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Cross-Well Seismology: A New Production Tool
Cross-well seismic imaging is a new technique for obtaining better reservoir characterization than is possible with surface seismic data alone. We anticipate that this technique will be used routinely in a few years in support of production efforts. The most immediate application is likely to be the monitoring and evaluation of thermal enhanced oil recovery (EOR) efforts such as the Duri steamflood project. Many problems associated with reservoir definition such as the determination of the continuity of bedding between wells may also be addressed.
There are several factors which compound to limit the resolution capability of surface seismic methods. The low frequencies (typically 10–50 Hz) place limits on the size of the features which can be resolved. The distances the energy travels coupled with the energy loss of a reflection reduce the signal-to-noise ratio. The noisy oil field environment is also a limiting factor for surface seismology.
In cross-well seismology we place the source in one well and the receivers in an adjacent well an either side of the region of interest. This reduces the distance which the energy travels, is usually below the weathering layer, and helps to isolate the experiment from environmental noise. Currently, the data are acquired using a downhole airgun and standard one-level clamped geophones. A future downhole acquisition system may consist of a swept-frequency, vibratory source which is non-destructive, and a 100-level receiver string of clamped, three-component geophones.
Two-dimensional velocity images can be constructed for the region between the boreholes using a technique known as tomographic inversion. The traveltimes for the direct arrivals of the P- and S-waves, combined with the predicted raypaths, serve as the basis for this reconstruction. With this information alone we can usually construct a good image of the acoustic velocities, but the constraints from a sonic log in each of the wells and a geologic model for the region between the wells will improve the results. In the future we hope to be able to combine the velocity image from the direct arrivals with the migrated crosswell reflection data for an even more detailed image.
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