Evaluation Support Scotland

ABSTRACT Emerging completion technologies are placing more severe demands on seal systems required for downhole use. The move towards higher production pressures and temperatures means that seal design could become the limiting factor for the exploitation of future wells. Conventional seal design practice is not supported by a fundamental base of knowledge, although there is considerable experience and empirical data at existing operating conditions. Consequently there is little basis for predicting seal behaviour under conditions more extreme than those currently experienced. This paper presents an extract from a larger programme of work aimed at developing an improved understanding of seal system materials and design that determine seal behaviour under severe conditions. A dynamic seal test facility has been designed and built to examine a range of commercial and specially-made seals under various conditions up to those met in severe service. Two typical test sequences are described. Seal stacks tested dynamically in wet Methane/H2S at 100°C(212°F) and 34 MPa (5,000 psi) have shown some leakage and mechanical damage after less than 50% of the required service lifetime. The leakage rates observed are higher under dynamic conditions than static, and were sensitive to both temperature and pressure changes. The presence of H2S did not appear to influence sealing behaviour or damage. The importance of seal shape under load, thermal expansivity and thermally-induced changes in mechanical properties have begun to be defined in terms of how such factors contribute to seal damage and consequent leakage. THE PROBLEM Oil and gas production technologies rely on the function of downhole seal systems for critical components such as flow control valves, safety valves, and to seal between elements around the production tubulars within the casing. These types of seal share a need for high reliability and longevity whilst operating in harsh, often changing, well environments. In polished bore receptacle (PBR) completions, these requirements must be met by a system also capable of allowing the large dynamic movements associated with pressure and temperature fluctuations in the string. The relative motion between seal and bore to be accommodated can be many feet in each excursion and can total hundreds of feet during the required PBR seal lifetime. These operational needs have guided the development of today's dynamic PBR seal systems towards the use of elastomers, engineering polymers and composites: Considerable field experience, coupled with simulated well testing, has built up an empirical base of knowledge from which specific designs have evolved. This approach has been sufficient to guarantee a minimum seal life time for selected service environments, i.e. the duration of a qualification test. This method cannot be used to define expected seal life under realistic, variable and long term conditions, nor does it provide fundamental information to allow improved seals to be developed for more extreme conditions. With the move towards harsher production environments, it is possible that dynamic seal design could become a limiting factor. This now makes it necessary to develop a more fundamental, and hence predictive, understanding of dynamic seal behaviour under high-pressure/high-temperature (HP/HT) conditions.