Intra-survey pressure variations – implications for 4D seismic interpretation

During the time taken for seismic data to be acquired, reservoir pressure and saturation may fluctuate as a consequence of field production and operational procedures. This has consequences for the quantitative analysis of 4D seismic data and particularly for understanding of the pressure signal that diffuses rapidly into the reservoir over a time-scale of hours or less. A modelling study using actual acquisition data (permanent seabed sensors and also towed streamers), reveals that the signature of pressure variations in the pre-stack domain is complex, and thus the resultant post-stack image is not representative of the true reservoir mechanisms that caused the pressure changes. This is of particular concern when trying to accurately resolve small pressure changes away from wells with post-stack data. It appears however that larger signals closer to the well may still be detected adequately. Our results have implications for post-stack quantitative 4D seismic analysis, as well as processing and acquisition workflows for detailed seismic time-lapse studies.