Estimation of two-phase fluid behavior in porous rocks based on rock properties

Abstract

This study investigates the potential of monitoring CO₂ behavior using seismic wave velocities. Specifically, the relationship between CO₂ saturation （S_CO2） in porous rocks and seismic wave velocities （V_p and V_s） was examined through core flooding experiments involving supercritical CO₂. Measurements of seismic wave velocities, electrical resistivity （ρ_e）, and strain （ε） were conducted simultaneously, and the results were analyzed using the Gassmann-CRM model. The key findings are as follows:

1. Sensitivity of V_p

V_p is highly sensitive to CO₂ saturation and distribution patterns. However, its sensitivity is significantly influenced by injection rates and the macroscopic capillary number （N_c）.

2. Behavior at low N_c values

When N_c is below 1 （N_c<1）, changes in V_p are limited. Conversely, N_c and seismic wave amplitude are more sensitive to CO₂ behavior under these conditions.

3. Effects of CO₂ injection rates

Higher CO₂ injection rates result in significant changes in cluster size and more pronounced V_p responses associated with S_CO2.

These results highlight the importance of integrating complementary methods, such as ρ_e and seismic wave amplitude, in addition to V_p, for effective monitoring of CO₂ reservoirs. For high-permeability reservoirs, using N_c as a key metric for monitoring strategies is particularly effective.