Deciding on the optimum design for any new well is a complex task, made often more difficult by a wide range of technical uncertainties. Getting the completion design right is particularly important for subsea wells, where the cost of intervention can be prohibitive.

After several years of production, it is common to encounter significantly different reservoir and well conditions than were initially predicted at the time of the original well completion and artificial-lift designs. Gas-lift is not always required until later in the well life. While some gas-lift valves have an impressive reliability record, some can fail or be deemed sub-optimal for the well conditions and necessitate retrieval to optimise well performance or to reinstate well integrity.

Retrievable straddle packers are commonly used to isolate water and/or gas production in the sandface completion or reinstate well integrity in the upper completion. This paper will review different methods used to reinstate gas-lift, and describe the gas lift design process and straddle configuration using case studies from the North Sea.

In particular, Premier Oil undertook a subsea well intervention campaign to restore production, which had ceased following the suspected failure of the gas-lift system. Analysis of the well performance data suggested that one or more gas-lift valves were inoperable and that the original valve depths were not ideal for the encountered reservoir conditions.

The operation was performed from a light well intervention (LWI) vessel, deploying a retrievable Interwell straddle and PTC GoLift™ Straddle system, installed in the tubing across and between the existing side pocket mandrels, creating a retrofit, triple packer, gas-lift straddle.

Premier performed a safe and successful LWI operation, returning the well back to its full production potential, having met all well intervention objectives. Running the retrievable straddle packer solution with PTC's GoLift™gas-lift system helped to optimise production and saved the need for a costlier rig-based upper completion workover.

This paper will further look at the design and development of these technologies, examine in depth one case study for the Brenda D3 design and installation.

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