Reducing Risk and Monitoring Water Injection Using Time-Lapse (4D) Seismic at the Ekofisk Field

Over the past decade, time-lapse (4D) seismic has evolved into a valuable reservoir-monitoring tool used on numerous fields throughout the world. 4D seismic has been used at the Ekofisk field to monitor water injection and reservoir depletion. The 4D seismic signal is complex and can be broken down into two main components. The first and most apparent of these being 4D travel time differences caused by the geomechanical effects of compaction and stress changes occurring in the reservoir and overburden. The second and subtler component of the 4D signal is an amplitude difference caused by impedance changes occurring as the reservoir responds to water injection and pressure depletion. The combined response from these two 4D attributes has been used to optimize well placement and reduce the risk of drilling into water swept areas of the reservoir.