Proper Performance Testing to Maintain Seal Integrity in Deepwater Completions
Authors: Slay Jeremy Buc , Ferrell Kent 
Published: October 2008

Selecting O-ring seal systems for well completions used in oil and gas production requires a thorough knowledge of a seal's physical properties and the conditions of the service environment to which it will be subjected. This paper focuses on the performance qualification of seal systems used in high-performance environments. Seal reliability and seal lifetime improvements are particularly crucial for the oil industry as failure dramatically increases rig costs. Thus, careful analysis of the performance envelope for suitable seal materials must be performed, and performance testing that simulates the field environments must be conducted.

This paper covers the topic of seal-system performance testing, which should be investigated in addition to basic material properties. Performance tests include extrusion resistance, rapid-gas decompression, compression set, and compressive-stress relaxation. Another important area of interest involves high-pressure / low-temperature (HPLT) performance of seals.

Introduction

When designing equipment for oil and gas production, engineers often depend on rubber and plastic sealing systems. To ensure successful tool operation, it is important to be aware of the performance envelope of the system to be used and all potential causes of seal failure. Typical modes of failure include extrusion, explosive decompression, and loss of seal force from stress relaxation or low temperatures.

Material engineering is an integral part of tool design and combines mechanical sealing performance requirements with material science to create a functional piece of equipment that satisfies application requirements. Materials must be strong enough to sustain service forces, which include tension, compression, and shear. The rubber and plastic must be chemically compatible with corrosive waters, simulation fluids, solvents, etc. that might lead to seal degradation. In many cases, materials must also comply with regulatory codes, statutes, and customer-mandated requirements.

Service conditions that must be considered include the fluids to be sealed, temperature range, and the working pressure. The temperature range is important, because seal integrity is influenced by both the minimum and maximum temperature as well as thermal cycling. The pressure range, cyclic events, and decompression rates are also important.

Purpose of Seals

In the petroleum industry, a seal is a device designed to prevent the movement of fluid from one area to another, such as making a connection fluid-tight. The fluid can be a liquid, gas, or supercritical gas. The broad categories of seals are known as static and dynamic. Static seals feature sealing surfaces that do not move relative to one another, while dynamic seals feature sealing surfaces that do move relative to one another. The conflicting requirements of a dynamic seal call for a compromise to ensure good contact pressure with minimum friction and rubbing wear. It is important to also consider whether or not the seal system requires external force inputs to energize the seal as in the case of a downhole packer element.