Clair Field: Development of a Waterflooded Fractured Reservoir

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 96316, "Clair Field - Managing Uncertainty in the Development of a Waterflooded Fractured Reservoir," by P.J. Clifford, SPE, A.R. O'Donovan, SPE, K.E. Savory, SPE, G. Smith, and D. Barr, BP plc, prepared for the 2005 Offshore Europe, Aberdeen, 6–9 September. Clair is the largest naturally fractured reservoir developed in the U.K. Recovery is by waterflood. Its 28-year appraisal period reflects the high reservoir complexity, relatively poor-quality conventional-seismic image, and uncertain effect of the conductive fractures. Clair is not predrilled, and the preferred well locations and design will change. Introduction Clair is a naturally fractured Devonian sandstone reservoir with a 1,970-ft reservoir interval containing 23°API oil 47 miles west of Shetland in 500 ft of water. Clair was discovered in 1977 and appraised by a series of vertical wells with disappointing rates. Field potential finally was demonstrated by the drilling of high-angle appraisal wells 206/8-9z and 206/8-10z, culminating in an extended well test (EWT) of 206/8–10z in 1996, which produced 500,000 STB of oil and demonstrated communication with a minimum of 500 million STB. The EWT led to sanction of the Clair Phase 1 development in 2001 and first oil in February 2005. The Clair Phase 1 development comprises the core, graben, and horst fault blocks (Fig. 1) believed to contain approximately 1.5 million STB of oil. Appraisal of the remaining areas of the field is continuing and may lead to sub-sequent development phases. Phase 1 is a single fixed-platform development. The field will be developed by drilling 22 additional wells over a 3-year period. The principal recovery mechanism is waterflood, with a planned ratio of approximately 2 producers per injector. Matrix permeability averages approximately 30 md, and oil viscosity is 3.5 cp at reservoir conditions. Recovery strategy begins with development of the more productive relatively high-permeability fluvial sands of Unit V in the large core segment and proceeds with development of the interbedded lake-margin fluvial sands of Unit VI on the basis of learning from Unit V wells. Drilling in the smaller and less appraised graben and horst blocks proceeds in parallel with the later core development. Natural-Fracture Description. The controlling feature of reservoir development in Clair is the system of natural fractures. Core and image logs demonstrate widespread granulation seams, conductive fractures, and cemented fractures. Use of seismic techniques to distinguish conductive from nonconductive faults and fractures is a key methodology in Clair reservoir description. To describe the possible fracture distributions, and to allow their incorporation into reservoir models, a number of scenarios have been generated. In these scenarios, stochastic distributions of conductive faults or joints have been generated with use of conditioning to seismic data, and these have been upscaled into fracture permeability arrays for incorporation into dynamic reservoir models.