Gravel pack design

A gravel pack is simply a downhole filter designed to prevent the production of unwanted formation sand. The formation sand is held in place by properly sized gravel pack sand that, in turn, is held in place with a properly-sized screen. To determine what size gravel-pack sand is required, samples of the formation sand must be evaluated to determine the median grain size diameter and grain size distribution. The quality of the sand used is as important as the proper sizing. The American Petroleum Institute (API) has set forth the minimum specifications desirable for gravel-pack sand in API RP 58, Testing Sand Used in Gravel-Packing Operations.

Formation sand sampling

The first step in gravel-pack design is to obtain a representative sample of the formation. Failure to analyze a representative sample can lead to gravel packs that fail because of plugging or the production of sand. Because the formation sand size is so important, the technique used to obtain a formation sample requires attention. With knowledge of the different sampling techniques, compensation can be made in the gravel-pack sand size selection, if necessary.

Produced samples

A produced sample of the formation sand is easily contaminated before it reaches the surface. Although such a sample can be analyzed and used for the gravel-pack sand size determination, produced samples will probably have a smaller median grain size than the median of actual formation sand. The well’s flow rate, produced fluid characteristics, and completion tubular design influence whether a particular size is produced to surface or settles to the bottom of the well. In many cases, the larger sand grains settle, so a sample that is produced to the surface has a higher proportion of the smaller-size sand grains. This is the reason that the surface sample is not a good representation of the various sizes of formation sand. Also, the transport of sand grains, through the production tubing and surface flow lines, may result in broken sand grains, causing the presence of more fine and smaller grains.

Bailed samples

Samples collected from the bottom of a well using wireline bailers are also relatively easy to obtain, but these too are probably unrepresentative of the size of the actual formation sand. Bailed samples are generally biased to the larger-size sand grains, assuming that more of the smaller grains are produced to surface. Bailed samples also may be misleading in terms of grain size distribution. When closing the well in to obtain a sample, the larger sand grains settle to the bottom of the well first, and the smaller sand grains fall on top of the larger ones. This results in a sorting of the formation sand grains into a sample that is not representative the formation sand. The use of bailed samples may result in the design of larger than required gravel-pack sand that can result in sand production (small formation particles passing through the gravel pack) or plugging of the gravel pack (small formation particles filling the spaces between the gravel-pack sand grains).

Sidewall core samples

Sidewall core samples are obtained by shooting hollow projectiles from a gun lowered into the well on an electric line to the desired depth. The projectiles remain attached to the gun with steel cables, so that when the gun is pulled from the well, the projectiles are retrieved with a small formation sample inside. Taking sidewall core samples is generally included in the evaluation stages of wells in unconsolidated formations; these are the most widely used sample types for gravel-pack sand design. Although more representative than produced or bailed samples, sidewall core samples can also give imprecise results because the volume in each sidewall sample is small. When the projectiles strike the face of the formation, localized crushing of the sand grains occurs, producing broken sand grains and generating more fine particles. The core sample also contains drilling mud solids that can be mistaken for formation material. Experienced lab analysts can separate the effects of crushing and mud solids prior to evaluating the sample, thus improving the quality of the results.

Conventional core samples

The most representative formation sample is obtained from conventional cores. In the case of unconsolidated formations, rubber sleeve conventional cores may be required to assure sample recovery. Although conventional cores are the most desirable formation sample, they are not readily available in many wells because of the cost of coring operations. Coring in sand-producing formations is also plagued with poor recovery. If available, small plugs can be taken under controlled circumstances at various sections of the core for a complete and accurate median formation grain size and grain-size distribution determination.

Other samples

From time to time, operators have no formation sample. In this event, rely on any of the samples from offset wells. If the formation of interest has gravel-pack completions in nearby fields, rely on these. If there is still no information, select a relatively small gravel that will control most formation sand, or consult an expert.

