Time-lapse crosswell seismic provides an efficient way to monitor the migration of a CO2 plume or its leakage after CO2 injection into a geologic formation. Recently, crosswell seismic has become a powerful tool for monitoring underground variations, using the concept of a virtual source, with virtual sources positioned at the receivers installed in the well and thus the positions of sources and receivers can be invariant during monitoring. However, time-lapse crosswell seismic using vertical wells and virtual sources has difficulty in describing the front of a CO2 plume, which usually is parallel to the vertical wells, and in obtaining sufficient ray coverage for the first-arrival tomography. These problems arise because of the theoretical downward-illumination-directivity limitation of the virtual source. We have developed an effective monitoring method that uses virtual sources and two horizontal wells: one above and one below the CO2 sequestration reservoir. In our method, we redatum the traces that are recorded at geophones in horizontal wells from sources on the surface. The redatumed traces then become virtual traces recorded at geophones in the lower well and sent from virtual sources at the positions of the geophones in the upper well. The geometry of our method has advantages for locating the front of the CO2 plume, which is normal to the horizontal wells, compared with either real or virtual sources. The method also is advantageous in acquiring full ray coverage between the wells, and that coverage is superior to coverage acquired using vertical crosswell seismic with virtual sources. In addition, we can avoid problems related to any potential change in the medium above the reservoir and in the source and receiver positions. The results of applying our method to synthetic data that simulate CO2-sequestration monitoring show that the front of a CO2 plume in the reservoir is depicted accurately in a velocity tomogram. The new method also can be used to monitor a reservoir during production of heavy oil.

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