The article identifies the most common factors which can play a role in premature failures of packer seals:

1. Installation procedures

• Storage damage: ageing (heat, sunlight or radiation); distortion (poor support, heavy loads).
• Friction damage: non-uniform rolling or twisting, or abrasion by un-lubricated sliding.
• Cutting by sharp edges: Inadequate taper on corners, sharp edges on ports, seal grooves etc.
• Lack of lubrication.
• Presence of dirt.
• Use of incorrect installation tools.

2. Operational factors

• Inadequate duty definition: Composition of the fluids, normal working conditions or transient conditions.
• Seal peeling due to localised rolling as pressure changes.
• Extrusion due to expansion of the seal (swelling, thermal, explosive decompression) or due to compression.
• Too short decompression times leading to blistering.
• Wear and tearing due to insufficient lubrication.
• Wear damage due to pressure fluctuations.

3. Service life

During normal operation, the service life of a polymeric seal is limited by ageing and wear. The temperature, operating pressures, number of cycles (rotations, sliding, mechanical stress) and the environment have an influence on the total service life. Ageing can be a physical phenomenon such as a permanent deformation, or can be due to a reaction with chemicals in the environment. Wear can be caused by rubbing of the seal against another surface in dynamic applications, or by strong pressure fluctuations in static applications. The wear resistance increases usually with increasing hardness of the seal material. Corrosion of the metallic parts and lack of lubrication of the surface increase the wear rate.

4. Minimum and maximum temperature

The sealing ability of elastomers decreases strongly if the temperature is lower than the recommended temperatures, due to a loss in elasticity. The low temperature properties can play an important role in the selection process for elastomeric seals for sub-sea applications in cold oceans. At high temperatures accelerated ageing occurs. The maximum temperature for elastomers varies between 100 and 300°C. Elastomers which can be operated around 300°C tend to have poor overall strength and poor wear resistance. In the design of the seal, room must be reserved to allow expansion of the elastomer due to an increase in temperature (thermal expansion of seal materials is approximately one order of magnitude larger than that of steels).

5. Pressure

The pressure exerted on the seal can result in a permanent deformation of the seal (compression set).The compression set must be limited in order to guarantee leak free operation. Another problem which can arise at high pressures, is swelling (10-50%) of the elastomer volume by absorption of well fluids from the environment. Limited swelling is acceptable if the seal design has allowed for it.

6. Pressure differentials

The elastomer must have an excellent extrusion resistance if there is a large pressure differential over the seal. Extrusion is the most common cause of failure in high pressure seals at high temperatures. The extrusion resistance of a seal may be increased by increasing its hardness. Harder seals need higher interference and assembly forces for effective sealing. The sealed gap must be made as small as possible requiring narrow tolerances during manufacture.

7. Pressure cycles

Pressure cycles can lead to degradation of the elastomer by explosive decompression. The severity of the damage to the elastomer will depend on the composition of the gases present on the seal material and on how fast the pressure changes. The more homogeneous elastomeric materials (e.g. Viton) are more resistant to explosive decompression than elastomers (such as Kalrez and Aflas) which usually contain many small cavities. Decompression occurs predominantly in gas lift applications. If pressure cycles occur, a tight seal gland is desirable because it limits the seal inflation during decompression. This requirement conflicts with the necessity to have room for thermal expansion and swelling of the seal. In dynamic applications a tight seal gland may result in wear or binding of the elastomer.

8. Dynamic applications

In dynamic applications the friction of the seal with the rotating or reciprocating (sliding) shaft can cause wear or extrusion of the elastomer. With a sliding shaft, rolling of the seal can also occur, which can easily result in damage. A demanding situation is the combination of high pressures and a dynamic application. In order to improve the extrusion resistance of a seal its hardness is often increased. A higher hardness implies also that higher interference and assembly forces are needed which result in higher friction forces. In dynamic applications seal swell should be limited to 10-20%, as swell will result in an increase in the friction forces and in wear of the elastomer. An important property for dynamic applications is a high resilience, i.e. the ability to stay in contact with a moving surface.

9. Seal seat design

The seal design must allow for (10-60%) swelling of the elastomer in oil and gas. If not enough room is available the extrusion of the seal will occur. Another important parameter is the size of the extrusion gap. At high pressures only very small extrusion gaps are allowed resulting in a requirement for tight tolerances. In a number of cases anti-extrusion rings can be applied. The design of the seat should also take into account the installation requirements of the seal. During installation elastic elongation (stretch) should not result in permanent deformation and the elastomer should not be damaged by sharp corners. It is worthwhile to note that gland-seal designs are inherently safe, as the seal is not stretched during installation, which is the case in a piston seal design. On the other hand, gland seal designs are more difficult to manufacture and are difficult to access for cleaning and for seal replacement.

10. Compatibility with hydrocarbons, CO2 and H2S

The penetration of hydrocarbons, CO2 and H2S into the elastomer results in swelling. Swelling by hydrocarbons increases with pressure, temperature and aromatic content. The reversible volume increase is accompanied by a gradual softening of the material. Swelling by gases such as H2S, CO2 and O2 increases with pressure and decreases slightly with temperature. Pressure changes after swelling of the seal can result in decompression damage to the seal. H2S reacts with certain polymers, resulting in cross-linking and therefore irreversible hardening of the seal material. Deterioration of elastomers in seal tests (and possibly also in service) is generally less than in immersion tests, probably due to the protection offered by the seal cavity to chemical attack.

11. Compatibility with well treatment chemicals and corrosion inhibitors

Corrosion inhibitors (containing amines) and treating completion fluids are very aggressive against elastomers. Due to the complex composition of the corrosion inhibitors and well treatment chemicals it is advised to determine the resistance of the elastomer by testing.