Sieve analysis

A sieve analysis is a laboratory routine performed on a formation sand sample for the selection of the proper-sized gravel-pack sand. A sieve analysis consists of placing a formation sample at the top of a series of screens that have progressively smaller mesh sizes downwards in the sieve stack. After placing the sieve stack in a vibrating machine, the sand grains in the sample will fall through the screens until encountering a screen through which certain grain sizes cannot pass because the openings in the screen are too small. By weighing the screens before and after sieving, the weight of formation sample, retained by each size screen, can be determined. The cumulative weight percent of each sample retained can be plotted as a comparison of screen mesh size on semilog coordinates to obtain a sand size-distribution plot, as shown in Fig. 1. Reading the graph at the 50% cumulative weight gives the median formation grain size diameter. This grain size, often referred to as d50, is the basis of gravel-pack sand size-selection procedures. Table 1 provides a reference for mesh size vs. sieve opening.

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  Fig. 1—Sand size distribution plot from sieve analysis.

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  Table 1

If possible, a sample should be taken every 2 to 3 ft within the formation, or at least at every lithology change. The minimum size of the formation sample required for sieve analysis is 15 cm³. Sieving can be performed either wet or dry. In dry sieving (the most common technique), the sample is prepared by removing the fines (i.e., clays) and drying the sample in an oven. If necessary, the sample is ground with a mortar and pestle to ensure individual grains are sieved rather than conglomerated grains. The sample is then placed in the sieving apparatus that uses mechanical vibration to assist the particles in moving through and on to the various mesh screens. Wet sieving is used when the formation sample has extremely small grain sizes. In wet sieving, water is poured over the sample while sieving to ensure that the particles do not cling together.

Gravel pack sand sizing

There have been several published techniques for selecting a gravel-pack sand size to control the production of formation sand. The most widely used sizing criterion1 provides sand control when the median grain size of the gravel-pack sand, D50 , is no more than six times larger than the median grain size of the formation sand, d50 . The upper case D refers to the gravel, while the lower case refers to the formation sand. The basis for this relationship was a series of core flow experiments in which half the core consisted of gravel-pack sand and the other half was formation sand. The ratio of median grain size of the gravel-pack sand and median grain size of the formation sand was changed over a range from 2 to 10 to determine when optimum sand control was achieved.

The experimental procedure consisted of measuring the pack permeability with each change in gravel size and comparing it to the initial permeability. If the final permeability was the same as the initial permeability, it was concluded that effective sand control was achieved with no adverse productivity effects. If the final permeability was less than the initial permeability, the formation sand was invading and plugging the gravel-pack sand. In this situation, sand control may be achieved, but at the expense of well productivity.

Fig. 2 illustrates the results of core flow experiments for a particular gravel/sand combination. As shown in the plot, the permeability of the pack increases up to a median gravel/sand size ratio of 6 but decreases as the ratio increases further. The permeability decreases to a minimum as a 10:12 ratio is reached; then, it increases. The explanation for this behavior is that the permeability increases as the gravel/sand size ratio increases up to a ratio of about 6, which reflects the increasing permeability of the larger gravel (i.e., at a gravel/sand ratio of one, the gravel is the same size as the formation sand). At a gravel/sand size ratio of 6, the formation sand grains bridge on, rather than into the pore structure of the gravel, which is the correct gravel size that provides the highest permeability. However, as the gravel size becomes larger and the ratio increases, the formation begins to bridge within the pore structure of the gravel, thereby decreasing the pack permeability. At a ratio of 10:12, the formation sand has moved well into the pores, decreasing the permeability substantially. As the gravel becomes larger, a reversal occurs because now the formation sand can move both into and through the pore structure of the gravel. At ratios in excess of 15, the formation sand can flow through the gravel with ease. As Fig. 2 indicates, at gravel/sand ratios less than 10:12, there is sand control, whereas at ratios larger than 12, there is no sand control.

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  Fig. 2—Effect of gravel-sand ratio on sand control permeability.

