Seismic reservoir monitoring
Figure 11.5-6 shows a difference section from the time-lapse data as in Figure 11.5-4 following cross-equalization. This difference section exhibits a strong amplitude anomaly at the reservoir level situated at the salt flank. Such an amplitude difference may be attributed to changes in the reservoir conditions as a result of production . Because of a wide range of factors associated with acquisition and analysis of the 4-D data, in addition to the difference data volume, the individual data volumes themselves are also visualized and interpreted.
The example of cross-equalization shown in Figure 11.5-7 relate to a steam injection project. Note the differences in the time slices from the image volumes associated with the 1996 survey and 1997 survey before and after cross-equalization. The bubbles correspond to the location of the injection wells.
Figure 11.5-4 Time-lapse 3-D seismic data: (a) an inline section and (b) a time slice from the time image volume associated with the 1979 survey; (c) an inline section and (d) a time slice from the time image volume associated with the 1991 survey. The interior of the broken yellow line defines the overlapping area between the two surveys. See text for details. ; figure courtesy Rickett, Stanford Exploration Project, and 4th Wave Imaging; data courtesy Chevron.)
Figure 11.5-5 Amplitude spectra of the two time-lapse 3-D seismic data as in Figure 11.5-4 (a) before and (b) after cross-equalization; (c) a difference section following cross-equalization. ; figure courtesy Rickett, Stanford Exploration Project, and 4th Wave Imaging; data courtesy Chevron.)
Figure 11.5-6 Difference section after cross-equalization of the time-lapse data as in Figure 11.5-4. ; figure courtesy Rickett, Stanford Exploration Project, and 4th Wave Imaging; data courtesy Chevron.)
The 4-D seismic anomalies are characterized as differences between time-lapse 3-D data that are present after cross-equalization as exemplified by Figure 11.5-5. Calibration of these anomalies often is ambiguous, in that, they may be attributable to changes in one or more of the reservoir conditions, such as change in fluid saturation caused by water displacing oil, pore pressure change caused by injection, or a temperature change caused by steam injection .
Although significant progress has been made in the 4-D seismic method, its value in determining dynamic reservoir properties is just beginning to be demonstrated. The information regarding the dynamic reservoir properties much sought after by the production engineer includes changes in oil saturation, water saturation, and pore pressure. Future developments in seismically driven reservoir characterization and monitoring should contribute significantly to optimum management of oil and gas fields.
- Rickett and Lumley, 1998, Rickett, J. and Lumley, D. E., 1998, A cross-equalization processing flow for off-the-shelf 4-D seismic data: 68th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 16–19.
- Ecker, 1999, Ecker, C., Lumley, D. E., Tura, A., Kempner, W., Klosnky, L., 1999, Estimating separate steam thickness and temperature maps from 4-D seismic data: An example from San Joaquin Valley, California: 69th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 2032–2034.