Video 4D Vertical Seismic Profiling
Geophysicist Katie Mahoney discusses WesternGeco 4D VSP.
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.
The primary objective of 4D feasibility studies is to synthesize seismic amplitude responses to variations in reservoir conditions. From this, we can determine
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.
We employ a diverse range of feasibility studies toward determining the most appropriate acquisition, processing, and inversion parameters for each 4D survey.
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.
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.