ABSTRACT

The move towards higher production pressures and temperatures means that seal design for downhole usage may become a limiting factor for the exploitation of future wells. To determine factors associated with seal behaviour under severe (HP/HT) conditions, a dynamic seal test facility has been built to simulate downhole conditions up to 15,000psi and 250ºC. A number of novel seal designs have been developed and tested for HP/HT service in downhole completions using this test facility. An important aspect of test procedure is whether the test is performed under absolute or differential pressure conditions. Previous work has emphasised absolute pressure conditions. This paper compares absolute and differential conditions and investigates which of these represents the more severe case.

INTRODUCTION

Polished Bore Receptacle (PBR) designs for oil and gas well completions, are being used under increasingly severe operating conditions. A critical part of such completion systems is a dynamic seal stack assembly. Cost effective oil and gas production in such completions relies on reliable performance of the seals in the PBR assembly continuing through out the well lifetime. The function of the seals is to provide pressure containment while allowing significant changes in the production tubular length without unduly stressing either the tubing or the completion assembly. This is achieved by allowing the seals (usually fixed to the production string) to slide within a polished tube which provides the sealing contact surface. Changes in length of the tubing string maybe caused by pressure fluctuations in the production fluid or temperature changes, due either to natural cooling or heating of the well fluids or stimulation of the well, e.g. by water injection. These changes in length can be several metres during a single excursion and may total hundreds of metres during the life of the PBR seal system. PBR seals maybe required to operate initially under high pressure and high temperature (HP/HT) conditions. However, during the life of the well conditions may change to the extent that the seals come to operate at significantly reduced temperatures and with the overall balance of pressures reversed, i.e. sealing in the opposite direction to when they were commissioned.

There is a requirement for a better understanding of seal life under these conditions, which may combine dynamic movement. One solution is to develop a qualification test to provide information on the expected service life of seals, initially on a worse case basis. A test rig has been developed to reproduce downhole conditions and a test procedure has been developed to simulate operational conditions as part of a two year project.

Initial seal stack tests1,2 were performed under absolute pressure conditions, since this was considered to be the worst case. Subsequently, it has been shown, however, that testing under full differential pressure conditions will result in earlier seal failure for otherwise similar test conditions.

PBR seals comprise a number of separate elements manufactured from a range of elastomers, engineering polymers and/or composites. These individual elements are often repeated to provide a seal stack.

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