Abstract
We measured the changes in P-wave velocity that occur when injecting CO2 in gaseous, liquid, and supercritical phases into water-saturated anisotropic sandstones. P-wave velocities were measured in two cylindrical samples of Tako Sandstone, drilled along directions normal and parallel to the bedding plane, using a piezo-electric transducer array system. The velocity changes caused by CO2 injection are typically -6% on average, with maximum values about -16% for the case of supercritical CO2 injection. P-wave velocity tomograms obtained by the differential arrival-time method clearly show that CO2 migration behaviour is more complex when CO2 flows normal to the bedding plane than when it flows parallel to bedding. We also found that the differences in P-wave velocity images were associated both with the CO2 phases and with heterogeneity of pore distribution in the rocks. Seismic images showed that the highest velocity reduction occurred for supercritical CO2 injection, compared with gaseous or liquid CO2 injection. This result may justify the use of the seismic method for CO2 monitoring in geological sequestration.
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