We conducted a comprehensive experimental study on the effect of the CO2 injection rate (flow rate: represented by the macroscopic capillary number, Nc) on CO2 saturation (SCO2) and the CO2 distribution in porous, brine-saturated Berea sandstone. We measured two independent geophysical parameters, P-wave velocity (Vp) and electrical resistivity (ρ*), to monitor the two-phase flow. Vp showed clear dependency on SCO2 in the transition zone between the capillary limit (CL) and the viscous limit (VL), but not near the CL. The Vp−SCO2 relationship showed that the characteristic size of connected CO2 volumes decreased with increasing Nc, whereas resistivity increased continuously with increasing SCO2. Therefore, resistivity is sensitive to SCO2 under flow conditions between the CL and VL. The ρ*–SCO2 relationship showed that CO2 penetration increased with increasing Nc. These results indicate that we can use Vp and ρ* to monitor CO2 flow in pore spaces because the injected CO2 forms pathways and replaces brine under a wide range of Nc values.
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