Mapping Exploitation-Unduced Velocity Changes in the Shallow Overburden of an Hydrocarbon Reservoir Using Correlation of Seismic Noise

Abstract:

We used two “vintages” of ambient seismic noise recorded at the Valhall Life of the Field Seismic network in 2004 and 2005 to perform a passive time-lapse imaging of the subsurface. First, the cross-correlations between each pair of stations were computed for both vintages to extract Scholte waves. Second, the relative velocity variations between the 2004 and 2005 cross-correlations were measured on the ballistic waves using the Moving-Window Cross-Spectral technique. Finally, the best-quality relative velocity variation measurements were regionalized using a modified eikonal tomography technique. The results, albeit noisy because of the short duration of the available records, show a large patch of increased seismic velocity in the southern part of the network and a weaker anomaly in the northern part. The increase of velocity can be attributed to the subsidence caused by the exploitation of the flanks of the Valhall reservoir with new wells. Our results are in good agreement with other time-lapse results using Scholte waves from active or passive datasets. The proposed technique could be used for continuous monitoring of the hydrocarbon reservoir under exploitation and of the CO2 sequestration sites.