August 2018, Vol. 245, No. 8
Features
Unconventional Well Demands and EOR Drive Innovation in Subsea Sealing Technology
By Andrew Longdon, UK Technical Manager for Trelleborg Sealing Solutions
As the subsea industry continues to evolve to help meet the demands of oilfield operators, there is now an even greater requirement for ground-breaking sealing technology.
Maximizing the yield from wells is a high priority for operators. To this end, exploration continues to move to greater subsea depths and there is an increased emphasis on Enhanced Oil Recovery (EOR) processes, which are able to 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 being more frequently undertaken and the associated subsea intervention needs to be even more sophisticated than previous attempts.
Unconventional wells are 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 HP/HT.
Conventional subsea operating temperatures of up to +120 °C/ +248 °F often increase in unconventional wells to over +200 °C/ +392 °F in deep reservoirs, while typical pressures have on occasion trebled from a previous standard of 69 MPa/ 10,000 psi to 207 MPa/ 30,000 psi.
Safety Critical
Seals are critical elements within oil and gas systems, particularly valves and downhole drilling, completion and intervention tools. They ensure that oilfield equipment is working to its optimum capacity, as well as being the primary barrier to preventing any system fluid loss or system fluid contamination from external sources.
When working in the subsea environment, the seal function becomes more significant still. Subsea seals are the principal components that prevent hydrocarbon leakage from oilfield completion or production equipment into the world’s delicate oceans and as such, perform a vital role in meeting environmental concerns, while also 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 both material and seal profile technology are required in order 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 then 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 TFE/Propylene copolymer and Perfluoroelastomer (FFKM) are available for RGD resistance.
Demanding Enhanced Oil Recovery Applications
These newly produced elastomer materials feature low compression set characteristics. This material property is particularly relevant to EOR applications, where seals may be required to function for much longer durations than in traditional interventions, while also coping with HP/HT conditions. The ability of the elastomer to resist compression set and hence maintain a large degree of the latent internal sealing forces, is critical in ensuring 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.
While recognizing the traditional standards used in the industry, such as API 6A, ISO 23936, Norsok M710 etc., and having a portfolio of materials that satisfy 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 example exhibit exceptional low temperature sealing performance making them suitable for use within high pressure / low temperature applications. These materials can be ideal where 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, giving excellent results for use in dynamic applications while under higher pressure and temperature conditions.
Pushing the Envelope
Whether a design engineer has a need for a standard O-Ring or a custom-engineered seal, oil and gas equipment makers will continue to heavily reply 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 serving 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 of fluids. P&GJ
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