Seal Life Prediction and Design Reliability in Downhole Tools
Authors: Lehr Doug , Furlan Wayne 
Published: October 2017

Downhole tools that perform a sealing mission are frequently expected to have a 20-year service life. For any seal to perform over its intended service life, its materials must possess longevity and its configuration must perform reliably. This paper will describe how the developers of downhole seal systems combine advanced seal material selection methods with reliability-based design to achieve function, longevity, and reliability goals for offshore applications.

Accelerated Life Testing (ALT) using Arrhenius principles results in life-predictive models with significant limitations. However, Compressive Stress Relaxation (CSR) and other methods now provide improved models for elastomeric materials. CSR measures the residual sealing force in a seal specimen that has been compressed over time at elevated temperatures. Seal materials selected using CSR and other methods can then be used to design seal systems that satisfy requirements for form, function, reliability, and service life.

The process described will demonstrate how CSR and other material testing protocols are combined with configuration testing and Design for Reliability (DfR) to develop reliable seal systems.

CSR was used in a deepwater Gulf of Mexico (GoM) application to determine the best elastomeric material for seal configurations within a subsurface tubing-mounted safety valve. The CSR results challenged the existing material selection paradigm. However, the results of the DfR laboratory test program using the elastomer recommended by CSR led to the discovery of a configuration stability problem. Material evaluations and lab testing will be discussed in the case study.

ALT on several elastomeric materials used in high-temperature (HT) retrievable packers revealed a potential longevity problem at the target temperature rating and service life. This problem would potentially have affected the packing elements, O-ring, and T-seal containment rings. Without ALT this "performance gap" may have gone undiscovered. The results of the ALT that led to this finding will be discussed.

CSR evaluations on fluoroelastomers suggest that some types possess significantly better service life than others. This challenges some subject matter experts (SME) opinions regarding material selection and has implications for configurations involving O-rings, chevron seals, T-seals, and larger-volume packing elements. The CSR evaluations and comparisons between various fluoroelastomers will be presented.

All these results have led to improved decisions in polymeric materials selection and seal design. These results will be also be discussed in the context of DfR.

CSR techniques are relatively new to the oilfield industry, but they enable objective prediction of service life for seal materials. When combined with reliability-based seal design and testing protocols, the result is a seal system that achieves performance and reliability goals for today's long-term offshore applications. The practicing engineer will benefit from case studies that document how this new knowledge is used to develop the highest reliability packer, safety valve, and accessory seals.