Sand and Gravel - A Study of Their Permeabilities
Authors: Sparlin Derry D. 
Published: December 2007

This paper was prepared for the Society of Petroleum Engineers of AIME Symposium on Formation Damage Control, to be held in New Orleans, La., Feb. 7–8, 1974. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon requested to the Editor of the appropriate journal, provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
An extensive study of the permeabilities of gravels and mixtures of sand and gravels has been completed. The measurements were made in a 6-foot section of 7-inch diameter casing on 1/2-cubic foot samples of gravel. Permeabilities and sieve analyses of common Permeabilities and sieve analyses of common sizes of commercially available gravel were measured and found to be far from ideal.
Permeability measurements and sieve analyses were also made on many mixtures of sand and different gravel sizes. The detrimental effect of mixing sand and gravel on production capacities is shown by the production capacities is shown by the permeability measurements and by calculations permeability measurements and by calculations relating the data to gravel packed wells.

Introduction
Both field experience and laboratory tests have shown that production can be severely reduced by damaged gravel packs. Damage can occur either during placement or after the well is put on production. It can be prevented by planning the gravel packing operations to insure that the gravel retains its high permeability after the well is on production. This paper shows how much production. This paper shows how much damage can occur from allowing the gravel to mix with formation sand or from using poorly sieved gravels. poorly sieved gravels.

BACKGROUND
Layers of formation sand will usually have widely varying permeabilities with horizontal permeability being greater than vertical permeability. That is because most sands were initially deposited in flowing water over a horizontal plane such as a river bed or delta; and as flow rates changed, the size of sands deposited changed. Cores show that there are commonly differences in permeability as large as tenfold from one permeability as large as tenfold from one sand layer to the next. A core with a sand of 200 md permeability might have sand with 20 md or 2000 md only 1 foot away.
As the unconsolidated formation sand layers mix together, it is no wonder that production declines. When the natural production declines. When the natural arrangement of sand grains is disturbed, it is more difficult to flow through that sand.