THE important factors in any study of 3. To investigate sand migration in the the screening of sand with a gravel enve-gravel screen and the effect of flow velocity lope, as applied to use in oil wells, are: (1) and gravel size upon sand migration. sand size and shape, (2) gravel size and shape, (3) ratio of gravel size to sand size, (4) flow velocity, (5) viscosity of the fluid. Perhaps the first extensive investigation of the function of gravel in screening sand was performed by Coberly and

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... gner,l from which the following conclusion was drawn: "The diameter of the largest grains in a single classified sand suitable for gravel packing is approximately 13 times the grain size at the ten-percentile analysis of the formation sample." This applies to sands classified by Tyler screens and is for gravel grains of the maximum size that could be used to form a stable bridge. Field practice has shown that the screening effect of a given gravel for a given sand apparently varies with the flow velocity. 2 Also, it has been assumed by some operators that the thickness of the walls of the gravel envelope surrounding the well has little effect upon the screening process. It is thought by others that a narrow envelope of gravel might form an ineffective screen. Thus the purposes of this work are: 1. To investigate the effect of gravelsand size ratio upon screening action under the condition of varying flow velocities. 2. To investigate the effect upon screening action of varying the thickness of the gravel screen.