Optimising Slot usage on a Minimum Facilities Platform
Authors: Avocato N.S. , Jackson J.R. , Jones I.G. , Murphy S.J. 
Published: December 2007

N.S. Avocato, SPE, J.R. Jackson, SPE, I.G. Jones, and S.J. Murphy, SPE

Using a creative and innovative approach, Chevron UK Ltd. has increased the number of well slots on Alba Northern Platform (ANP) by over 20%, at minimum cost. ANP is using four 46" OD caissons each housing two 20,000 bpd wells.

The Close Proximity Well (CPW) project is a refinement of an application seen in some Gulf of Mexico and mud-slide areas, whereby multiple wells are drilled from the same caisson. The main differences are in the caisson size, the proximity of the wellheads, and the fixed platform retrofit application in that the ANP was originally planned and constructed for single slots.

This paper will describe the planning, design and the execution of the Close Proximity Well Project. It will demonstrate how these new practises have accelerated completion of the Alba Field phased development using an existing platform. The first CPW will have been completed by the time of the presentation.

Introduction

The Alba Field is located in Block 16/26 of the UK Continental Shelf, lying about 130 miles north-east of Aberdeen in 453 feet of water. The reservoir is a long, narrow Eocene sandstone deposit located at a depth of approximately 6,300' TVDSS. The field was discovered by the 16/26-5 well in 1984 and field development plans were initiated in 1987. A location map is shown in Figure 1.

A two phase development was selected in 1990 because of uncertainties in the geology, reservoir performance and the processing of the Alba produced fluids. The scheme would allow experience from the first phase to be used to optimise the second phase.

In August 1991, a 9 slot subsea template was installed at the Alba Northern Platform location. Five wells were predrilled from a semi-submersible rig between June 1992 and March 1993. The 24 slot platform was installed in July 1993 and tie-back operations began in October that year.

Alba Northern Platform came on stream in January 1994, producing approximately 75,000 bpd of 20 API oil to a permanently moored Floating Storage Unit with shuttle tanker operations transporting the oil to ports in Western Europe and beyond. A minimum facilities platform was required to ensure the commercial viability of the project.

A total of 12 wells have been drilled though April 1995, 9 horizontal producers and 3 gravel packed injectors. Peak production has been over 100,000 bopd.

The Challenge. In July 1993 the Alba Southern Area Development study began screening options for exploiting the estimated 400 MMBBLS STOOIP in the southern portion of the field. It was recognised that the use of existing Alba Field infrastructure was crucial to maximising the economics of the development. The challenge was that Alba had a minimum facilities platform, designed with only 24 slots as shown in Figure 2.

Several means of obtaining more slots were considered. Conductor slot sharing was identified as the preferred method since it required no change to the Alba well completions and employed proven oilfield concepts and techniques with little reliance on undeveloped or unproven equipment.

The Solution. 46" diameter pipe was driven to a competent formation approximately 250' below the mudline. This caisson provides environmental protection and support to house two 18" conductor strings set at the normal conductor depth of 1300'. All subsequent operations are then as per the standard Alba well.

The two wells were kicked off from the base of the 46" Although the wells are being drilled from one caisson, the separation at the 18" shoe is similar to the 25' normally seen at this depth (1300') on other adjacent slots. It is the proximity of the two wells within the caisson and the well heads on the platform that lead to the term Close Proximity Wells (CPWs).

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