Facing extreme heat, pressure and chemical action all at once isn't a job for the faint-hearted. But the rubber seals used in the energy industry are designed to do it with aplomb.
There's more to the humble rubber seal than meets the eye. Especially when it's put to use in the oil field. This is where the trifecta of heat, pressure and corrosive chemical action comes into play. And in a lot of ways, the yield, efficiency and safety of the entire operation boils down to the rubber seals and gaskets that hold everything in place.
Not just any standard-edition rubber seals, though. The kind that are used in oil and gas wells are made of highly resistant and reliable elastomeric compounds formulated especially for use in the energy industry. These elastomers can be derived from both natural rubbers and synthetic polymers, as long as they display the required temperature, pressure and chemical resistance.
Let's get to know some of these elastomers.
Affordable and easy to process into both standard and custom forms, nitrile is by far the most popular oil field elastomer. It checks all the boxes in terms of strength, displays decent resistance to temperatures up to 125°C, and withstands most chemicals except hydrogen sulfide and strong acids.
There are various types of nitriles, each containing a different ratio of acrylonitrile and butadiene. Nitriles with relatively higher acrylonitrile levels are more oil resistant. But on the flipside, increasing acrylonitrile levels too much can compromise the low-temperature properties of the compound. At present, nitriles with 41% acrylonitrile are most commonly used in energy applications.
Highly Saturated Nitrile (HNBR):
In layman's terms, HNBR can be understood as nitrile's cousin with better heat resistance. The butadiene units of nitrile are hydrogenated, and the resultant compound is cured with peroxide to further improve its properties. HNBR is compatible with a wider range of chemicals, and can be safely used in temperatures as high as 200°C. As an added bonus, it doesn't lose its hardness or modulus with rising temperatures. It costs a bit more than nitrile, but still falls within the moderately priced bracket.
Priced on the slightly higher side as compared to NBR and HNBR, fluorocarbons are extremely resistant to oils, fuels and acids. Some of them can withstand temperatures up to 250°C. Though fluorocarbons often require a little extra processing, they tend to be readily available in both standard and custom forms and as such have found a wide range of uses in the oil field.
Elastomeric seals, and how they survive the oil field:
For the most part, oil field elastomers take the shape of static or dynamic seals. And the challenges they need to address are unique to the energy industry.
To begin with, there's the daunting combination of high temperature, pressure and chemical action. Oil well seals may routinely encounter temperatures higher than 200°C along with a differential pressure of 70MPa, while simultaneously being in contact with highly acidic or caustic fluids. They also have to tackle low oxygen levels in down-hole applications, and hold up in spite of unpredictable physical and chemical changes.
Generally speaking, both static and dynamic seals used in the oil field can be categorized as either compliant or energized. Compliant seals are designed to ‘comply' with a known setting; they include the likes of O-rings, V-packing and swab cups. Energized seals rely on an external force to cause a distortion and create an initial seal; they include pipe rams and packer elements.
Both kinds of seals need different materials and different approaches to design. Compliant seals are relatively easy to make, but they also pose a higher risk of seal loss with physical or chemical changes. Energized seals are harder to design, involving a series of calculations to ensure that the distortion load creates an initial seal. But on the plus side, they're better than compliant seals when it comes to surviving physical and chemical changes.
Using both compliant and energized seals in a single setup is a common challenge encountered in the oil field. For instance, let's say that a packer (energized seal) is set in place by applying 10,000 kg of pipe weight. The weight distorts the rubber element with respect to the casing wall, creating an initial seal in the process. Over time, this distortion continues to compensate for any physical changes in the rubber, keeping the seal intact.
Meanwhile, the O-rings (compliant seals) in the packer are also going through physical changes. Except in their case, there's no way to compensate for these changes. After a while, the O-ring loses its shape and the packer starts leaking.
Leaking O-rings aren't the biggest problem for down-hole packers. Their very design needs to cater to seemingly contradictory parameters. The packer must be able to deform enough to create the initial seal, while also offering enough extrusion resistance to prevent loss of sealing force over time. This is only possible when the elastomer used has the right modulus – which in turn requires some tinkering with the elastic and viscous properties of the material.
Case in point – the blow-out prevention (BOP) packer. Among the largest rubber components in the oil field, BOPs can use up to 500 kilos of rubber at a time, which in turn is bonded to hundreds of kilos of metal for support and extrusion resistance. The rubber must be able to elongate and close upon itself or on the drill stem without rupturing. It must later revert to the original internal diameter so the drill stem and casing can be inserted without causing damage.
Another type of seal that needs this kind of flexibility and resistance is the inflatable packer. As the name suggests, it much be able to pass through extremely constricted openings, and then go through hydraulic inflation to seal off the oil well. In the process, the rubber may need to elongate to 5 times its original size, without tearing or giving in to extreme temperatures or harsh chemicals.
Needless to say, these complex applications require a fair bit of design and planning. More than anything else, they require specific formulations that not only meet the present-day demands of the energy industry, but also allow constant improvements, upgrades and customization.