IN BRIEF

The Geo People has extensive experience of 4D projects: supplying specialist survey QC, project management, PAM operators, PAM equipment, and training for what are very complex surveys, often in close proximity to FPSOs FSOs, FLNGs and/or other platforms. In particular, survey QC is critical to ensure optimum repeatability of sail line direction, source direction and frequency range.

Seismic re-processing QC of old data is also provided, so that the data matches the superior processing routines of today’s monitor survey data.

IN DETAIL

4D survey design

Many kinds of feasibility studies are used to determine the optimum processing parameters, acquisition set up and inversion parameters. The aim is to repeat the previous seismic survey and identify changes in HC pressures, volumes and their migration.

Repeatability for a true 4D signal

4D seismic, also called 'time-lapse' seismic, requires an initial/baseline seismic survey of a subsurface area with a subsequent 'monitor' survey of the same area, copying as precisely as possible the initial acquisition survey parameters and conditions. This method allows comparison of seismic over intervals of months or years to detect changes resulting from the production of hydrocarbons or the injection of fluids.

How much the baseline survey geometry can be replicated in subsequent surveys is critical to the design of a 4D monitor survey. Often, the baseline survey is found to be poorly sampled, compared to present best industry practice. Offset distribution and fold of coverage may also fall short of present standards. Although processing techniques should ideally be identical for baseline and repeat surveys, baseline survey data is sometimes re-processed, to better match the present processing techniques.

Operational constraints and errors in repeatability

Orientation of the seismic lines must be identical for baseline and monitor surveys, so when the baseline survey is carried out, it is important to know where any subsequent infrastructure will be.

The acquisition system contains inherent perturbations resulting from positioning accuracy, receiver sensitivity/calibration and source calibration. Isometric seismic technologies attempt to reconstruct the wave-field to better reveal the subtle variations in seismic responses related to changes in reservoir fluids and pressure.

Seismic resolution

It is important that the frequency of the recovered seismic signal is high enough to image the smallest significant depth interval of a geological layer. Matching seismic data and borehole data will show whether the smallest significant layer is being imaged or not and if the resolution of the significant layer cannot be resolved, it is still possible to make useful observations of the subsurface.

It is also important to undertake noise analysis of the seismic data, to determine its effect on the imaging of the geological events.