In practice, the proper gravel-pack sand size is selected by multiplying the median size of the formation sand by 4 to 8 to achieve a gravel-pack sand size range, in which the average is six times larger than the median grain size of the formation sand. Hence, the gravel pack is designed to control the load-bearing material; no attempt is made to control formation fines that make up less 2 to 3% of the formation. This calculated gravel-pack sand size range is compared to the available commercial grades of gravel-pack sand. Select the available gravel-pack sand that matches the calculated gravel-pack size range. In the event that the calculated gravel-pack sand size range falls between the size ranges of commercially available gravel-pack sand, select the smaller gravel-pack sand. Table 2 contains information on commercially available gravel-pack sand sizes.

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  Table 2

Note that this technique is based solely on the median grain size of the formation sand with no consideration given to the range of sand grain diameters or degree of sorting present in the formation. The sieve analysis plot, discussed earlier, can be used to obtain the degree of sorting in a particular formation sample. A near vertical sieve analysis plot represents good sorting (most of the formation sand is in a very narrow size range) vs. a highly sloping plot, which indicates poorer sorting as illustrated by curves “A” and “D,” respectively, in Fig. 1. A sorting factor, or uniformity coefficient, can be calculated as

....................(1)

where

• C_μ = sorting factor or uniformity coefficient,
• d₄₀ = grain size at the 40% cumulative level from sieve analysis plot,
• d₉₀ = grain size at the 90% cumulative level from sieve analysis plot.

If C_μ is less than 3, the sand is considered well sorted (uniform); from 3 to 5, it is nonuniform, and if greater than 5, it is highly nonuniform.

Gravel pack sand

The productivity of a gravel-packed well depends on the permeability of the gravel-pack sand and how it is placed. To ensure maximum well productivity, one should use high quality gravel-pack sand. API RP 58, Testing Sand Used in Gravel Packing Operations, establishes rigid specifications for acceptable properties of sands used for gravel packing. These specifications focus on ensuring the maximum permeability and longevity of the sand under typical well production and treatment conditions. The specifications define minimum acceptable standards for:

• Size and shape of the grains
• Amount of fines and impurities
• Acid solubility
• Crush resistance

Only a few naturally occurring sands are capable of meeting the API specifications without excessive processing. These sands are characterized by their high quartz content and consistency in grain size. Table 3 gives the permeability of common gravel-pack sand sizes conforming to API RP 58 specifications (data from Sparlin, Gurley, and Cocales).

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  Table 3

Once the sieve analysis has been performed and plotted, the remainder of the gravel-pack sizing can be performed graphically. The gravel-pack sand size is determined by multiplying the median formation grain size by 6. This value is the median gravel grain size. With a straight edge, construct the gravel curve so that its uniformity coefficient, C_μ, is 1.5. The actual gravel size can be determined by the intercept of gravel curve with the 0 and 100 percentile values. Select to the nearest standard gravel size. The screen slot width is typically half the smallest gravel size selected but should not exceed 70% of the smallest grain diameter. While it may appear that this design is conservative, it will not restrict productivity and allows for variances in screen tolerances. The diameter of the screen should allow for at least 0.75-in. clearance from the casing inside diameter (ID). Fig. 3 is an example gravel-pack design.

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  Fig. 3—.Effect of gravel-sand size ratio on sand control and productivity.

Gravel pack sand substitutes

Although naturally occurring quartz sand is the most common gravel-pack material, many alternatives exist. These include:

• Resin-coated sand
• Garnet
• Glass beads
• Aluminum oxides

Each of these materials offers specific properties that are beneficial for given applications and well conditions. The cost of the materials ranges from 2 to 3 times the price of common quartz sand.

Nomenclature

Cμ	=	sorting factor or uniformity coefficient
d40	=	formation sand diameter, 40 percentile
d90	=	formation sand diameter, 90 percentile

Noteworthy papers in OnePetro

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External links

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See also

Gravel pack equipment and tools

Gravel placement techniques

Slotted liners and wire wrapped screens

Prepacking perforations with gravel

Well preparation for gravel packing

Openhole gravel packing

History of gravel packs

Sand control techniques

PEH:Sand_Control