Effect of Gravel on Unconsolidated Sands

Abstract

The important factors in any study of the screening of sand with a gravelenvelope, as applied to use in oil wells, are:gravel size upon sand migrationsand size and shapegravel size and shaperatio of gravel size tosand sizeflow velocityviscosity of the fluid. Perhaps the first extensive investigation of the function of gravel inscreening sand was performed by Coberly and Wagner,' from which the followingconclusion was drawn: "The diameter of the largest grains in a singleclassified sand suitable for gravel packing is approximately 13 times the grainsize at the ten-percentile analysis of the formation sample." This appliesto sands classified by Tyler screens and is for gravel grains of the maximumsize that could be used to form a stable bridge. Field practice has shown thatthe screening effect of a given gravel for a given sand apparently varies withthe flow velocity. 2 Also, it has been assumed by some operators that thethickness of the walls of the gravel envelope surrounding the well has littleeffect upon the screening process. It is thought by others that a narrowenvelope of gravel might form an ineffective screen. Thus the purposes of thiswork are:To investigate the effect of gravel-sand size ratio upon screening actionunder the condition of varying flow velocities.To investigate the effect upon screening action of varying the thickness ofthe gravel screen.To investigate sand migration in the gravel screen and the effect of flowvelocity and gravel size upon sand migration. Description of Apparatus In an attempt to reproduce field conditions for the purposes of thisinvestigation, a flow tube was constructed, according to the manner of Uren, torepresent a radial drainage segment of an oil sand. To arrive at the dimensions of the flow tube, a radial drainage segment ofrectangular cross section was assumed to define an area of 2 sq. in. on theface of a well 6 in. in diameter. From this the dimensions of the segment werecalculated out to a distance of 5 ft. from the face of the well. By computingcross-sectional areas at a number of sections along the rectangular segment andtransforming these areas into the circular form, a drainage cone was obtainedhaving everywhere the same cross-sectional area as the drainage element ofrectangular cross section. To construct the apparatus, two sections of pipe of 7 5/8 in. inside diameter, one 13 in. and the other 4 ft. long, were joined by a bolted flange. A mandrelhaving the dimensions of the drainage cone was centered in the pipe and a neatcement mixture was cast around it. A protective gasket was placed between thetwo sections of pipe so that the cement body could be separated at that point.When the mandrel was withdrawn a drainage cone was obtained A round slotted screen, which could be screwed up against the end of thedrainage cone, held the gravel in place. T.P. 1195