Schlumberger

4D Survey Evaluation & Design

4D survey design to ensure maximum repeatability

We employ a diverse range of feasibility studies toward determining the most appropriate acquisition, processing, and inversion parameters for each 4D survey.

Repeatability for a true 4D signal

Extraction of the 4D signal requires the subtraction of two seismic volumes (baseline and monitor) over the same spatial reference but acquired at different times. The repeatability of the seismic method for both acquisition and processing is an important factor in determining the true 4D signal, over and above acquisition and processing artifacts.

Critical in the design of a 4D monitor survey is the extent to which the baseline or legacy survey geometry should be followed, keeping in mind that the legacy survey may often be poorly sampled relative to the industry’s currently accepted best practices for sampling, offset distribution, and fold of coverage. Processing techniques should also be identical on baseline and repeat surveys. Reprocessing of the baseline survey may be required to accurately match the latest processing techniques used on the repeat surveys.

Operational constraints and errors in repeatability

Repeat surveys must follow the orientation of the baseline survey to ensure maximum repeatability. Not only must we consider these issues at the time of the baseline survey but also project forward to the proposed dates of subsequent repeat surveys. It is particularly important to understand long-term plans for additional infrastructure likely to be added to a producing field during the interval between baseline and subsequent surveys.

Analysis of the acquisition system determines the inherent perturbations present owing to positioning accuracy, receiver sensitivity and calibration, and source calibration. Our Q-Marine point-receiver marine seismic system sets the standard for repeatability, and our IsoMetrix marine isometric seismic technology builds on the Q-Marine system’s field-proven elements to enable reconstructing the wavefield at any desired grid. This raises 4D repeatability to an unprecedented level and better reveals subtle variations in seismic responses related to changes in reservoir fluids and pressure.

Seismic resolution

To monitor change in reservoir beds, the recoverable seismic frequency band should include frequencies that are sufficiently high to resolve the required geological intervals. Existing data is analyzed and matched with well data to determine the thickness of the reservoir units and the ability of the seismic bandwidth to image them.

It is possible to observe change in the subsurface even if the exact layer in which it occurred is not resolved. These observations may still provide useful information. General imaging quality should also be assessed to determine inherent noise levels and their effect on the detection of 4D events.

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