Monitoring the Ormen Lange Field with 4D Gravity and Seafloor Subsidence

The primary development drilling at the Ormen Lange field at the Norwegian continental shelf concluded in 2014. Reservoir monitoring is employed to reduce subsurface uncertainties and assist with late field-life development decisions. Geophysical methods used include 4D seismic, seafloor geodesy and field-wide 4D gravity and subsidence surveys. This abstract presents the results on both seafloor subsidence and 4D gravity from the surveys performed in 2012 and 2014. These results are provided at a lower cost and significantly faster turnaround compared to 4D seismic.

While 4D seismic does not provide sufficient sensitivity to seafloor subsidence at Ormen Lange because of oceanographic variations, 4D gravity and subsidence surveys provide a clear picture, that is key for understanding and monitoring reservoir compaction.

4D gravity results, in turn, provide a valuable input to understanding mass changes in the reservoir. This input has less lateral resolution than that from 4D seismic, but as gravity is sensitive to mass changes, it provides immediate insight into the energy balance of the field and can be quickly integrated into history matching workflows. The integration of gravity and time-lapse seismic provides a reduction of uncertainties on aquifer influx and strength, with increased confidence through the consistency of independent methods.