Thin-Section and Permeability Studies Call For Smaller Gravels in Gravel Packing
Authors: Gulati Mohinder S. , Maly George P. 
Published: December 2007

The paper discusses the results of an experimental investigation designed to find the most effective gravel-to-sand size ratio for an effective gravel pack. It was found that a gravel-to-sand size ratio of less than 6, and preferably 4, gives a stable pack. Angular gravel and a uniform formation sand promote the pack stability. The experimental results were confirmed by visual examinations of thin sections.

Introduction
Two general methods of sand control, bridging and sand consolidation, have been extensively used in oil and gas wells. Bridging of sand grains may be accomplished by the use of screens, gravel packs, or plastic-coated gravel. Gravel packing is one of the more plastic-coated gravel. Gravel packing is one of the more popular methods of achieving sand control for relatively popular methods of achieving sand control for relatively thick sands in unconsolidated sand formations. There are a number of criteria that must be satisfied for a successful gravel pack. Some of these areratio of gravel size to formation-sand size;gravel quality (angularity, shape, strength, size degradability, contamination, and solubility);type and cleanliness of fluids used during underreaming and gravel placement;hole cleanout;gravel-to-fluid ratio;slot size and type of liner or screen; andover-all job planning, supervision, and quality control.
Our experimental work was designed to answer the following questions.What is the extent of gravel-permeability reduction as a function of gravel-sand size ratio?What is the effect of gravel angularity on bridging?Under what conditions does and badge on and within the gravel?What is the effect of sand-grain uniformity, defined by the uniformity coefficient of the sand, on the gravel permeability?
An accurate screen analysis of producing formations is necessary for correct gravel sizing. The Tyler or U. S. standard screen scale is used for determining the range of formation-sand sizes. A formation-sand sample is washed, dried, weighed, separated, and passed through various-sized screens. The screen sizes are designated in Tyler or U. S. standard numbers for ease in classification. For example, a 6 Tyler screen has 0.131-in. openings. The material is progressively passed through the largest to the smallest screen. The sand on each screen is weighed and a plot is constructed showing cumulative percentage retained vs grain size. percentage retained vs grain size. Sand size is generally characterized by the 10-percentile point of the sieve analysis, or the size of sand particle at its coarsest 10 percent by weight. This does not take into consideration the size of the remaining finer 90 percent when, in fact, the size of these may be a controlling factor. The amount of fines can be defined by introducing another parameter, called the uniformity coefficient or C-ratio, defined as C = D(40)/D(90), where D(40), is the grain diameter at the 40-percentile point and D(90), is the grain diameter at the 90-percentile point. A C-ratio less than 3 describes a uniform or well-sorted sand, while a C-ratio greater than 5 describes a nonuniform or poorly sorted sand. An important consideration in designing a successful gravel pack is to determine the correct gravel-to-sand size ratio.
JPT
P. 107