Estimation of pore-pressure change in a compacting reservoir from time-lapse seismic data

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An approach is developed to estimate pore-pressure changes in a compacting chalk reservoir directly from time-lapse seismic attributes. It is applied to data from the south-east flank of the Valhall field. The time-lapse seismic signal of the reservoir in this area is complex, despite the fact that saturation changes do not have an influence. This complexity reflects a combination of pressure depletion, compaction and stress re-distribution throughout the reservoir and into the surrounding rocks. A simple relation is found to link the time-lapse amplitude and time-shift attributes to variations in the key controlling parameter of initial porosity. This relation is sufficient for an accurate estimation of pore-pressure change in the inter-well space. Although the time-lapse seismic estimates mostly agree with reservoir simulation, unexplained mismatches are apparent at a small number of locations with lower porosities (less than 38%). The areas of difference between the observations and predictions suggest possibilities for simulation model updating or a better understanding of the physics of the reservoir.

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