Reservoir Characterization of Ekofisk Field: A Giant, Fractured Chalk Reservoir in the Norwegian North Sea - Upscaling
Authors: Agarwal B. , Thomas L.K. , Sylte J.E. , O&apos;Meara D.J. 
Published: December 2007

The Ekofisk reservoir is a high porosity, low matrix permeability naturally fractured chalk with extremely complex structure and reservoir distribution. To facilitate reservoir management decisions, a 3-D reservoir fluid flow model is used as a tool to determine optimum well placement and predict future performance. A major multi-disciplinary reservoir characterization effort has been in progress on the Ekofisk Field. The objective of the project was to improve the existing reservoir description using all available data, and to construct and history match a new 3-D flow model using this updated, detailed reservoir description. Because of the impact heterogeneity has on preferential fluid flow direction, significant attention has been given to capturing as much of the intrinsic heterogeneity as possible, both laterally and vertically, in the new 3-D geological model. In the upscaling process to the fluid flow model, therefore, preservation of this heterogeneity is critical. Standard upscaling techniques based on simple averages of permeability may not suffice; for this reason an alternative approach was employed, a technique based on fluid flow, or flux, within the geological model.

Flux-corrected upscaling uses simplified flow simulations for calculating effective properties of complex, three-dimensional reservoir models and addresses several of the drawbacks of standard methods. The method imbeds information on the most important factors affecting recovery efficiency; reservoir architecture and heterogeneity, well placements and constraints, and aquifer location and extent.

Comparisons between the geological and fluid flow models show that the intrinsic heterogeneity of the transmissibility is preserved in the upscaling process.

Introduction

The Ekofisk Field, located in the Norwegian Sector of the North Sea was discovered in 1969. The reservoir consists of two fine-grained limestone producing formations, the Ekofisk formation (Danian Age) and the Tor formation (Maastrichtian Age), separated by a thin, impermeable Tight Zone. The reservoir was initially overpressured and contained an undersaturated oil at 50 Mpa (7120 psi) and 131 C (268 F) at a datum elevation of 3170 m (10,400 ft) subsea The bubble point pressure was approximately 38 Mpa (5545 psig). Production started from the chalks in 1971. Current estimates indicate about 7 billion barrels of oil originally in place. The field has produced over 1.2 billion barrels of oil to date from a total of 160 wells. Current production from about 67 deviated and horizontal wells is about 240,000 barrels of oil per day and 600 MMCFD of gas. Reinjection of excess natural gas has been ongoing in Ekofisk since 1975 with 1.28 TSCF of gas injected to date. A pilot water injection project was initiated in 1981 in the highly fractured Tor formation and in the Lower Ekofisk in 1986. Fieldwide water injection began in 1987. Current water injection rates are 840,000 bwpd into 37 active injection wells. A number of additional improved oil recovery techniques are being evaluated and currently ongoing is a Water-Alternating-Gas (WAG) pilot in the southern area of the field. Fig. 1 shows a structure map of the Ekofisk Field drawn on the top Ekofisk formation.

Reservoir characterization on Ekofisk was directed at gaining a detailed understanding of reservoir hydrocarbon volumes, the architecture of the reservoir and at fully describing the heterogeneity and anisotropy of reservoir parameters. P. 311^