Seismic and strain detection of heterogeneous spatial distribution of CO₂ in high- permeable sandstone

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 (V_p) and strain under low capillary number (C_a) flow conditions. We set three V_p-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 V_p 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.