Fit for Purpose Time Lapse Seismic at F6

F6 is Shell’s largest gas field in the offshore Sarawak, with a GIIP (Gas Initially In-Place) of around 7Tcf (Trillion Cubic Feet). Production commenced in 1987 from a single platform located above the central pinnacle of the carbonate build-up. By 2006, 4Tcf had been produced, but scope for redevelopment still remains in the form of further drilling and late life compression. However, the biggest uncertainty is the strength of the aquifer and the movement of the GWC (Gas Water Contact). Pulsed neutron logging indicates a GWC rise near the main producing area beneath the platform but a large uncertainty remains towards the flanks. The first 3D seismic survey over F6 was acquired in 2002 (15 years after production start-up), which means there is no suitable pre-production 3D baseline survey. Prior to commitment to the project, a feasibility study was carried out within SSB (Sarawak Shell Berhad) using the 2002 dataset as the 3D baseline. The objectives were primarily, to determine if production-related sweep signals are observable on seismic and if so, when is the ideal time to carry out the monitor survey in order to impact business decisions. The feasibility studies showed that a time-lapse seismic response could be expected in 2006 but highlighted that the timelapse<br>signals on the flank may be weak to image. This is largely driven by the two different reservoir model inputs in the feasibility study, which could be a uniform rise of the GWC or cone-shape rise of the GWC. Both models also have significant impact on recoverable gas on the flanks as illustrated as in Figure 1. Finally, the changes in the velocity and impedance from both models were found to be small (<5%) and thus, requires good repeatability and good signal-to-noise ratio (S/N) seismic data in order to observe time-lapse signals. Considering the possibilities from the study and the business impact of additional recoverable volume at the flank, the time-lapse seismic was a timely exercise to help understand the extent of water influx and derisk future investment on the field.