Movement of fine grains in the vicinity of well screens

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Abstract
A great number of laboratory experiments have been performed to study the flow generated suffusion process in soil samples supported by perforated screens. Both parallel and radial flow were tested (a typical example of the latter is a well-screen supporting the material of an aquifer). The soil samples tested were theoretically divided into two parts: a coarse grained fraction which behaves as a self-supporting skeleton, and a fine-grained fraction filling the pores of the skeleton. Experimental results showed that the process of soil suffosion occurs with increasing flow velocity. At low velocities the fine grains maintain their original position and the flow can be described by Darcy's law. Reaching a critical velocity vcr(1) the fine grains start to move, partially plugging the pores of the skeleton, while resistance of the sample is increasing. Reaching a second critical flow velocity vcr(2) results in the development of microchannels, through which fine grains may move out from the layer. The quantity of these fine grains depends on the composition of the soil sample, the flow rate and the slot-diameter of the well-screen. On the basis of the experimental results empirical formulae have been developed for calculating the critical velocities vcr(1) and vcr(2), and the optimum slot sizes of the screen.

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