Abstract

The Andrew Field produces from a Palaeocene sandstone saturated oil reservoir, in UK North Sea Blocks 16/27a and 16/28. Cyrus is a small subsea tieback, 7km NE of Andrew, containing undersaturated oil. Both fields are operated by BP Exploration. Field development was sanctioned in 1994, some 20 years after discovery. During those 20 years, numerous development concepts failed to achieve the expected economic return. However, in 1992 the Andrew well count was reduced from 18 to 10 by changing from vertical to horizontal producers. In conjunction with an Alliance risk/reward approach to topsides construction, which was expected to significantly reduce Capex, development sanction was obtained.

Andrew development drilling started in September 1995, with three pre-drilled template wells. These were tied-back to the platform in 1996 and followed by further platform-drilled producers. A 1993 horizontal appraisal well was also tied back as an Andrew producer. Two horizontal subsea producers were drilled on Cyrus in 1996, replacing two earlier horizontals wells which had suffered rapid water production. First oil was achieved in June 1996, 6 months ahead of programme. Current production is at the expected plateau level of about 58 000 bbl/D from Andrew and 12 000 bbl/D from Cyrus. Reducing well numbers and using only horizontal wells relies on the long term performance of these wells for economic success. Andrew/Cyrus is amongst BP's first all-horizontal field developments. Key design decisions started at the sandface completion: openhole sand control screens were considered because of sand production concerns; the cost and difficulties of cementing and perforating long horizontal liners were reviewed; the savings from using pre-holed, uncemented liners were set against the concerns over long-term production and external casing packer (ECP) reliability. Multilaterals were also considered in detail. The outcome of this work was to install pre-holed liners with cement inflated ECP's in both Cyrus wells, but to use cemented and perforated liners on Andrew.

The Andrew Well Engineering Alliance was created, and was aligned to BP's key business objectives through the 'minimum performance standards' of well on target, % of the horizontal section contributing to flow, zonal isolation and data acquisition. These measures were the basis of triggers for gainshare payments. Maximising the % contributing to flow led to the world's first through-tubing, underbalanced, single trip, no kill, horizontal perforating system using a hydraulic workover unit and formation isolation valve. The success of this approach has been confirmed by recent coiled tubing production logging (CT-PLT).

This paper discusses the reservoir uncertainties and anticipated well management challenges. It describes the balance of factors that impacted the design of the horizontal completions. Initial well construction experiences and the first 6 months of production are described.

Development Outline

Figure 1 gives an outline of the development area. The Andrew topsides were installed in May 1996, with the gas and oil export pipelines and subsea bundle tie-back of Cyrus completed a few weeks later.

Andrew reserves are estimated at 112 × 106 bbl. Development drilling on Cyrus has suggested that the sanction estimate of 24 × 106 bbl was optimistic, with the structure being somewhat smaller than expected. Cyrus 're-development' is through primary depletion from two new horizontal wells, replacing the previous horizontals. The Andrew reservoir, a four-way dip-closed structure with a 58m oil column between gas cap and aquifer, is being developed with ten horizontal producers. It is expected that aquifer support will be sufficient to maintain reservoir pressure. Whilst the development includes a vertical gas management well completed in the gas cap, there are no plans for water or gas injection wells for pressure maintenance.

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