We have carried out laboratory measurements of P-wave velocity and deformation strain during CO 2 injection into a porous sandstone sample, in dry and water-saturated conditions. The rock sample was cylindrical, with the axis normal to the bedding plane, and fluid injection was performed from one end. Using a piezoelectric transducer array system, we mapped fluid movement during injection of distilled water into dry sandstone, and of gaseous, liquid, and supercritical CO 2 into a

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... sample. The velocity changes caused by water injection ranged from 5.61 to 7.52%. The velocity changes caused by CO 2 injection are typically about -6%, and about -10% for injection of supercritical CO 2 . Such changes in velocity show that the seismic method may be useful in mapping CO 2 movement in the subsurface. Strain normal to the bedding plane was greater than strain parallel to the bedding plane during CO 2 injection; injection of supercritical CO 2 showed a particularly strong effect. Strain changes suggest the possibility of monitoring rock mass deformation by using borehole tiltmeters at geological sequestration sites. We also found differences associated with CO 2 phases in velocity and strain changes during injection.