ABSTRACT

The BP Schiehallion and Foinaven oilfields West of Shetland are benign shallow reservoirs, but lie in a harsh subsea environment, with 350-550m water depth. There have been up to three 4th generation semi-submersible rigs operating yearround in these fields since development drilling began in 1995. The two fields currently deliver about 250 Mstb/day from 2 FPSOs, 25 gas-lifted oil producers, 15 water injectors and 2 gas disposal wells. As the end of development drilling approaches, the rig programme will focus on infill and satellite targets over the next few years.

With operating costs approaching US$250,000 per day per rig, the major part of well cost is rig time, and so rig time savings represent very significant cost savings. High angle and extended reach wells are usually required for these fields, putting completion packers beyond wireline access. The early completions used extensive coiled tubing operations to run and pull plugs as part of completion installation and often suffered from non-productive time and waiting on weather associated with the coiled tubing work. Completions could take up to 15 days, excluding the subsea tree installation, highlighting the potential savings of interventionless completion installations.

In 1999, following experience on 25 West of Shetland wells with other interventionless systems, the newly developed SB-3H packer was used for the first time. The primary method of packer setting is by rupture disk against an atmospheric chamber. There is an independent, secondary, hydraulic setting mechanism. The new interventionless hydrostatic packer can reduce rig time to set the packer from as much as 6 days (working with coiled tubing on floating rigs in poor weather) to less than 30 minutes. On top of these immediate cost savings, eliminating intervention brings significant health and safety advantages.

This paper describes development and application of the interventionless hydrostatic packer. Several other changes in completion practices are also mentioned, which together with the SB-3H packer have cut completion installation times by up to 50%.

Introduction

The West of Shetland Foinaven and Schiehallion reservoirs are benign, shallow turbidite Tertiary sands, requiring high angle wells to access reserves and improve production rates(1). Figure 1 shows a typical high angle well profile for a West of Shetland well. Such profiles prevent wireline access to the packer / liner top. The sands are also weak, around 500-1000 psi unconfined compressive strength, requiring sand control completions in all oil producers and in most water injectors.

From the start of planning development wells, there was considerable focus on efficient drilling(2) and completion installation and on designs to last for life of field. The field developments are high profile and a significant milestone for the UK industry, opening new oilfields in a new province.

Premium connections were used throughout the production casing and completions. Thirteen-percent chrome metallurgy was used for production wells, while water injectors used Inconel jewellery with lined or coated carbon steel tubulars. All wells used surface-controlled subsurface safety valves and permanent packers without an expansion joint. The oil producers also have conventional gas lift mandrels and permanent pressure/temperature gauges. Conventional dualbore subsea trees were used, 5×2 for Foinaven and 7×2 for Schiehallion. High rig rates provide a clear focus for avoiding workovers and interventions.

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