Already a leading player, StatoilHydro has now made the leap from qualitative to quantitative time-lapse seismic - from identifying drained parts of a reservoir to producing saturation maps.
Time-lapse or 4D seismic involves comparing the results of 3D seismic surveys repeated at considerable time intervals (e.g. before a field starts producing versus various post-production stages): time is the fourth dimension.
Robust differences seen between the survey results are attributed to fluid changes and/or changes in reservoir pressures.
Prinsipp for og eksempel på 4D-seismikk
4D seismic technology can thus be used to monitor variations caused by production and/or the injection of fluids and gas to improve recovery.
Up until now, most of our research has been focussed on qualitative aspects:
- Differentiating between drained and un-drained portions of a reservoir to locate remaining pockets of oil and gas
- Mapping fault-bounded reservoir compartments
- Inferring whether faults allow fluid to flow through them or not
Recently, however, attention has been turned to developing techniques for extracting quantitative information, including uncertainties.
The approach is to integrate the seismic data with all other available information (from cores, wells, production and reservoir simulation).
An example of this was a 4D seismic-based map of saturation made for a part of the Statfjord field (i.e. a map showing the relative proportions of oil and water).
Subsequent drilling proved that this map was more accurate than that derived from conventional reservoir simulation. A similar exercise is now being done for the structurally more complex Gullfaks field.
The next step is to enter the realm of seismic ‘history matching’ by linking 4D seismic to flow simulation.
If successful, this will dramatically improve forecasts of how a reservoir behaves during production and the updating of geological and petrophysical reservoir models.
