4D Survey Planning & Design

Presurvey evaluation to determine project parameters

The planning process for a 4D survey is more involved than planning a regular exploration 3D survey.

Presurvey evaluation and design studies are critical in determining 4D survey acquisition, processing, and inversion parameters.

The goal of successful 4D studies is to increase production and cost savings through better planning of production and injection wells and greater understanding of reservoir characteristics.

4D survey planning and design studies assist in increasing production and cost savings.

4D feasibility studies

The primary objective of 4D feasibility studies is to synthesize seismic amplitude responses to variations in reservoir conditions. From this, we can determine

the type of 4D signal that change in the rocks and fluids will generate
whether that signal will be detectable above the noise
the most realistic 4D response.

The synthesis can be based on either a zero-dimensional rock physics model or 3D reservoir simulator predictions of reservoir conditions. The first step is to build a rock physics model of the reservoir and then investigate the uncertainty of the model to variations in reservoir parameters such as saturation and pressure.

4D survey evaluation and design

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

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.

From 2D and 3D processing and imaging through to marine isometric seismic technology, WesternGeco has been at the forefront of depth migration technologies and broadband processing.

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.

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.