Estimation of recovery time and self-flushing outside and inside the well screen

Abstract

With respect to the detailed survey of underground water contamination, a question arises about the effect of the monitoring well equipment on the representativeness of the underground water samples. The well screen is one of the key well parts. The active and passive sampling in this part of the well has an influence on the contaminant stratification or redistribution. Should we take the original sample, one of the possibilities, according to the author, is to observe the so-called regeneration time. This experiment aspired to determine how great was the actual influence of the well screen and what time delay should be kept after inserting or handling with the sampling equipment. For these purposes, a physical model of the flow medium was built with a well screen mounted. A series of tracing experiments at changing parameters of the porous medium hydraulic conductivity and well screen perforation served to measure the time decrease in the NaCl tracer concentration inside the well screen. The measured data were approximated analytically using the so-called dilution method; the flow speed inside the well screen was obtained as a function of the perforation and hydraulic conductivity of the medium. Furthermore, the so-called drainage effect of the well was taken into account, as it is also influenced by the well screen. It is value was compared with the analytical and numerical solution. The Visual Modflow software was used for numerical modelling, which enabled modelling of the designed equipment and verification of the basic flow parameters of the model. For the given conditions, the drainage effect α = 2,1 was set as a basic value at which the well screen induces zero resistance at approx. 4 % perforation. From the obtained functional relations, coefficients for conversion between the investigated parameters were derived and the tracer delay time inside the well screen. was qualified.. The second stage was the evaluation of the extent to which a change in the natural state failure will manifest itself outside the well screen by inserting sampling equipment. The derived radius of the affected zone around the well screen, which is a function of the volume of water displaced by hydrostatic pressure and the surrounding material porosity, helped to establish the time in which the failure state recovers theoretically. This resulted in the total regeneration time tC as a total of the time in minutes inside and outside the well screen. The paper aims to point to this time parameter, its suitability and possible approach to the determination in place. This is not, however, a valid value or calculation, but experimental data obtained at a narrow-band hydraulic conductivity and gradient, by interpolation and in ideal laboratory conditions. The regeneration time obtained in this way can only be recommended as a rough estimate.