Combined Effects of Bottom Flow and Wellbore Mixing on the Single-well Push-pull Test Using Large-diameter Well

The single-well push-pull (SWPP) test is widely used for determining hydrogeological parameters such as dispersivity coefficient due to its advantages of low cost, short duration, and simple operation. Conducting SWPP experiments in large-diameter wells, commonly used in arid and semi-arid regions, often faces the combined effects of leakage at the well bottom and wellbore mixing. To address this, this study constructed a three-stage solute transport model for the SWPP test that considers both the influences of flow at the well bottom and wellbore mixing effects. The model was solved using the finite difference method, and its accuracy was verified against the analytical solutions of previous seepage and solute transport models. Subsequently, the study explored the seepage characteristics near large-diameter wells and the impact mechanism of well size on the SWPP test based on this model. The research results showed that: (1) The newly proposed SWPP test model, which considers well bottom flow, matches well with previous analytical solutions in depicting the drawdown process during pumping and the solute transport process at the well screen, indicating the accuracy of the model’s construction and solution. (2) As the length of the well screen gradually decreases, the influence of the well bottom flow on the entire flow field gradually increases, changing the flow field from radial to spherical. (3) Compared to a small well diameter, a larger well diameter results in a smaller breakthrough curve (BTC) value for solutes during the injection phase, but a larger BTC during the extraction phase, indicating that the mixing process within the well has a more significant impact on the SWPP test than the leakage process at the well bottom. (4) When the well depth is about half of the well diameter, creating a quasi-spherical flow field, the leakage process at the well bottom may have a more significant impact on the SWPP test than the mixing process. For the simulated scenarios set in this study, even in the absence of bottom flow, the previous analytical solution, due to the neglect of wellbore miscibility, still have an error of 152.5% in the interpretation of dispersivity coefficient. Moreover, as the burial depth of the wellbore bottom gradually decreases, the error in the previous analytical solution caused by the bottom flow also increases gradually.