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In 4D seismic, the velocity model used for imaging and reservoir characterization can change over calendar time as the reservoir is produced. This is particularly true for heavy-oil reservoir produced by steam simulation (EOR). We propose an automatic 4D update of the 3D velocity model using an efficient technique based on 4D pre-stack time migration (4D-PSTM) that describes the pre-stack differential kinematic effects by matching the 4D dataset. On real continuous 4D seismic data, the 4D-PSTM allows us to quantify interval velocity variations that can be used to map temperature changes in the reservoir in agreement with petro-elastic model expectations.

A continuous reservoir monitoring system has been installed for Shell, on a heavy-oil onshore field situated in the Netherlands, to re-develop oil production by Gravity-Assisted Steam Drive. The challenge was to continuously monitor using seismic reflection the expansion of the steam chest injected in the reservoir during production. The main problems for onshore time-lapse seismic are caused by near-surface variations between base and monitor surveys which affect the seismic signal coming from the reservoir. In our system, a set of permanent shallow buried sources and sensors has been installed below the weathering layer to both mitigate the near-surface variations and minimize the environmental footprint. The very high sensitivity of our buried acquisition system allows us to track 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. A daily 4D movie of the reservoir property changes allows us to propose a scenario that explains the unexpected behavior of the production and confirms that the steam does not follow the expected path to the producer wells but rather a more complicated 3D path within the reservoir.

To evaluate repeatability of emerging seismic technologies for future 4D seismic reservoir monitoring studies on Middle East carbonate reservoirs, a seismic field acquisition test was conducted over an onshore field in Saudi Arabia. The effects of near-surface complexity (in the form of sand and karsts) as well as large surface temperature variations are illustrated and quantified by 4D attribute analysis using permanent piezoelectric seismic sources. Even though measured repeatability does not reach values observed in non-desert environments, we show that burying receivers dramatically improves the wavelet amplitude stability. As the complex near-surface scattering layer appears to be thicker than initially expected with the presence of karsts down to a depth of 40 m, we conclude that deeper burial of sources and receivers below the most complex part of the near surface may potentially let us use lower-fold seismic data for reservoir monitoring in complex near-surface desert environments.