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.

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