Over the years, upstream oil operations have improved their efficiency and capability of pulling greater quantities of oil out of the ground. Engineers are achieving this by drilling deeper, using enhanced oil recovery (EOR) and running at higher temperatures than ever before. Although effective, these tactics can pose challenges to the seals used in pumps and compressors, as they often create demanding environments for the seals to operate in.
More Than Just Oil
The oil content found in wells varies and can depend on the location, climate and elevation—as well as other factors. A common variation in crude oil is whether it is considered heavy or light. Light crude oil contains mostly hydrocarbons. As impurities in the oil increase, it becomes heavier, thicker and more viscous and is referred to as heavy crude oil. Impurities can include heavy metals, aromatic compounds (chemicals like toluene, xylene and benzene) and resins, making the heavy crude oil less valuable and more time-consuming to process. Tar sands, bitumen and asphalt are all considered heavy. The heavier the crude oil is, the more challenging it is for the seals in the processing operation, as abrasives and sticky resins tear and destroy seals, while aromatic compounds can cause potential chemical attacks.
From well to well, the amount of sulfur content can also vary. Oil with low amounts of sulfur is called sweet crude oil. Sour crude oil, in contrast, has high amounts of sulfur, which can be dangerous. The greater the sulfur content, the higher the probability of the presence of hydrogen sulfide (H2S), which is a poisonous, corrosive and flammable gas. Both sulfur and H2S have a signature rotten egg smell, which makes them easily identifiable.
However, H2S is dangerous and can be deadly at higher concentrations. Even in small amounts, H2S can chemically attack seals—while polytetrafluoroethylene (PTFE) is fully compatible, nitrile rubber (NBR) is typically compatible in concentrations less than 10 parts per million (ppm), fluoro-elastomer (FKM) less than 2,000 ppm and hydrogenated nitrile butadiene rubber (HNBR) less than 50,000 ppm (5%).
Enhanced Oil Recovery
In many oil wells, only about 10% of the oil is simply pumped out of the ground. Enhanced oil recovery (EOR) is typically used next, which is where a separate line is drilled, and steam, water, carbon dioxide (CO2) or other media is injected behind the oil to loosen it up and make it easier to remove. The type of media injected into the oil well can have a significant impact on what the optimal seal material should contain, so it is important to understand the effects of each.
Water injection is the most popular EOR method. While the water used for injection can come from outside sources, produced water is often used (which is excess water pumped out of a different well) and can contain oil, gas, H2S and abrasive particles—such as rust, sand, salts, asphaltenes and waxes. These particles can shorten the lifespan of a seal. In addition, water is considered nonlubricating, so seals that work in oil may stick and tear when used with high amounts of water.
Steam injection, another EOR method, is used for heavy crude that is thick and viscous. The steam injected produces wet heat that loosens up the heavy crude oil. With steam, there are additional considerations of having higher temperatures, plus chemical resistance concerns of steam compatibility with the seal material.
With EOR, carbon dioxide can also be injected behind the oil to push it out. Oil wells often contain groundwater, and when carbon dioxide contacts water it can form carbonic acid, so it is important to confirm chemical compatibility. A larger issue with carbon dioxide is explosive decompression, which is when pressurized gases permeate rubber seals. If a rapid reduction of pressure occurs, the gas inside the rubber can expand violently, creating a fissure resulting in immediate leakage. For carbon dioxide or any gas EOR, it is important to choose seals that have excellent permeation resistance, or minimal porosity, to prevent gas from entering the seal.
In addition to artificially injected gases, naturally occurring gases like methane, butane, propane and natural gas may also be present. In general, gases can be challenging for seals. Gases have a smaller particle size than liquids, which makes them more difficult to seal. Gases are also nonlubricating, so seals—particularly rubber seals—are often damaged as the moving equipment rips and tears the dry rubber.
The issue of nonlubricating materials is compounded when there are high speeds involved or long stroke lengths. High speeds can generate friction, so seal lubrication is extra important in these circumstances. Long stroke length is another concern that many may not know. If the media itself is not lubricating, when the equipment is traveling long distances and rubbing against the seal, any initial lubrication present can be pulled off the seal resulting in tearing or damage. For both gas and long stroke applications, it is recommended to use a lantern ring to inject grease into the seal cavity to keep it lubricated. If this is not possible, use a seal material with the best dry running functionality.
Selecting the Best Seal for the Job
While great for basic oil applications, NBR and mixed rubber seals can have difficulties when used in EOR or other applications that see more than just sweet crude oil. For pumpjack stuffing box packing, plunger and transfer pump packing and compressor seals, there are three main materials that work well in these difficult applications: HNBR, fluoroelastomer (Viton) and PTFE. These materials can also be combined with fabric to provide additional reinforcement for higher pressures.
HNBR, Viton and PTFE have different temperature ratings, chemical and permeation resistance and friction characteristics. There are many resources online to look up chemical resistance ratings for these materials, but as a general rule, PTFE has the best chemical and temperature resistance, then Viton and finally HNBR. Why not just use PTFE for everything?
PTFE alone can be prone to deformation and can lose its resilience over time, resulting in a need for frequent retightening. PTFE with fabric reinforcement typically ends up being hard, and while being tough, it does not achieve an excellent seal. When comparing elastomers, HNBR is tougher and more abrasion resistant than Viton, plus it does well in nonlubricating applications and
has excellent permeation resistance for gas applications.
Fabric combined with rubber or PTFE will make the material stronger, more durable, tougher and stiffer. These all look like good characteristics; so, why choose not to use fabric reinforcement? The answer to that lies in the flexibility of the product. Seals that have a signature v-ring type shape are pressure-activated, which means they use the system pressure to flare open the “V” to achieve a seal. Low pressure is a concern for these types of seals because the pressure might not be high enough to flare the sealing lips open. This is where fabric and the actual hardness of the material come into play.
Just adding fabric to the rubber or plastic will make it stiffer, but there are lots of different fabric types to choose from that can have varying effects. Plus, it is important to choose a fabric that is chemically compatible with the media. There are also varying durometers or hardness of both rubber and plasticthat you can choose based on the application’s needs.
In low-pressure applications, generally, a soft material without fabric will seal the best, but as the pressure increases, fabric reinforcement and harder materials will need to be used. This can get complex if the pressure varies from low to high or if the equipment is older and worn. In these applications, harder seals might not be the best choice for sealing.
A Combination of Seals for Success
In most applications, the seal that seems to work the best is a combination of different materials and stiffness, creating a layered effect where the harder seals are
on the outside and the softer seals are on the inside. This creates a balanced solution that optimizes the strengths of each type of seal. The soft seals conform to any imperfections in the equipment and allow the potential achievement of zero leakage. The hard seals see the brunt of the pressure and are the first line of defense against abrasives, protecting the softer inner seals.
This layered stuffing box packing set design has been used to successfully seal many difficult upstream oil production applications. It has sealed steam applications with up to 30% H2S at over 500 F (260 C). It has excelled in a 300 F (149 C) transfer pump running abrasives at high speeds. It has outlasted traditional packing in a compressor sealing non-lubricating well gases including H2S.
Selecting a seal can be complex and oil well pumping even more so, due to the many variables to consider and the differences seen in each application. It is important to confirm media chemical compatibility, along with other application details such as temperature and pressure, when selecting material(s) for a seal.
Additionally, it is always recommended to consult with the seal manufacturer for the correct sealing product recommendation.
