Meeting tough process sealing challenges

The oil and gas industry is on a quest for new oil resources, and companies are being forced to drill deeper in new, more difficult formations, leading to upstream operating temperatures in excess of 200 C (392 F) and pressures above 29,000 psi.

EOR methods such as injection of steam or CO₂ under high pressure, presence of higher concentrations of hydrogen sulfide (H₂S) in the wells and amine-based corrosion inhibitors can seriously challenge the polymers used downhole to seal drilling equipment, particularly in cases where a sudden drop in pressure can cause seal damage from rapid gas decompression (RGD).

These more aggressive chemical environments are beyond the mechanical, chemical and thermal resistance of most commonly used sealing elastomers, including hydrogenated nitrile rubber (HNBR) and fluoroelastomer (FKM).

Damaging RGD

RGD can lead to seal damage when a sudden pressure drop causes gas dissolved within the elastomer to expand, building up internal stresses that can form blisters or internal cracks.

NORSOK standard M-710, Rev. 2, October 2001, and TOTAL general specification GS EP PVV 142, Rev. 8, the internationally accepted industry test standards for nonmetallic sealing materials, are used as benchmarks of performance of elastomer seals in critical oil and gas service and are considered essential to qualifying an elastomer seal for RGD environments.

Consequences of inadequate seal selection

Premature damage and unexpected failure are the most obvious consequences of selecting a seal material lacking the three key parameters of mechanical, chemical and thermal resistance.

The outcome is likely to be unscheduled maintenance. Take the case of a sudden seal leak in drilling equipment due to RGD. The operator must factor in the time and cost of tool removal, repair, reinstallation and, certainly, a day lost.

High-performance elastomers

Elastomer seals are widely used in petroleum exploration, refining, distribution and transportation equipment. Figure 1 ranks the principal elastomers in terms of heat and oil resistance, with the highest performing elastomers grouped within the green area.

Seals of properly compounded HNBR provide good chemical, thermal and RGD resistance for standard wells operating at around 150 C (302 F), but higher temperatures often necessitate fluoroelastomers formulated for RGD resistance. However, HP/HT and ultra-HP/HT conditions go beyond the thermal capabilities of most HNBR and FKM elastomers and require perfluoroelastomers (FFKMs).

FFKM seals such as DuPont’s Kalrez offer the highest resistance to elevated service temperatures, aggressive amine-based corrosion inhibitors and high levels of H₂S.

FFKM sealing parts

Kalrez FFKM parts combine the resilience and sealing force of a traditional elastomer with the chemical inertness and thermal stability of polytetrafluoroethylene (Teflon), help reduce unscheduled downtime, extend seal life and save cost in harsh industry environments.

The benefits of these parts include temperature stability from -42 C (-43 F) to 327 C (620 F), resistance to more than 1,800 chemicals and aggressive fluids, excellent sealing force retention, low compression set, and elastic recovery. Kalrez 0090 K312 “A” O-rings, specifically developed for oil and gas applications, have attained the best possible NORSOK M710 rating of “0000” for RGD resistance.

Typical uses

O-rings: The high-hardness O-rings are used in upstream applications where elevated temperature, chemical and extrusion resistance is necessary. They are often installed with backup rings where pressures and seal designs require.

Packers: The parts can be made in sizes up to 10 in. in diameter and 5.9 in. long for long-term service in retrievable packers.

Valves: Large valves in H2S service between upstream and downstream processes require the chemical resistance of FFKM sealing parts. The O-ring seals can be supplied in nonstandard sizes up to 1.5 m (5 ft) in diameter for such applications.

Boots: Kalrez boots as small as 1.6 in. in length and 0.4 in. in diameter are specified for downhole submersible pump connections.

Seal selection and total system cost

When considering the economics of elastomer seals, it is vital to look beyond the cost of the seal itself and compare total system cost, i.e., the costs of the seal plus installation plus downtime plus loss of production plus cleanup.

A case study illustrates the point. Elastomeric seals used in a mixed ethylene oxide and amines process pump operated by an oil and gas processor failed every 15 days. The user decided to switch to FFKM O-rings, enabling a realistic cost comparison to be made over one year of operation.

The comparison looked at the cost of shutdowns caused by the failure of each O-ring. Each FFKM O-ring provided 3.5 months of service—seven times longer than the previous elastomeric O-rings. Factoring in a conservative downtime cost of $1,300 per shutdown, covering maintenance costs and production losses, the new O-ring delivered savings of about $28,800 per year for one pump alone.

Acknowledgment
This article is based on the June 26, 2014, webinar “Solving sealing challenges in upstream and downstream environments” hosted by Smithers Rapra and presented by Jean-Luc Matoux of DuPont. The webinar and the whitepaper of the same name are downloadable at downloads.intertechpira.com/solving-sealing-challenges-upstream-downstream-environments-webinar/