Improper Formation Sampling Leads to Improper Selection of Gravel Size
Authors: Maly G.P. , Krueger R.F. 
Published: December 2007

The only sure way of determining the proper gravel size for 100-percent sand control is from full-interval coring. The next best alternative is to select the smallest practical gravel size.

Introduction
The success of a gravel-packing operation that is, the degree of sand control and the maintenance of high well productivity is affected by many factors related to gravel size and quality and the nature of the completion operations. All the factors are important and interrelated, and failure to pay close attention to any one of them may lead to poor results, even if all the others are optimized. We cannot emphasize this strongly enough. For example, we have found in field treatments that supposedly "clean" placement fluids may sometimes severely plug the placement fluids may sometimes severely plug the gravel pack or the formation sand or both. Such plugging, of course, negates the attention paid to plugging, of course, negates the attention paid to all other factors.
We will discuss in this article only the effect of formation sampling on the determination of proper gravel size, We will show also that some practices for determining formation-sand size may offer a misleading basis for selecting gravel size.
From many studies of field results we have concluded that most gravel-pack completions are only partially effective. But gravel-pack plugging and partially effective. But gravel-pack plugging and the associated loss of well productivity is usually unrecognized. One of the many contributing factors is use of the improper gravel size. Many operators arbitrarily select a large gravel (for example, 6–9 mesh) because the particular size was beard to be satisfactory in a neighboring field. Others are content to determine gravel size on the basis of a few sidewall cores, a composite sample thereof, or produced sand. We will show that these practices can be dangerously wrong because they can lead to improper selection of gravel size and this in turn leads to gravel-pack plugging (lowered well productivity) or liner erosion plugging (lowered well productivity) or liner erosion (shortened pack life) or both. The net result is often early pack failure and an expensive workover.

Determination of Gravel Size
It is common practice to select the gravel size required for formation-sand control from a screen analysis of formation core samples. This practice originated in Coberly's laboratory studies. Coberly's investigations led to an extremely important concept: as flow conditions change, the stable sand bridges that are necessary for sand control may temporarily break down, only to re-form "almost instantly." He developed the idea of using the 10-percentile point of a formation sand screen analysis to establish an empirical gravel:sand size ratio that would provide a stable bridge. We have designated these ratios, or gravel design factors, by the letter F in this paper.
Studies by several investigators have led to a wide range of gravel-size recommendations, ranging from 2 to 13 times the 10-percentile sand size. In some areas, large gravel sizes (for example, 6–9 and 8–12 mesh) are arbitrarily used without analysis of formation grain size.
JPT
P. 1403