In
jection of carbon dioxide into depleted oil fields or deepsaline aquifers represents one of the most promisingmeans of long-term storage of this greenhouse gas. Whilethe ultimate goal of CO
2 in
jection in the subsurface ismineral storage of CO
2 as carbonates, short-term (<50 year)storage of in
jected CO
2 is most likely to be accomplishedby ionic trapping of CO
2 as bicarbonate ions (HCO
3-)and hydrogeological trapping of molecular CO
2. Here, wedemonstrate a technique for quantifying ionic trapping ofin
jected CO
2 as HCO
3- using geochemical data collectedprior to and during 40 months of CO
2 in
jection into ahydrocarbon reservoir at the International Energy Agency(IEA) Weyburn CO
2 Monitoring and Storage Pro
ject,Saskatchewan, Canada. As a result of in
jection of CO
2with a low carbon isotope ratio (
13C value), fluid and gassamples from four selected production wells showed anincrease in HCO
3- concentration and a decrease in
13Cvalues of HCO
3- and CO
2 over the observation period. Isotopeand mass balance calculations indicate that, after 40months of in
jection, ~80% of the HCO
3- in the reservoirbrines sampled from the four wells formed via dissolutionand dissociation of in
jected CO
2. This chemical andisotopic technique should be applicable to CO
2 in
jectionand storage in oil fields and in deep saline aquifers, providedthere is sufficient carbon isotopic distinction betweenin
jected CO
2 and baseline aquifer HCO
3- and CO
2.
