Evaluation of a resistivity model derived from time-lapse well logging of a pilot-scale CO2 injection site, Nagaoka, Japan

► A rock physics model for resistivity at a CO2 injection site was constructed. ► A Waxman–Smits model was applied to fit the shaly sandstone log data. ► The effect of dissolved CO2 on the resistivity model was also considered. ► The model could estimate CO2 saturation at the site with sufficient accuracy. This paper presents a rock physics model describing the relationship between resistivity and CO2 saturation at the Nagaoka CCS test site in Japan. 10.4kt CO2 were injected at Nagaoka between July 2003 and January 2005, with repeated logging performed both during and after the injection period. The target reservoir at Nagaoka is shaly sandstone, in which the conductance of clay minerals has a significant effect on log resistivity. We analyzed open-hole log data in order to estimate shale volume and distribution, and applied Waxman–Smits model to fit the data. For the known observables in the model we used the resistivity log and water saturation derived from the neutron log, with the saturation exponent and the correction value for shale conduction then obtained using the time-lapse logging data. According to the modeled results, the area characterized by smaller saturation exponents corresponded to that with minor changes in resistivity. Dissolved CO2 was also found to reduce the resistivity of formation water. Model modification taking this effect into account was thus carried out, although the essence of the original Waxman–Smits model was retained. The performed analyses provide evidences of successful residual and solubility trapping currently in progress at Nagaoka.