In this study, we attempt to clarify the heterogeneous CO2 distribution in homogeneous and high-permeability porous sandstone (Mt. Simon sandstone: 105 mD) using both physical rock experiments and fluid mechanical analyses. In particular, the experiments involve CO2 injection into porous sandstone with measurements of P-wave velocity (Vp) and strain under low capillary number (Ca) flow conditions. We set three Vp-measurement lines and two strain gauges (vertical and horizontal) at the center of the core. We also monitor changes of flow rate, volume, and differential pressure between the two pumps during CO2 injection. The Vp values of all channels show slight changes (under 4%). By contrast, the strain measurements indicate a substantial expansion in both directions. It is proposed that the injected CO2 reaches the counter end of the core specimen immediately and forms few large-percolation clusters in porous sandstone. The CO2 that follows flows only through clusters as pathways without capillary resistance. These CO2 flows produce spatially heterogeneous distributions of CO2 in the porous sandstone and make it difficult to monitor and detect CO2 by seismic wave velocities only.