Time-lapse seismic - 3-D Seismic Interpretation - page 182

1993 Pre-production

1999 Post-production

2000 Post-production

Fig. 8.3 Time-lapse sections. Note the initial brightening in the upper level, followed by slight

dimming.

resolution to understand the 3-D subsurface flow patterns. Also, it would be useful to
have some way to anticipate gas or oil breakthrough into an oil-producing well before
it happens. Time-lapse seismic can provide a way to get almost real-time data on these
fluid movements. Although the idea has been known for many years and can be applied
to isolated 2-D seismic lines, it is only in the last few years that the availability of high-
quality 3-D seismic has made the technique routine. For example, BP acquired its first
3-D survey primarily for 4-D effects and for commercial as opposed to experimental
purposes in 1999, and then built up activity quickly to acquire five surveys for 4-D effects
in 2001 in the North Sea alone (Ritchie et al. , 2002 ) . Shell had acquired 25 4-D surveys
worldwide by the end of 2001, and acquired a further 25 in 2002 (de Waal & Calvert,
2002) . Like other users, Shell regards time-lapse as proven technology for monitoring
fluid movements in thick clastic reservoirs offshore. The challenge is to extend the
method to pressure monitoring, to carbonate and to thin-bedded clastic reservoirs, and
to the onshore (where significant seismic data quality issues arise).
There are several strands to be combined in any time-lapse study.
The rock physics that predicts what amplitude and travel-time effects will be produced
by changes in fluid fill and reservoir pressure.
Seismic acquisition and processing issues: we need to distinguish genuine changes
in the reservoir from differences in the seismic data due to differences in acquisition,