Analysis of alternative push-pull-test-designs for determining in situ residual trapping of carbon dioxide

Carbon dioxide storage in deep saline aquifers is a promising technique to reduce direct emissions of greenhouse gas to the atmosphere. To ensure safe storage the in situ trapping mechanisms, residual trapping being one of them, need to be characterized. This study aims to compare three alternative single-well carbon dioxide push-pull test sequences for their ability to quantify residual gas trapping. The three tests are based on the proposed test sequence by Zhang et al. (2011) for estimating residual gas saturation. A new alternative way to create residual gas conditions in situ incorporating withdrawal and a novel indicator-tracer approach has been investigated. Further the value of additional pressure measurements from a nearby passive observation well was evaluated. The iTOUGH2 simulator with the EOS7C module was used for sensitivity analysis and parameter estimation. Results show that the indicator-tracer approach could be used to create residual conditions without increasing estimation uncertainty of Sgr. Additional pressure measurements from a passive observation well would reduce the uncertainty in the Sgr estimate. The findings of the study can be used to develop field experiments for site characterization.