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Abstract Tracer methods represent techniques commonly used for the characterization and for the monitoring of transport processes in geo-reservoirs (e.g., CO2 storage). The current short communication addresses the development of a conceptual, mathematical and numerical model for a new tracer class (KIS tracers, Schaffer et al., 2013) useful for the characterization of fluid–fluid interfacial areas during supercritical CO2 injection into deep saline aquifers. This tracer type has the potential to quantify the amount of fluid–fluid interfacial areas, important for the quantification of reactions at the fluid interface, which can implicitly lead to optimized injection strategies, a better assessment of the extent of the CO2 plume and of the storage efficiency. The presented modeling approach overcomes the drawback of the current standard multiphase multicomponent models, which ignore kinetics of mass transfer over the interfacial area between the CO2 and brine and consider only the volumetric fraction of the fluids or their mass and molar fractions, respectively. In this model, the concept of a specific interfacial area obtained from pore network modeling is used to complement the constitutive relationships between capillary pressure and saturation. It is a two-phase four component flow and transport model with a kinetic mass transfer of tracers between the two fluids and taking the dissolution of CO2 in brine into account. Two numerical simulation scenarios are shown as examples for the design of experimental work in laboratory and eventually in the field. The modeling approach follows the assumptions of previous experimental work (Schaffer et al., 2013). Their implications are investigated by sensitivity analyses to narrow the physical range of reaction rates for further experiments and molecular tracer design. Both examples indicate that the tracer concentration is sensitive with respect to the interfacial area and, therefore, KIS tracer utilization both in the lab and in the field appear to be feasible for implementation.

The storage of supercritical carbon dioxide in deep saline aquifers requires new techniques to assess plume spreading, storage efficiencies and operational strategies after and during injections. In this work, a new class of reactive tracers (KIS tracers) planned to be used for the characterization of interfacial areas between supercritical CO2 and formation brine is presented. The implementation of a time-dependent hydrolysis reaction at the interface enables to investigate the development of the CO2/brine interface. Besides the basic concept for these novel tracers and the methodology for a suitable target molecular design, the desired tracer properties as well as the exemplary synthesis of first promising compounds are presented here. Additionally, the first experimental results of an analog study in a static two-phase batch system are shown and evaluated with a newly developed macroscopic model. Subsequently, the numerical forward modeling of different functions for the interfacial area change is described. The first results are promising and show the potential for new applications of KIS tracers after further research.

AbstractThis paper presents the experimental plans and designs as well as examples of predictive modeling of a pilot-scale CO2 injection experiment at the Heletz site (Israel). The overall objective of the experiment is to find optimal ways to characterize CO2 -relevant in-situ medium properties, including field-scale residual and dissolution trapping, to explore ways of characterizing heterogeneity through joint analysis of different types of data, and to detect leakage. The experiment will involve two wells, an injection well and a monitoring well. Prior to the actual CO2 injection, hydraulic, thermal and tracer tests will be carried out for standard site characterization. The actual CO2 injection experiments will include (i) a single well injection-withdrawal experiment, with the main objective to estimate in-situ residual trapping and (ii) a two-well injection-withdrawal test with injection of CO2 in a dipole mode (injection of CO2 in one well with simultaneous withdrawal of water in the monitoring well), with the objective to understand the CO2 transport in heterogeneous geology as well as the associated dissolution and residual trapping. Tracers will be introduced in both experiments to further aid in detecting the development of the phase composition during CO2 transport. Geophysical monitoring will also be implemented. By means of modeling, different experimental sequences and injection/withdrawal patterns have been analyzed, as have parameter uncertainties. The objectives have been to (i) evaluate key aspects of the experimental design, (ii) to identify key parameters affecting the fate of the CO2 and (iii) to evaluate the relationships between measurable quantities and parameters of interest.