Abstract
The Valhall field has been on production since 1982 and produced in excess of 800 mmboe. The primary reservoir is the over-pressured, high porosity and fractured Upper Cretaceous Tor formation. The field compacts during depletion, resulting in subsidence in the overburden.
In 2003 a permanent seismic array, LoFS, was installed across the field. The main objective of the system was to support the water flood program. To date15 seismic surveys have been acquired and the LoFS program has proved to deliver valuable information.
In monitoring of the Valhall water-flood program we were facing two fundamental technical challenges; poor p-wave seismic quality in part of the field and the presence of very thin dense, zones with pervasive fault induced fracturing. These fractures act as conduits and distribute the water away from the well.
Fortunately, the type of seismic recordings achieved using the permanent system are favorable for the use of full waveform inversion, and we have been able to produce velocity models containing much higher resolution than conventional ray-method based tomography. This model supported focusing of primary seismic reflection such that the presence of the reservoir could be interpreted from 4D seismic images, as the noise that still dominated the conventional 3D images appears to be highly repeatable and was reduced in the inherent differencing process.
