Transport of a tracer in a two-phase flow system

Chemical tracers have been extensively used in a wide range of environmental fields, including carbon dioxide (CO₂) geosequestration studies. We performed a series of tracer release experiments at Darcy-scale to understand the migration behavior of a tracer in a two-phase (scCO₂ and water) flow system.  The key features of the breakthrough curves (BTCs) that include the tracer arrival time, peak concentration, and tailing pattern were examined. The features of the BTCs show distinct differences depending on whether the domain was previously swept by CO₂. When the tracer is released before CO₂ sweeps the formation, the tracer plume is narrowly concentrated in the vicinity of the CO₂-water transition and the BTCs reflect the effect of local heterogeneity. The lack of a significant difference in tracer test BTCs conducted after CO₂ flooding suggests that a series of tracer release tests can be applied to the identification of newly generated flow paths.