Maximizing the yield from wells is a high priority for operators. To this end, exploration continues to move to greater subsea depths and increasingly focuses on enhanced oil recovery (EOR) processes, which can substantially extend global oil reserves.
A move to the unconventional
Accessing unconventional wells — those previously believed to be too deep for existing capabilities — is now more frequently undertaken, and the associated subsea intervention needs to be even more sophisticated than in previous attempts.
An unconventional well is essentially any reservoir that requires special recovery operations outside the traditional operating practices. This means processing technology must be able to withstand high fluid pressures while subjected to very high-temperature conditions, often coined high-pressure, high-temperature (HPHT).
Conventional subsea operating temperatures of up to +120°C/+248°F often increase in unconventional wells to more than +200°C/+392°F in deep reservoirs, while typical pressures have on occasion trebled from a previous standard of 69 megapascals (MPa)/10,000 psi to 207 MPa/30,000 psi.
Seals are critical elements in oil and gas systems, particularly in valves and downhole drilling, and completion and intervention tools. They ensure that oil field equipment is working to its optimal capacity, as well as acting as the primary barrier to preventing any system fluid loss or system fluid contamination from external sources.
When working in the subsea environment, seal function becomes more significant still. Subsea seals are the principal components that prevent hydrocarbon leakage from oil field completion or production equipment into the world’s delicate oceans and as such, they perform a vital role in meeting environmental concerns while ensuring workforce safety compliance. This is in addition to enhancing performance of the tools themselves.
Some standard sealing solutions and materials are unable to withstand the extreme operating temperatures and pressures of drilling in greater water depths and to reach deeper into the reserves. Therefore, specialized solutions in material and seal profile technology are required for the industry to start exploring unconventional wells.
Recognizing this, new material and product solutions focus on specific oil and gas issues such as rapid gas decompression (RGD). This phenomenon occurs when an elastomer has been subject to high pressures for a sustained period of time, driving gas deep into the structure of the polymer. If the system pressure is released relatively quickly, this trapped gas can expand significantly before it has a chance to escape from the material matrix, potentially damaging elastomer seals by ripping them apart from the inside.
Special compounds from hydrogenated nitrile butadiene rubber (HNBR) to fluoroelastomer (FKM) to tetrafluoroethylene/propylene copolymer and perfluoroelastomer (FFKM) are available for RGD resistance.
Demanding EOR applications
These newly produced elastomer materials feature low compression set characteristics. This material property is particularly relevant to EOR applications, in which seals may be required to function for much longer durations than in traditional interventions while also coping with HPHT conditions. The ability of the elastomer to resist compression set and hence maintain a large degree of the latent internal sealing forces is critical to ensure that the seal continues to function correctly across the range of energizing pressures for extended tool operating lifetimes. Specifically engineered for the offshore and subsea industry, the materials match up to the most demanding of upstream requirements and are ideally suited for challenging EOR systems.
Standard versus special
While recognizing the traditional standards used in the industry such as API 6A, ISO 23936 and NORSOK M710, and having a portfolio of materials that satisfies these, specific applications sometimes require materials that go beyond the standards. These are particularly prevalent when dealing with unconventional wells and often require tailored materials. Applications may have, for example, exceptionally high methane content in the well or a focus on compression set properties at high temperatures for long endurance capability.
Some specialist HNBR materials, for instance, exhibit exceptional low-temperature sealing performance, making them suitable for use in HPHT applications. These materials can be ideal when equipment is stored topside in cold climates and then sent downhole, where pressures rise quickly but the equipment temperature increase lags behind. The influence of pressure on the glass transition/cold-temperature flexibility of an elastomer can have serious consequences to the performance of seals in such applications.
Furthermore, HNBR materials exhibit superior low compression set performance and high-temperature sealing capability, making them eminently suitable for operating for extended lifetimes in aggressive well environments. Additionally, high mechanical-strength HNBR grades provide outstanding wear and abrasion capabilities, producing excellent results for use in dynamic applications while under HPHT conditions.
Specialist FKM materials have been developed that exhibit superior methanol resistance and optimal chemical resistance for EOR applications. Industry standards such as NORSOK M710 require testing to certain levels of methanol concentration and temperature, for example. However, in high-methanol applications, the actual well conditions present differently from the industry standard. For such wells, it means that the materials, even though they meet standards, would not necessarily achieve performance criteria.
Pushing the envelope
Whether a design engineer needs a standard O-ring or a custom-engineered seal, oil and gas equipment makers will continue to heavily rely on sealing material experts to optimize the performance of seals in equipment.
While oil and gas applications face increasingly critical challenges, the upside is that equipment manufacturers who serve them are thinking out of the box — from the developments of subsea robots working underwater to 10,000-foot depths to the use of seismic imaging systems to see below the seabed. Sealing material developers are following suit by pushing the envelope with advanced sealing materials and profiles to handle the most extreme of temperatures, the highest of pressures and prolonged exposure to the most aggressive fluids.