A continuous reservoir monitoring system has been installed for Shell, on a medium heavy-oil onshore field situated in the north-east of The Netherlands, in the context of re-development of oil production by Gravity Assisted Steam Flood. The challenge was to continuously monitor, with seismic reflection, the lateral and vertical expansion of the steam chest injected in the reservoir during production over more than a year.

The main problems for onshore time-lapse are caused by near-surface variations between base and monitor surveys which affect the seismic signal coming from the reservoir. Here, a set of permanent shallow buried sources and sensors have been installed below the weathering layer to both mitigate the near surface variation and minimize the environmental footprint.

The very high sensitivity of our buried acquisition system allows the tracking/monitoring of very small variations of the reservoir physical properties in both the spatial and calendar domains.

The 4D reservoir attributes obtained from seismic monitoring fit the measurements made at observation, production and injector wells (pressure, temperature and oil/water production).

A "daily 4D movie" of the reservoir properties changes allows the proposition of a scenario which explains the unexpected behavior of the production and confirms that the steam does not follow the expected path to the producer wells but rather takes a more complicated 3D path within the reservoir.

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