Application of Push-Drift-Pull Tracer Tests at a CO2 Storage Environmental Test Site and Identification of Factors Affecting Breakthrough Curves

Abstract

A single well push-drift-pull tracer test is a convenient and cost-effective tool to characterize hydrogeological features of the subsurface aquifer system although it has a limitation on the tested results by various experimental designs. In this study, application of the single well push-drift-pull tracer test at the carbon capture and storage (CCS) environmental monitoring field site was performed for deciding suitable location of monitoring wells prior to the CO2 leakage experiments. Laboratory-scale push-pull experiments with numerical simulation were also conducted to evaluate the factors affecting the result of push-drift-pull test and to suggest a proper test design for obtaining reliable results from the push-pull test. By applying the single well push-pull tracer tests at the field with two tracers (salt and SF6), local-scale estimates of the hydraulic properties (linear velocity, effective porosity) of the aquifer were obtained at the study site. Mass recovery percentage of the volatile tracer (SF6) was lower than that of the non-volatile tracer (salt) and degassing of SF6 was seemed to be intensified as drift time increased. Based on the obtained hydraulic properties and tracer mass recovery rates, an effective CO2 monitoring network including unsaturated zone boreholes was installed at the study site. To discover the affecting factors on the results of the push-drift-pull test, a series of laboratory-scale push-drift-pull tests were conducted with a sand tank under various controlled conditions such as variable drift time, tracer density, hydraulic gradient, and well penetrating depth. The results of the sensitivity analysis from the laboratory-scale tests presented the importance of sampling interval in most conditions except the penetrating depth. Several linear velocities were computed from the numerical simulation to investigate the effect of tracer density and pumping rate. The linear velocity was underestimated when input tracer concentration was increased because solute travel distance and direction during drift time were dependent on the density of tracer plume. For the pulling phase, reasonable pumping rate must be applied to extract the majority of injected tracer mass in order to obtain reliable center of mass time (tcom) and analyze solute transport properties. Therefore, the factors that affect test results should be considered carefully to design a push-drift-pull tracer test.