Subtle Reservoir Barriers as Control of Fluid Flow in the Ekofisk Field

Tor Strand, Thorsten Eiben
ConocoPhillips Norway

The giant Ekofisk Field is located in the Norwegian sector of the North Sea. After more than 30 years of production, two billion barrels of oil have been produced, but still approximately one billion barrels of reserves remain. The reservoir has been extensively water-flooded. Three billion barrels of water has been injected.

The reservoir in the Ekofisk Field consists of clean chalk with porosities typically around 25-40%. The matrix permeability is low, in the order of 1-2 mD. However, the effective permeability is enhanced due to extensive fracturing.

On a large scale, the reservoir displays a well-bedded layering. The two main reservoir units, the Tor Formation of Maastrichtian age and the Ekofisk Formation of Danian age, consist mainly of extensively re-sedimented chalk. These two reservoir units are separated by a tight zone consisting of more pelagic deposits of argillaceous and silica-rich chalk. On a smaller scale, the reservoir shows a rather chaotic internal pattern with debris flows, slumps, slides and turbidites interbedded with more pelagic layers. The overall depositional pattern is interpreted to represent rapidly deposited units, separated by thin layers reflecting more quiet deposition.

Early in the production history the reservoir behaved as two fairly homogeneous reservoir units, with no significant internal pressure differences. In the more mature production stage of the field, subtle depositional and/or diagenetic features have turned out to play an important role in the behaviour of the reservoir. Occasionally, very thin layer-parallel silica-cemented bands may form important pressure barriers, difference observed across these thin layers are up to 400 psi pressure. Such cemented bands also seem to be able to act as barriers to water flow, and may form both top or base of water-zones. The siliceous bands are probably formed in association with quiet depositional periods. Other types of heterogeneities are formed by depositional lobes or slide blocks which may form juxtaposed units with rather different reservoir properties. Faults and fractures form another set of heterogeneities which may act either as barriers or as conduits to fluid flow. It has also recently been observed that new fractures may be formed during production of the field.

Figure 1 - Example of thin, field-wide silica-cemented band in Upper Ekofisk reservoir which can form barrier to pressure and fluid flow.