Abstract

The Schiehallion field lies in water depths of 1,150 to 1 475 ft and is situated west of the Shetland Islands. The reservoir was discovered in 1993 and first brought on stream in 1998. It is the largest field operated by BP in the region and is one of the largest producing fields in the UK sector. Currently there are 19 production and 19 water-injection wells in the field, which have yielded more than 250 million BOE to date.

At the outset, the North West Area Development (NWAD) of the Schiehallion field was planned to have four penetrations, consisting of two producers, each with its accompanying injector; however, design and economic reviews indicated that development with a multilateral well and matching injectors was more viable. The selected multilateral system consisted of a hollow whipstock through which perforations would be made after the completion was installed. While the perforating gun and low-side weight bias orientation system are not new technologies, they still required substantial qualification because of the unique construction of the well, driven primarily by well control policies and the requirement for an intelligent completion to facilitate independent flow control of both laterals. The resultant integration of technologies supplied by several service companies demanded rigorous testing and qualification of the various components. Several tests were performed to validate the viability of the completion design, such as perforating gun suitability; confirmation that minimal tubing deformation would occur; perforation hole sizes; the likelihood and containment of perforation debris; and testing of a straddle system required to bridge off the tubing perforations to isolate and control the mother bore production. The test program culminated in a full-scale stackup of the intelligent completion within a test well, followed by perforating and subsequent quantification of the debris generated and captured.

This paper details the development trials carried out on the perforation system required for this multilateral system to meet the needs of drilling and completing a subsea multilateral producer with an intelligent completion. In addition to discussing the physical testing, this paper summarizes the results obtained from erosion modeling and productivity evaluation, which also influenced the final well design.

Introduction

The Schiehallion field is located, along with the Foinaven and Loyal fields, approximately 103 nautical miles west of the Shetlands Islands, offshore UKCS. The development of these fields has been based upon two floating production storage and offtake (FPSO) vessels receiving oil from clusters of subsea wells via both rigid flowlines and flexible risers. Shuttle tankers offload oil from Foinaven to the Flotta terminal and to the Sullom Voe terminal for Schiehallion and Loyal fields. All these fields inject seawater, and either re-inject produced gas or export it to the Magnus field system for use in an enhanced oil recovery (EOR) scheme.

The NWAD development of Schiehallion field was originally conceived as a four-penetration project consisting of two producing wells, each with its own dedicated water-injection well; however, it was decided early in the planning of the project that the producer penetrations should be in the form of a multilateral well design incorporating a hollow whipstock. The original well design allowed the lateral legs to produce and commingle through a single completion string to surface; however, during the detailed planning of the well, this design philosophy was challenged and adapted to facilitate remote downhole flow control (DHFC) of the production of each lateral.

Consequently, "Phase 1a" of the Schiehallion NWAD was planned to access reserves in the Segment 2 and 3 T35 Sands, using one multilateral producer and two water injectors. The multilateral producer, FP02, would have one leg in the Segment 2 fault block and one leg in the Segment 3 fault block, with each leg being supported by a water injector (Fig. 1). Well FP02 was drilled in the summer of 2006, using the Paul B. Lloyd Junior semi-submersible rig, and subsequently completed with DHFC equipment to optimize well operability and reserves recovery.

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