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Line 48: + + + + − + − − [[file:cross-borehole-tomography-in-development-geology_fig3.png|thumb|{{figure number|3}}Time lapse tomograms acquired at 6 month intervals across a thermal EOR project. (After <ref name=pt07r25 />.)]] − + − − [[file:cross-borehole-tomography-in-development-geology_fig4.jpg|thumb|{{figure number|4}}Single image tomograms documenting stratigraphy and reservoir zones. Heterogeneity of the lower reservoir is documented by the contrast in log curves and the corresponding change in the tomographic velocity fields. (After Justice et al., 1990.)]]
Cross-borehole tomography (view source)
Revision as of 13:55, 21 January 2014
, 13:55, 21 January 2014→Survey types
==Survey types==
==Survey types==
[[file:cross-borehole-tomography-in-development-geology_fig3.png|left|thumb|{{figure number|3}}Time lapse tomograms acquired at 6 month intervals across a thermal EOR project. (After <ref name=pt07r25 />.)]]
Two major types of tomographic surveys are in use today. These consist of single tomograms for imaging reservoir characteristics, and time-lapse tomography for imaging time evolution processes in reservoirs such as those associated with [[enhanced oil recovery]] (EOR).
Two major types of tomographic surveys are in use today. These consist of single tomograms for imaging reservoir characteristics, and time-lapse tomography for imaging time evolution processes in reservoirs such as those associated with [[enhanced oil recovery]] (EOR).
[[file:cross-borehole-tomography-in-development-geology_fig4.jpg|thumb|{{figure number|4}}Single image tomograms documenting stratigraphy and reservoir zones. Heterogeneity of the lower reservoir is documented by the contrast in log curves and the corresponding change in the tomographic velocity fields. (After Justice et al., 1990.)]]
In time-lapse tomography, a baseline survey is taken, ideally before an EOR (or similar) project begins. The resulting tomograms may or may not be interpreted at this time. After an appropriate time interval, during which the EOR program is operating and inducing changes in the reservoir, a second survey is taken. The resulting tomograms will be of little additional value unless the changes in the reservoir have also induced changes in the seismic velocity field. Fortunately, there are some well-documented situations in which this does occur (see Nur, 1984; <ref name=pt07r25>Justice, J. H., Vassiliou, A. A., Singh, S., Logel, J. D., Hansen, P. A., Hall, B. R., Hurt, P. R., Solanki, J. J., 1989, Acoustic tomography for monitoring enhanced oil recovery: Leading Edge, v. 8, p. 12–19., 10., 1190/1., 1439605</ref>, <ref name=pt07r26>Justice, J. H., Vassiliou, A. A., Mathisen, M. E., Singh, S., Cunningham, P. S., Hutt, P. R., 1992, Acoustic tomography in reservoir surveillance: Society of Exploration Geophysicists, Reservoir Geophysics, p. 321–334.</ref>, <ref name=pt07r27>Justice, J. H., Mathisen, M. E., Vassiliou, A. A., Shiao, I., Alameddine, B. R., Guinzy, N. J., 1993, Crosswell seismic tomography in improved oil recovery: First Break (in press).</ref>. For example, heating of heavy oils by steam injection can produce large reductions in the seismic velocity field in the affected part of the reservoir.
In time-lapse tomography, a baseline survey is taken, ideally before an EOR (or similar) project begins. The resulting tomograms may or may not be interpreted at this time. After an appropriate time interval, during which the EOR program is operating and inducing changes in the reservoir, a second survey is taken. The resulting tomograms will be of little additional value unless the changes in the reservoir have also induced changes in the seismic velocity field. Fortunately, there are some well-documented situations in which this does occur (see Nur, 1984; <ref name=pt07r25>Justice, J. H., Vassiliou, A. A., Singh, S., Logel, J. D., Hansen, P. A., Hall, B. R., Hurt, P. R., Solanki, J. J., 1989, Acoustic tomography for monitoring enhanced oil recovery: Leading Edge, v. 8, p. 12–19., 10., 1190/1., 1439605</ref>, <ref name=pt07r26>Justice, J. H., Vassiliou, A. A., Mathisen, M. E., Singh, S., Cunningham, P. S., Hutt, P. R., 1992, Acoustic tomography in reservoir surveillance: Society of Exploration Geophysicists, Reservoir Geophysics, p. 321–334.</ref>, <ref name=pt07r27>Justice, J. H., Mathisen, M. E., Vassiliou, A. A., Shiao, I., Alameddine, B. R., Guinzy, N. J., 1993, Crosswell seismic tomography in improved oil recovery: First Break (in press).</ref>. For example, heating of heavy oils by steam injection can produce large reductions in the seismic velocity field in the affected part of the reservoir.
With at least two time-lapse surveys in hand, the differences in the successive images (Figure 3) should relate to changes induced in the reservoir by the process, all other factors being constant.
With at least two time-lapse surveys in hand, the differences in the successive images ([[:file:cross-borehole-tomography-in-development-geology_fig3.png|Figure 3]]) should relate to changes induced in the reservoir by the process, all other factors being constant.
Single tomographic images, however, are often difficult to interpret. Changes in the observed seismic velocity field across the reservoir may be due to the combined effects of lithology, pore fluids, and the physical state of the reservoir. Often geological stratification is easily discernible (Figure 4) due to the contrast in seismic velocities characteristic of different types of rocks The results of faulting and other structural features in the reservoir can also sometimes be seen, when associated with a sufficient contrast in seismic velocities.
Single tomographic images, however, are often difficult to interpret. Changes in the observed seismic velocity field across the reservoir may be due to the combined effects of lithology, pore fluids, and the physical state of the reservoir. Often geological stratification is easily discernible ([[:file:cross-borehole-tomography-in-development-geology_fig4.png|Figure 4]]) due to the contrast in seismic velocities characteristic of different types of rocks The results of faulting and other structural features in the reservoir can also sometimes be seen, when associated with a sufficient contrast in seismic velocities.
==Applications==
==Applications==