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Cross-borehole tomography can be thought of as an extension of sonic logging to the reservoir cross section between two boreholes. The information obtained from cross-borehole tomography, when properly interpreted in the context of all available information, can often be invaluable for preparing accurate [[geological cross sections]] for reservoir development and planning.

Cross-borehole tomography can be thought of as an extension of sonic logging to the reservoir [[cross section]] between two boreholes. The information obtained from cross-borehole tomography, when properly interpreted in the context of all available information, can often be invaluable for preparing accurate [[geological cross sections]] for reservoir development and planning.

Important differences exist, however, between cross-borehole seismic velocity images and sonic logs. Sonic logs usually measure velocity of sound in rocks at very high frequencies (5 to 40 kHz), whereas seismic tomography measures the velocity of sound at seismic frequencies, usually in the frequency range of 20 Hz to as high as several kilohertz. Sonic logs represent the measurement of velocities at the wellbore and are usually plotted as a continuous curve in depth. Because seismic tomography measures the two-dimensional velocity field between the wellbores, it is usually represented by a color-coded map in which a color is assigned to the seismic velocity at each point. This map, or plot, is referred to as a ''tomogram''. Other displays such as contour plots can also be used, but this is not common.

Important differences exist, however, between cross-borehole seismic velocity images and sonic logs. Sonic logs usually measure velocity of sound in rocks at very high frequencies (5 to 40 kHz), whereas seismic tomography measures the velocity of sound at seismic frequencies, usually in the frequency range of 20 Hz to as high as several kilohertz. Sonic logs represent the measurement of velocities at the wellbore and are usually plotted as a continuous curve in depth. Because seismic tomography measures the two-dimensional velocity field between the wellbores, it is usually represented by a color-coded map in which a color is assigned to the seismic velocity at each point. This map, or plot, is referred to as a ''tomogram''. Other displays such as [[contour]] plots can also be used, but this is not common.

==Tomographic data acquisition==

==Tomographic data acquisition==

==Survey types==

==Survey types==

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file:cross-borehole-tomography-in-development-geology_fig3.png|{{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_fig3.png|{{figure number|3}}Time lapse tomograms acquired at 6 month intervals across a thermal EOR project. (After Justice et al.<ref name=pt07r25 />)

file:cross-borehole-tomography-in-development-geology_fig4.jpg|{{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.)

file:cross-borehole-tomography-in-development-geology_fig4.jpg|{{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.<ref name=pt07r25 />)

</gallery>

</gallery>

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).

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.<ref name=Nur_1984>Nur, A. M., 1984, Seismic monitoring of thermal enhanced oil processes: Society of Exploration Geophysicists Expanded Abstracts, 54th Annual Meeting, p.337-340.</ref><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.<ref name=Nur_1984>Nur, A. M., 1984, Seismic monitoring of thermal enhanced oil processes: Society of Exploration Geophysicists Expanded Abstracts, 54th Annual Meeting, p.337-340.</ref><ref name=pt07r25>Justice, J. H., A. A. Vassiliou, S. Singh, J. D. Logel, P. A. Hansen, B. R. Hall, P. R., Hurt, and J. J. Solanki, 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., A. A. Vassiliou, M. E. Mathisen, S. Singh, P. S. Cunningham, and P. R. Hutt, 1992, Acoustic tomography in reservoir surveillance: Society of Exploration Geophysicists, Reservoir Geophysics, p. 321–334.</ref><ref name=pt07r27>Justice, J. H., M. E. Mathisen, A. A. Vassiliou, I. Shiao, B. R. Alameddine, and N. J. Guinzy, 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 ([[: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.

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.

==Tomography data analysis==

==Tomography data analysis==

[[file:cross-borehole-tomography-in-development-geology_fig5.jpg|thumb|{{figure number|5}}Integrated data display documenting cross-borehole tomography interpretation across thermal EOR project. Log-defined stratigraphic units and fluid saturation zones correlate with tomographic velocity fields and provide the basis for interpretation of reservoir and fluid properties. (From Justice et ai., 1990.)]]

[[file:cross-borehole-tomography-in-development-geology_fig5.jpg|thumb|300px|{{figure number|5}}Integrated data display documenting cross-borehole tomography interpretation across thermal EOR project. Log-defined stratigraphic units and fluid saturation zones correlate with tomographic velocity fields and provide the basis for interpretation of reservoir and fluid properties. (After Justice et al.<ref name=pt07r25 />)]]

Tomography interpretations need to document reservoir properties and/or the production process in a way that development geoscientists and engineers can steadily understand and use in subsequent production planning. Before interpretation, tomography data quality and potential interpretation pitfalls should be evaluated. Appropriate tomography display parameters also need to be selected. Interpretations, which can then be completed based on correlations with reservoir data and models, should be presented using integrated data displays.

