Numerical simulation of single-well push-pull tests in a radial two-zone confined aquifer

A single-well push-pull test is one of the most effective ways to estimate aquifer transport parameters. However, a patchy aquifer of finite thickness may surround the test well due to a gravel pack, mud invasion and stress redistribution during the well construction, which is usually neglected for single-well push-pull tests. In such a case, the aquifer should be regarded as a radial two-zone system, i.e., patchy zone and aquifer formation zone. In this study, a numerical model of a single-well push-pull test for a radial two-zone confined aquifer was developed using finite-element COMSOL Multiphysics. Two special cases, i.e., uniform patchy aquifer and non-uniform patchy aquifer, were considered. For the uniform patchy aquifer, results indicate that larger values of effective porosity and dispersivity in the patchy zone results in larger values of the breakthrough curves in the pumping phase. Patchy-aquifer thickness was also noted to have considerable impact on the breakthrough curves. As for the nonuniform patchy aquifer, the variation of hydraulic conductivity, dispersivity and effective porosity in the patchy zone leads to a change of concentration along the z-direction around the well screen, and as such, a smaller dispersivity of patchy regions results in more fluctuation in the concentration curve, while little impact can be found for a larger dispersivity. In addition, results showed that the average concentration over the entire screened section with an equivalent uniform patchy aquifer can be applied to interpret the data of a single-well push-pull test for the case of a nonuniform patchy aquifer.