4D Survey Evaluation and Design

WesternGeco has a holistic approach to 4D survey evaluation and design (SED). The feasibility study results and the reservoir properties to be ultimately delivered will determine the survey acquisition, processing, and inversion parameterization. The influence of seismic processing parameterization is considered during acquisition design.

Repeatability

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

Acquisition is best conducted with identical parameters with identical systems. It is, therefore, important to consider only those acquisition systems/methods that are not approaching obsolescence. Critical in the design of a 4D monitor survey will be the extent to which the baseline or legacy survey geometry should be followed, bearing in mind that the legacy survey may often be poorly sampled relative to the generally accepted best practices in the industry for sampling, offset distribution, and fold of coverage. Processing techniques should also be identical on baseline and repeat surveys. Where processing systems evolve and supersede that used on the baseline volume, reprocessing of the baseline survey will be required to accurately match the latest processing techniques used on the repeat surveys.

More fundamental to the issue of repeatability is the orientation of the survey. Operational constraints often require surveys to be acquired in non-optimal orientation in terms of purely geophysical criteria. Obstructions, weather patterns, cost constraints, and maritime traffic can all influence the survey orientation. 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 date(s) of subsequent repeat survey(s). 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. Having considered the above sources of error in repeatability, we must also look at the acquisition system itself to determine the inherent perturbations present due to:

• Positioning accuracy
• Receiver sensitivity/calibration
• Source calibration

The WesternGeco Q-Marine system was specifically designed to set the highest standards of repeatability.

Seismic resolution

Temporal resolution is directly related to the high frequency content of the seismic signal. To monitor changes in reservoir beds, it is desirable that the recoverable seismic frequency band include frequencies high enough to resolve the geological intervals required. Existing data should be analyzed and matched with well data to determine the thickness of reservoir units and the ability of the seismic bandwidth to image them.

It is possible to observe changes in the subsurface, even if though the exact layer in which it occurred is not resolved. These observations may still provide useful information. Additionally general imaging quality needs to be assessed to determine inherent noise levels and the effect on the detection of 4D events.