Seismic questions

I have described some very complex and intriguing features visible in the time-lapse seismic images. These features are almost certainly associated with fluid-flow processes in the P and K reservoirs since they correlate with steam injection depths, and very few seismic changes are seen in the overburden images. Similar features are visible in the other time-lapse data sets, and evolve spatially with time. Such dramatic changes in seismic data during steam injection raise many interesting questions, including the following:

• Do the bright reflections and time delay in the reservoir indicate the presence of steam, or merely hot regions?
• Should we be able to distinguish separate fluid-fronts of water, oil and steam?
• Can we resolve the vertical and lateral distribution of steam in the upper and lower formations?
• What could be causing time pull-up (velocity increase) to the north and west of the injector?
• What is the explanation for the large polarity reversal to the south and east along the base of reservoir reflection?
• Why are changes within the reservoir confined to a radius of only 50 m, whereas changes below the reservoir extend at least 350 m out to the edge of the survey area?

In the following sections, I attempt to answer some of these questions by carefully considering a simple model of steamflood fluid flow, rock physics core measurements and analysis, finite-difference seismic modeling, and seismic image and velocity analysis.

 

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Figure 6 3-D stacked inline sections: before steam injection (left), after 5 months of steam (center), and the difference (right).

 

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Figure 7 3-D migrated inline sections: before steam injection (left), after 5 months of steam (center), and the difference (right).

 

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Figure 8 3-D migrated time slices at the base of the P reservoir (208 ms): before steam injection (top), and after 5 months of steam injection (bottom).