc.

PEEK

Polyetheretherkestone (PEEK) is a high temperature, crystalline aromatic polymer. The armoatic structure of this material is responsible for its performance at high temperature and in chemically hostile environments. This material is excellent for deep, hot, sour oil and gas wells. It can be used as back-up rings for O-rings and Vee-packing. This material offers a unique combination of properties with outstanding thermal characteristics and resistance to an extraordinarily wide range of solvents and proprietary fluids.

·PEEK is the registered trademark of ICI Americas

Ryton

Polyphenylene Sulfide (Ryton) can be compounded with a variety of materials to reduce the brittle nature of the Ryton and to improve the sealability of the compounds. The material has been used as back-up ring for Vee-packing and O-rings. Certain combinations of Ryton and other materials alter the brittle properties of Ryton and make it suitable for vee-ring seals at high temperature and pressure. Ryton can be used in temperatures to 450°F (232.2°C) and in pressures to 10,000 psi.

·Ryton is the registered trademark of Phillips Petroleum Company

Teflon

Other non-elastomer materials used in downhole applications are glass and molybdenum-disulfide-filled teflon. These materials can be used as primary seals when backed up by a harder material such as PEEK or Ryton.

Chemical resistance

The resistance to a range of relevant chemicals such as inhibitors, completion fluids, acids, CO2 and H2S has to be considered. Fluids which are present only during a short time interval may get trapped in the confined space of the seal seat, leading to a long time or even continuous exposure of the seal.

Mechanical properties

The polymer grade is finally selected by comparing the mechanical properties of the different grades with the mechanical requirements for the application (considering service pressure, pressure differentials, extrusion gap, dynamic requirements etc.).

2. Metal-to-metal seals

Metal seals are different from resilient seals in that they cannot easily flow into and fill the roughness between the mating surfaces to prevent fluid passage. They require a much higher contact pressure than resilient seals and it is found that contact pressures that produce only elastic deformation of the contact area do not suffice to establish a gas tight seal in a "dry" state. Seals that are wetted by a liquid film provide better sealing performance and the viscosity of the liquid plays an important role.

Static metal-to-metal seals can be energised to such an extent that they develop a high contact pressure, capable of providing bubble tight gas sealing. With dynamic metal seals permanent deformation of the metal seal contact surfaces is not acceptable. Hence, bubble tight gas sealing of metal dynamic seals that are not wetted with a liquid film seems to be a utopia.

3. Comparison Polymeric seals /metal seals

Polymeric seals

• needs a lower contact pressure to establish a reliable seal.
• Perfect sealing (bubble tight) can be achieved, even when used for dynamic applications.
• Polymeric, especially elastomeric, seals are very forgiving of manufacturing olerances and can cope to some extent with seal surface damage or wear.
• Handling and assembly precautions are not too critical.
• Dynamic seals made of polymeric material can cope better with water-based luids and are more forgiving on the requirement for fluid cleanliness than metal seals.
• Economically more attractive.

Metal seals

• better suited to high temperature and/or high pressure conditions.
• more resistant to chemical attack, for instance from well effluents with H2S and CO2, or from well treatments with Amine-based fluids.
• not effected by rapid gas decompression.
• do not suffer from creep or stress relaxation.
• Frictional behaviour is more constant and easy to predict.
• withstand greater forces.

4. Seal behaviour

Plastomeric seals or metal encapsulated polymeric seals act on plastic deformation and can also deform the confining interface (usually the grooves). When this occurs the space to be sealed is enlarged thereby decreasing the pressure rating of the assembly and also the bolt fatigue life. For example, flat washer-like seals are severely affected by the conditions described above.

Shallow tapered seals are being used successfully in the subsea environment: AX new style, CX, DX, FX, Grayloc, KX, NX, VX and VGX. In these seals, the separation forces are reduced as the seal circle is a minimum; closer to the parallel bore. The exception is CX, where to prevent key seating, the seal circle is larger.

To prevent buckling and cocking of the seal a centralising belt is necessary. This belt functions as the load flank (inner flank) of the API ring groove.

5. Seal selection criteria

In the selection of seals the following aspects should be considered:

• the most important function of a seal is that it should only seal;
• the seal should only be exposed to fluids, pressures and temperatures and not to load transfer, separation forces, bending moments and shear forces; such forces should be taken by the geometry of the connection.

The following API gaskets are examples of bad seal design:

• R. Which causes clear flange stand-off;
• RX and BX. Experience has revealed that these seals seldom achieve flange face to face contact.

These API gaskets transfer loads, align the mating members, provide shear resistance, are plastically deformed, and are often harder than the non-repairable confining groove, which will consequently be deformed. A typical example is groove deformation of the dual tree tops.

Zero flange stand-off has been proven impossible. Statements in vendor catalogues that they can achieve this should be examined closely and not automatically accepted.

6. Seal energisation

It must be possible for seals to be tested, and, as a consequence of this, they must also be re-energisable and/or retrievable. This is, however, not a feature that can be expected of integral hanger seals. The casing hangers can only be retrieved by splitting the casing, and the tubing hanger retrieved only after removing the entire tubing. Therefore it is recommended for all pack-offs to be retrievable without having to pull the confining strings. The present BRX seal and Gray's tubing hanger seal are, therefore, suboptimal on practical grounds.

Re-energisation can be achieved through reload. Torque, weight, and radial energisation are the most common energising methods. Of these, radial energisation is the most precise method, but also the method in which machining tolerances are more critical. All other methods rely on excessive force and unpredictable friction.

A good seal removes the necessity for re-energisation. However, some seals need re-energisation, either through plastic injection or through tie-down screws, which results in undesirable penetrations in the pressure vessel.

The use of elastomer in seals has its limitations:

• maximum pressure of 30,000 to 40,000 kPa (4000 to 6000 psi) depending on containment;
• maximum temperature of 200°C (400°F) for fluor elastomers with the right carbon black and particle size, and 150°C (300°F) for nitriles;
• incompatibility with H2S, CO2 and amines;
• explosive decompression;
• fatigue life;
• wear;
• friction;
• erratic energising behaviour. For example nitrile rubber behaves like a fluid under heavy pressure and as a solid under low temperatures. This has created many deformed casings which precluded access to the well bore.

7. Testing, field experience & seal design and development

It should be pointed out that for demanding applications, the suitability of a certain seal design combined with a seal material should be tested under realistic conditions or should be proven by field experience. Laboratory tests in some cases can under estimate the severity of the application, while in other cases the tests are over-conservative (e.g. swelling in immersion tests). As improvement in a certain property of the polymeric seal (by selecting another polymer or by additives) often leads to a reduction in other important properties, seal development requires careful attention. Applying a seal seat design, developed for more moderate service, in HTHP operations, may lead to unrealistic requirements being placed upon the seal material. Therefore, changing the seal seat design (e.g. reducing the extrusion gap to 0.1 mm or smaller or application of an anti-extrusion ring) should be considered, as it is often easier to adapt the metallic seat than to find a better elastomeric seal material.