f 4D Seismics - Principles And Application

The aspect of monitoring reservoirs clearly comprises all aspects of data acquisition, processing and<br>interpretation. Certain rules apply in order to keep the monitoring process stable, where the most important<br>ones are:<br>- to maintain the same 3D-acquisition geometry: the same geometry then guarantees that the same S/N-ratio in<br>terms of noise-suppression is achieved, one example being here e.g. the aspect of multiple-suppression for the<br>entire data-volume.<br>- to maintain the same processing flow: nevertheless of course e.g. residual statics have to be computed based<br>on the individual volumes of data. Another example here is the process of deconvolution.<br>- to use the first 3D-volume (usually called the base-line survey) to establish the basic processing flow as well<br>as to establish the underlying macromodel (velocities V(x,y,z) and velocity interfaces) and (if needed), by<br>sequence stratigraphic interpretation the 3D-litho model of the reservoir unit.<br>- to decide upon the monitoring quantity, i.e. which quantity to use for 4D-evaluation. The default here is to use<br>Acoustic Impedance (AI) (which means that after proper imaging the data is subject to a suitable inversion<br>algorithm, usually at post-migration stage).<br>Clearly the type of reservoir to be monitored has impact on the acquisition effort, since we have to distinguish<br>between two basic types of reservoirs:<br>- High-impedance reservoirs, i.e. carbonates. As a „rule of thumb“ the maximum changes here are up to 5% (in<br>acoustic impedance) and<br>- Low-impedance reservoirs, i.e. sandstones. The „rule of thumb“-Figure here is up to 10% of AI-change.<br>In any case for the actual reservoir under consideration its sensitivity to fluid substitution has to be evaluated,<br>usually based on the Biot-Gassmann-formalism for the Zero-Offset case. This part of a 4D-evaluation is called<br>the calibration part. If done properly, the outcome is a reliable estimate of the dynamic range of the changes in<br>acoustic impedance due to ongoing production. In addition one needs to estimate the lateral „area of change“ to<br>be expected by producing the reservoir<br>during the timespan in between the two seismic surveys.