Tomography interpretations need to document reservoir properties and/or the production process in a way that development geoscientists and engineers can steadily understand and use in subsequent production planning. Before interpretation, tomography data quality and potential interpretation pitfalls should be evaluated. Appropriate tomography display parameters also need to be selected. Interpretations, which can then be completed based on correlations with reservoir data and models, should be presented using integrated data displays.

Numerous interpretation pitfalls, which can make part or all of an interpretation uncertain, are also important to evaluate. Since velocity is affected by many factors, incomplete geological characterizations or log data increase the uncertainty of tomography interpretations. Sufficient well data are therefore necessary to support interpretation adequately. In reservoirs where EOR processes cause rapid changes, correlation of tomograms with older logs may not be valid. Efforts should be made to run logs at the same time the tomography data are acquired.

Numerous interpretation pitfalls, which can make part or all of an interpretation uncertain, are also important to evaluate. Since velocity is affected by many factors, incomplete geological characterizations or log data increase the uncertainty of tomography interpretations. Sufficient well data are therefore necessary to support interpretation adequately. In reservoirs where EOR processes cause rapid changes, correlation of tomograms with older logs may not be valid. Efforts should be made to run logs at the same time the tomography data are acquired.

Tomography display parameters are one of the more important factors governing the ease of interpretation and acceptance of results. To facilitate correlations with cross-borehole geology, tomogram velocity fields should be scaled to match major stratigraphic and reservoir units ([[:file:cross-borehole-tomography-in-development-geology_fig4.jpg|Figure 4]]). The definition of smaller velocity fields may be appropriate to show details such as reservoir heterogeneity within selected major velocity fields. Stratigraphic units such as formation tops and sedimentary facies should also be delineated. In addition to defining velocity fields that match geological units, the use of standard colors and/or graphic symbols for rock units is important. Cold colors (blues) should be used for high velocity fields with a transition to hot colors (reds) for low velocities. Using these guidelines for display should result in tomograms that closely resemble geological cross sections and are more readily understood and utilized by geologists, engineers, and management. Analysis of tomograms with reference to reservoir models and any available surface seismic should provide the basis for interpretations that can be summarized with integrated data displays, as illustrated in [[:file:cross-borehole-tomography-in-development-geology_fig5.jpg|Figure 5]]. Most important are the well data to velocity field correlations and the log to tomography correlations documenting lithology, porosity, fluid saturation, and temperature. The resulting log and tomography display should provide the data needed to qualitatively document cross-borehole structure, especially dip and faults, reservoir heterogeneity and homogeneity, fluid contacts and any EOR flood fronts.

Tomography display parameters are one of the more important factors governing the ease of interpretation and acceptance of results. To facilitate correlations with cross-borehole geology, tomogram velocity fields should be scaled to match major stratigraphic and reservoir units ([[:file:cross-borehole-tomography-in-development-geology_fig4.jpg|Figure 4]]). The definition of smaller velocity fields may be appropriate to show details such as reservoir heterogeneity within selected major velocity fields. Stratigraphic units such as formation tops and sedimentary facies should also be delineated. In addition to defining velocity fields that match geological units, the use of standard colors and/or graphic symbols for rock units is important. Cold colors (blues) should be used for high velocity fields with a transition to hot colors (reds) for low velocities. Using these guidelines for display should result in tomograms that closely resemble [[geological cross sections]] and are more readily understood and utilized by geologists, engineers, and management. Analysis of tomograms with reference to reservoir models and any available surface seismic should provide the basis for interpretations that can be summarized with integrated data displays, as illustrated in [[:file:cross-borehole-tomography-in-development-geology_fig5.jpg|Figure 5]]. Most important are the well data to velocity field correlations and the log to tomography correlations documenting lithology, porosity, fluid saturation, and temperature. The resulting log and tomography display should provide the data needed to qualitatively document cross-borehole structure, especially [[dip]] and faults, reservoir heterogeneity and homogeneity, fluid contacts and any EOR flood fronts.

==See also==

==See also==

[[Category:Geophysical methods]]

[[Category:Geophysical methods]]