|First Break, Vol 34, No 7, July 2016 pp. 51 - 59
In 2013 a seismic caprock monitoring system with 172 nodes was installed on the Oseberg oil and gas field in the Norwegian North Sea. It was specifically designed for active and passive seismic monitoring of a disposal injection well. The aim of the monitoring is for a safer operation with avoidance of leakages to the sea, and increased injection. Despite installation and yearly maintenance costs but owing to benefits in the operation of the injector, the system provides yearly savings of around $1.2 million. Currently, the system does most of the passive seismic analysis – microseismic and interferometry – in real time, and we present data examples from the last two years. Typical noise in the data (seismic shooting, platform generated noise and marine traffic) are removed and the signal-to-noise ratio is significantly improved, resulting in better detections and event location, especially for small microseismic events. Seismic interferometry is used to observe/detect changes in the water column and the shallow subsurface down to several hundred metres. Several offshore hydrocarbon-bearing fields have experienced fractures in the overburden from injection into disposal wells, including leakage to the surface. Although injection procedures to avoid this are effective, they significantly limit the rates and pressures of the injection. To overcome this, a permanent offshore seismic monitoring system using both active and passive seismic methods has been developed. It was installed at the seabed of the North Sea Oseberg field in 2013 with the main aim to control caprock integrity and ensure safe injection, which makes the system unique.