Why is it Possible to Produce Oil from the Ekofisk Field for Another 40 Years?

Abstract

Abstract The Ekofisk field, which is a naturally fractured chalk field, was the first offshore oilfield that was put on production in Norway in 1972. The field was produced by pressure depletion the first 10 years, and the initial recovery was estimated to only 17%. Due to the high porosity, up to 40%, significant compaction of the reservoir rock was observed. Injection of seawater was approved in 1983 and it was a great success. The expected oil recovery was adjusted to 46%. The Ekofisk formation is characterized as: low permeability in the matrix blocks (1–2 mD), high reservoir temperature (130 °C), low initial water saturation (5 -10%), and the rock appeared to be preferential oil-wet. Under such conditions, water-based EOR can only be successful if the fluid is able to change wetting properties of the rock in such a way as to increase the capillary forces. Intensive research during the last decade has proven that seawater at high temperatures acts as a "Smart Water" being able to improve the water wetness of carbonates. Even though the reservoir pressure was increased to the initial pressure, seawater had a weakening effect on the rock, and compaction is still taking place (about 10–12 cm/year) thus being a drive mechanism for oil recovery. The estimated oil recovery has now been increased to about 55%. Research show that seawater can even be modified to improve the oil recovery from the Ekofisk field, in a spontaneous imbibition process, by an additional 10% compared to ordinary seawater. The fundamental knowledge of the effect of seawater on oil recovery from the Ekofisk formation has been important when the decision recently was made to invest in extending the oil production from the field for another 40 years. Introduction Ekofisk was the first oil field that was put on production in Norway in 1972. It is a naturally fractured chalk reservoir consisting of three formations:Tor formation - preferentially water-wet,Lower Ekofisk - neutral to low water wetness,Upper Ekofisk - neutral to preferentially oil-wet. Important reservoir parameters are: matrix permeability 0.1–10 mD, porosity 30–33%, reservoir temperature about 130 °C, initial water saturation about 15–20%. The reservoir has an oil column of more than 300 m and lies 2900–3250m below sea level.1 The field was initially developed for oil recovery by pressure depletion with an estimated oil recovery factor of 17%. After producing the reservoir below the bubble point, the operator and authorities in 1987 agreed upon a water injection program for three reasons: (1) Pressure support; (2) Oil displacement; (3) Decrease or prevent compaction of the reservoir rock. The following experience showed that the displacement of oil by seawater was a great success, and it actually appeared to be more effective than expected from laboratory experiments. The expected recovery factor for Ekofisk has gradually been adjusted to slightly above 50%.