Reliable 4D Seismic Interpretation at the Njord Field - the Importance of the Petro-elastic Model

Seismic monitoring and 4D seismic have been very successful in the clastic reservoirs of the North Sea and the Norwegian sea during the last 10-15 years. The Njord field in the Norwegian sea is a difficult 4D case since results are counterintuitive: a relatively large interval velocity decrease is found in a location where it was thought that the pore pressure had decreased. The understanding of the data has recently improved, by constructing a simple pressure model that puts more weight on overpressure due to gas injection, and little weight on pressure depletion. A proper petro-elastic model was constructed, taking into account both saturation effects and pressure effects. The most important contribution to the new understanding was to study the correlation between the injection history and the 4D seismic response in 3 monitor surveys close to the injection wells. The new understanding of the data allows for a more reliable prediction of future 4D response, and may be important for the future development of the Njord field.