Assessing the usefulness of the isotopic composition of CO2 for leakage monitoring at CO2 storage sites: A review

Abstract Geological storage of injected CO 2 is a promising technology to reduce CO 2 emissions into the atmosphere. Tracer methods are an essential tool to monitor CO 2 plume distribution in the target formation and to enable tracking potential leakage of CO 2 outside the storage reservoir. Here, we demonstrate that the isotopic composition of CO 2 can serve as a suitable tracer at large CO 2 injection sites provided that the injected CO 2 is isotopically distinct from background CO 2 sources that are usually composed of dissolved inorganic carbon, bedrock-derived carbon, and soil CO 2 . Very favourable conditions for this tracer approach exist if δ 13 C values of injected CO 2 are more than 10‰ different from those of baseline CO 2 and other dissolved inorganic carbon species at the CCS site. In this case, changes in δ 13 C values accompanied with increasing concentrations of CO 2 or DIC in samples obtained regularly at monitoring sites within or above the storage reservoir indicate arrival of injected CO 2 . The proportion of injected CO 2 contributing to the obtained samples can be quantified when carbon isotope fractionation effects are either negligible or thoroughly known. We point out several areas where additional detailed information on carbon isotope effects during phase change, transport and geochemical reactions is desirable to refine this tracer approach for temperature, pressure and salinity conditions relevant for CO 2 storage sites. Oxygen isotope ratios of injected CO 2 were not found to be a conservative tracer due to oxygen isotope exchange between CO 2 and water on time scales of hours to a few days. δ 18 O measurements on CO 2 and H 2 O have, however, revealed pore space saturation with CO 2 and hence indicate the presence of injected CO 2 within CO 2 storage reservoirs. We suggest that the stable isotopic composition of injected CO 2 is a suitable tracer for assessing the movement and fate of injected CO 2 in the target reservoir and for leakage detection at CO 2 storage sites, provided that the injected CO 2 is isotopically distinct from background CO 2 sources. A key advantage is that this tracer approach does not depend on the co-injection of additional tracers and hence can be continuously used in large-scale commercial storage projects with CO 2 injection rates exceeding 1 million tonnes per year at reasonable cost.