Effective monitoring for CO2 sequestration with virtual sources

Abstract

Summary: Time-lapse crosswell seismic is an efficient way to monitor the migration of the CO2 plume or its leakage after injecting CO2 to the geological formation. Recently, it was applied to the monitoring with the concept of virtual source (VS) where virtual sources are positioned at the receivers installed in the well and thus the positions of sources and receivers can be invariant during the monitoring. However, time-lapse crosswell seismic with vertical wells and VS has difficulties in describing the front of CO2 plume, which is usually parallel to the vertical wells, and in obtaining sufficient ray coverage due to the theoretical limitation of VS. In this study, we present an effective monitoring method by using the VS method and two horizontal wells above and below the CO2 sequestration reservoir. In the method, we convert the data recorded at geophones in horizontal wells with sources on surface to the data recorded at geophones in the lower well with virtual sources at the positions of the geophones in the upper well. Under the geometry of our method, we have advantages in locating the front of CO2 plume, which is normal to the horizontal wells, and in acquiring full ray coverage between the wells. The results of applying our method to the synthetic data simulating CO2 sequestration monitoring show that the front of CO2 plume in the reservoir was precisely depicted in the velocity tomogram.