HYDRAULIC CONDUCTIVITY OF PREPACKED WELL SCREENS

This study was conducted to determine the hydraulic conductivity of commercially available prepacked well screens used for installing wells by direct push methods. Prepacked well screens tested were: two pairs of Geoprobeâ Systems prepacked well screens having the same sand pack material attached to slotted casing, having diameters of 2.3 cm and 3.3 cm and having lengths of 86 cm and 149 cm, respectively; and three pairs of GeoInsightâ prepacked well screens attached to slotted casing having a diameter of 2.5 cm and having a length of 144 cm but with 3 different gradations of sand pack material. The prepacked well screens were fitted with a riser and mounted vertically within a 20.3 cm diameter pipe filled with water. The water was then pumped from the riser and circulated back into the pipe outside the screen while the water level remained above the prepacks. Pressure transducers monitored water levels inside and outside the prepacked screen. The steady-state head drops between inside and outside of the prepacked screens at different flow rates were then used in steady-state ellipsoidal and radial flow models to calculate K. The K values obtained ranged from about 4 cm/s to 1000 cm/s. At discharge rates beyond 7.6 – 11.4 Lpm, depending on the prepacked screen characteristics, non-laminar flow occurred for the smaller diameter prepacked screens. Non-laminar flow for the two larger diameter prepacked screens occurred beyond a discharge rate of 13.3 Lpm. The observed non-laminar flow rates are consistent with Reynolds number calculations.  These non-laminar flow effects set a limit on the flow rate that can be used in performing either constant head tests (or slug tests) in the field using prepacked monitoring wells without correcting for frictional effects.