Abstract

We present results of the 4D seismic monitoring of the Ekofisk field waterflood. The thick, high porosity chalk reservoir compacts strongly in response to water injection1,4,6. This compaction has a clear effect on the 4D seismic data, which has a baseline 3D survey acquired at the beginning of the waterflood in 1989, and a monitoring survey acquired in 1999.

Ekofisk 4D seismic data has been a valuable qualitative tool for describing the spatial distribution of reservoir compaction and subsidence. The 4D data shows that some faults are significant barriers to water flow, and some reservoir compartments have been substantially compacted, from which we infer an increase in water saturation. These observations guide improvements to the reservoir simulation history match. The 4D seismic also helps explain recent drilling results and helps plan optimal well paths for future development drilling.

Recent work to incorporate the 4D seismic into a compaction model gives quantitative reservoir compaction results that agree well with geomechanical and log-based compaction models. The seismic data can be used to monitor reservoir compaction over long time periods and serves as calibration data for detailed compaction modeling.

Introduction

The Ekofisk Field in the Norwegian Sector of the North Sea was discovered in 1969 and began production in 1971. In 1984, Phillips recognized that the seafloor over Ekofisk field was subsiding and determined that the cause was reservoir compaction. Since then, the seafloor subsidence has been monitored by repeated bathymetry surveys, and the reservoir compaction is monitored by measuring the movement of radioactive bullets implanted in wellbores in and above the reservoir1. The cumulative maximum subsidence today is more than 8m above the crest of the field, corresponding to a maximum reservoir compaction of 10m. The reservoir compacts by two mechanisms-pressure reduction associated with the initial production phase from 1971-1989, and waterweakening associated with the waterflood from 1989-present. A 3D seismic survey covering Ekofisk field coincided with the start of the field-wide water injection program. Phillips acquired a repeat 3D seismic survey in 1999 to monitor the waterflood-induced compaction by 4D seismic analysis. The Ekofisk field seismic data is not mappable at the crest of the field, due to overburden velocity anomalies associated with gas and overpressure. This appears as a blank area in the middle of all the seismic maps presented in this paper.

Ekofisk 4D Seismic Observations

In 1997, Phillips completed a time-lapse seismic feasibility Study2 that indicated chalk compaction from water weakening could be observed with 4D seismic. The new seismic survey, completed in 1999, indeed shows compaction-related differences when compared to the baseline 1989 seismic survey.

Three main horizons are mappable on Ekofisk 3D seismic data. These are the top chalk reservoir, a tight zone separating the upper reservoir (Ekofisk formation) from the lower reservoir (Tor formation) and the base reservoir. The main 4D observation is a time shift of these main chalk horizons.

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