Experimental Study on CO₂ Monitoring and Saturation with Combined P-wave Velocity and Resistivity

As a part of basic studies with the purpose of better understanding to estimate the saturation of stored carbon dioxide (CO₂) with seismic velocity and resistivity, laboratory experiment has been conducted to monitor combined P-wave velocity and resistivity simultaneously in porous sandstone during CO₂ injection process. Experiments were conducted by simulating the reservoir condition with high temperature and high pressure in depth of about 1000m or more. Supercritical CO₂ (10MPa, 40°C) was injected into water-saturated sandstone. The results showed that resistivity increased monotonously throughout the injection period, and P-wave velocity and amplitude decreased drastically after starting injection of supercritical CO₂. CO₂ saturations estimated by both Resistivity Index (RI) and Gassmann fluid-substitution equations based on P-wave velocity and resistivity were very matched with CO₂ saturation from replaced brine in a sample. By comparing velocity-saturation relation with resistivity-saturation relation estimated by Gassmann and RI model, P-wave velocity becomes less sensitive when CO₂ saturation is over about 20%, while resistivity kept increasing with increase of CO₂ saturation. Resistivity can complement effectively the difficulty of P-wave velocity on quantifying CO₂ saturation from suggested equations.