Downhole completion equipment must operate under high pressures, elevated temperatures and in chemically aggressive environments
Downhole completion equipment must operate under high pressures, elevated temperatures and in chemically aggressive environments. Sliding sleeve seal assemblies are key components in intelligent completions, zonal isolation and flow control, enabling operators to selectively open or close communication with reservoir zones
The effectiveness of these valves depends largely on the seal assembly design, which ensures controlled actuation and prevents leakage during closed or partially open states. The challenges for sliding sleeve seals are they must pass ports on the mandrel during differential opening, which can result in seal failure due to erosion and mechanical damage.
FUNCTIONAL REQUIREMENTS
Typically, the seal assembly must be designed to withstand the following:
- Differential pressures up to 15,000psi
- Temperatures in excess of 175ºC
- Crossing ports under differential pressures up to 5,000psi
- Fluid compatibility with hydrocarbons, H2S, CO2, brine, acids, and stimulation fluids
- Resisting erosion during opening/closing of the sleeve
- Surviving multiple open/close cycles over the life of the well
- Low pressure gas testing
DESIGN CONSIDERATIONS
The seal geometry of choice is a Bi-directional Spring Energised Seal (SES) assembly consisting of primary SES, with secondary sealing provided by PTFE/TFM vee rings and PEEK vee rings offering support and protection whilst passing ports. A central PEEK double female adaptor is incorporated to provide stability to the complete assembly. Filled PEEK buffer rings are generally included within the seal assembly and are important in diffusing the pressure and protecting the seal components from erosion and damage.
The SES is the primary seal which incorporates a spring to provide static loading to the seal lips for reliable low pressure 200psi gas tests. The lips are designed with a unique geometry to ensure they pass the ports without being damaged. PEEK hat rings are used to support the axial load created by the pressure and to protect the SES sealing lips.
The PTFE/TFM and PEEK thermoplastic materials are compatible with the aggressive fluid environment, so there will be no chemical degradation of the seal components. Many PTFE/TFM compounds are available, but typically proprietary filled compounds are used to provide the strength and ability to better resist damage whilst providing a good seal. Depending on the pressure, temperature and running clearance, the hat rings and the female adaptors will be virgin, glass or carbon-filled PEEK.
HARDWARE CONSIDERATIONS
Regarding hardware, slots are preferred to holes as they offer the possibility for maximum seal contact area with the mandrel. Slots should be radiused with shallow lead-in angles, precision machining, and the seal surfaces finished to 8μ-inch to optimise seal performance.
REPUTATION MATTERS
All high-performance seal applications are different and consideration of seal geometry, material selection and hardware design are essential. Working with reputable seal suppliers with design experience and knowledge of critical downhole applications is the key to reliable sealing solutions.
Ian Gravill is general manager at KED-Seals: www.ked-seals.com
