A Novel Mobile-Immobile Model for Push-Pull Test with Delayered Extraction

Abstract:

Push-Pull test is a mature tracer test that could be used to estimate aquifer transport properties at the scale of meters by injection and then extraction at the same well. However, current mathematical model for Push-Pull test cannot capture the characteristics of a heterogeneous aquifer and does not consider the stage of tracer incubation in aquifer. A novel mobile-immobile model is proposed to explain the effect of tracer incubation in a confined heterogeneous aquifer. This model takes into account three stages, including tracer injection, incubation and extraction. Advection and dispersion both exist in the mobile zone. For the immobile zone, only dispersion is taken into consideration to interpret the effect of heterogeneity that impedes solute transport. The experimental data of three wells at a potential CO2 sequestration site with slightly fractured and heterogeneous aquifer were given to calibrate the model parameters. It is found that the decreasing rate of concentration is controlled by mass transfer rate between the mobile and immobile zones. The initial concentration of extraction decreases with longer incubation time and increases with higher injection concentration. The concentration decreasing rates of those three wells are almost the same before extraction volume/injection volume of 6, which implies that dilution dominates in this period. After that, the decreasing rate deviates with each other. A possible reason is that degradation plays more important role with longer reaction time.