Experimental study on CO2 monitoring and quantification of stored CO2 in saline formations using resistivity measurements

This paper reports CO₂ saturation estimations based on resistivity data obtained from laboratory measurements and induction logging results at the Nagaoka pilot CO₂ injection site. The laboratory experiments put in evidence that the presence of clay content tends to reduce the increase of resistivity caused by the displacement of brine by less conductive CO₂. As a result, CO₂ saturations estimated from resistivity measurements without any correction for the clay effect are considerably lower than the actual saturations. The resistivity index (RI) provides better estimates of CO₂ saturations than the Archie's equation because it requires the determination or assumption of only one rock parameter: the saturation exponent. CO₂ saturations estimated from the induction logging data acquired at Nagaoka are considerably lower than the neutron porosity changes due to displacement between brine and CO₂ in the reservoir. Even in the case of considering the De Witte's equation and the Poupon's to account for the clay effect, it was still difficult to get a good agreement with the neutron logging results. New relations based on the resistivity index with correction factors for the clay effect are developed and implemented in this study. One of these relations has proved to be effective to estimate CO₂ saturations in saline formations with high clay content.