Valhall Permanent Seismic Monitoring—Reducing Geological Model Uncertainties Using 4-D Seismic

Barkved, Olav Inge, Kjell Buer, Tron G. Kristiansen, BP Norge AS, Stavanger, Norway

To support continuous seismic reservoir monitoring and improve structural imaging, a permanent, multi-component seismic array has been installed at the Valhall field in the North Sea. We have acquired four seismic dataset since October 2003 and the plan to acquire the fifth during March 2004. Prior to this a conventional marine streamer 4D seismic survey were acquired in 2002, capturing the production history since 1992. The Valhall field is a thin, highly porous, low permeable chalk reservoir. Production from the reservoir results in a compaction, porosity reduction or hardening of the reservoir. The primary reservoir units contributing to flow are well defined in the 4D seismic images and allows of reducing uncer­tainties in top and base reservoir. Due to compaction, porosity logs from wells drilled into depleted needs to be corrected back to virgin conditions. The porosity / net stress relation is well established and used for this purpose. Changes in acoustic impedance from time­lapse seismic due to production provide an additional constraint, and are used to validate and improve the reconstructed porosities and thicknesses. Differential depletion across faults is results in uneven changes in acoustic impedance. The contrasts reflected in the 4D seismic images allows for improved fault/fracture definition. An order of magnitude higher permeability is seen in part of the field due to fractures. Observations from the well bores are linked to the overall structural interpretations, and detailed fracture patterns are already included in reservoir simulation models and used to define the drainage strategy. Time-lapse seismic will play an important role in validating these models.