Selection of Filter Pack and Slot Size

Published On: March 19, 2024By Categories: Drilling, Groundwater & Wells

It is critical to design a well from the formation into the well.

By Thom Hanna, PG

When designing a well, people often ask, “What slot size do I need if I want a certain amount of water that I need to pump from a well?”

Figure 1. Washed samples for logging and representative sample for sieve analysis.

There is a general feeling on design consideration that you should maintain flow velocity through the screen at 0.1 ft/sec, but this is not the determining factor in screen design. While we do not want to design a well that restricts the flow to the well or is difficult to develop, it is the opposite of this approach that needs to be taken. We need to start to design the well from the formation into the well.

The reality is the only purpose the screen has is to keep the formation or filter pack in place; the screen itself is not a filter in that sense. One of the goals in well design is to have the largest percentage of open area of the screen to allow proper well development. If you have ever tried to develop a screen with a low percentage of open area, you know how long it takes and how difficult it can be.

In this column, we will discuss how to design a well screen and filter pack for a typical completion in an unconsolidated formation.

There are common types of constructions. The first is called a naturally developed well and the second is an artificially filter packed well. In either type of completion, we want to create what I call a zone of enhanced hydraulic conductivity or permeability near the well screen.

Think about it: There is the same volume of water moving towards the well at 100 feet from the well as there is at a few inches from the well screen. Because the flow of water accelerates as it approaches the well, we want to design a filter pack that will prevent fines from being pumped from the well and allow the water moving into the well to flow under laminar or non-turbulent conditions to have an efficient well.

It only takes a few grains of filter pack to create a filter pack, but that will not result in a zone of enhanced hydraulic conductivity and would be difficult to place in the annulus of a well.

Representative Sample and Sieve Analysis

Figure 2. Typical sieve groups for sieving unconsolidated sediments (Sterrett 2007).

The first step is to collect a representative sample of the formation for which we will be designing the filter pack and screen. Both the slot size of the screen and the particle sizes of the filter pack are based on results of a sieve analysis conducted on sediment samples collected from the aquifer.

The quality of the sample will be dependent on the type of drill rig, the mud program (if using drilling mud), the nature of the formation, and the biggest variable of all—the person collecting the sample.

The most important fraction of the formation that needs to be collected is the finer fraction, which is the hardest portion of the sample to collect. No matter what the quality of the sample, we need to understand how the sample came from the aquifer at depth to the lab for sieve analysis—including the drilling process, how the sample was collected, and how representative the sample is of the aquifer. Well design decisions will be made, and we must account for what we might not know. Yes, it is important to know what you do not know!

The samples collected for sieve analysis should be representative of the formation across which the well will be screened. If the sample is too large for the sieves (more than one pound or 450 grams), it is best to reduce the size of the sample with a splitter or by quartering the sample.

The sample should be washed as necessary in the field to remove drilling fluids from the sample and placed in a well-marked sample bag for shipment (Figure 1). Typically, they are sent to a lab or a screen manufacturer that can provide this type of work or done by the drilling contractor.

In aquifers that are dominated by fine materials and the drilling fluids have bentonite, it can be difficult to collect a good sample. In these cases, you can collect the sample in a 5-gallon bucket and use a flocculent to decant the drilling fluid so you have a more representative sample.

Once the sample arrives at the lab, it is dried, and the fines are crushed to segregate them. Grain-size distribution is determined by passing the sediment sample through a series of stacked brass or stainless steel sieves (Figure 2), with the larger sieves on the top followed by finer mesh with a pan on the bottom that will collect the remaining fines.

Before sieving begins, the total sample weight must be recorded. The sample is then poured onto the top sieve and the set of sieves is shaken vigorously in a device that vibrates both vertically and horizontally for at least five minutes. The samples are typically sieved normally for 5 to 10 minutes per sample.

Figure 3. Samples being weighed in the lab.

After the shaking is completed, the contents of each sieve are weighed (Figure 3). Always be sure to dislodge any portion of the sample that is caught in the mesh. The weight of the material retained in the second sieve is then added to that of the material already in the weighing pan and the combined weight is recorded.

Each sieve then is emptied successively, and the cumulative weight of the particles retained on each sieve then is plotted as a percentage of the total sample weight against grain size in inches or in millimeters. From this curve, the degree of sorting and the average grain size are calculated (Figure 4).

Well Completion Types

Let’s talk about the two general types of well completions: natural developed and artificial filter packed.

Natural developed wells use the formation adjacent to the well to establish a filter pack by selectively removing the finer fraction of material close to the screen and leaving a coarser filter pack adjacent to the screen.

In wells with an artificial filter pack, an engineered filter pack is placed in the annulus between the borehole and screen. The type of completion is often dictated by the geology and drilling method.

Naturally Developed Wells

Figure 4. Slot opening of a 0.050-inch is selected from this grain-size distribution curve for a naturally developed well depending on the designer’s understanding of the well and aquifer.

In a naturally developed well, a portion of the finer formation material near the borehole is removed through the screen during development. This results in a zone of enhanced hydraulic conductivity extending outward from the screen. The increased porosity and hydraulic conductivity reduce near-well drawdown during pumping.

For non-homogeneous sediments, when the groundwater is not particularly corrosive and sample reliability is good, the typical approach is to select a slot which passes 60% of the material and retains 40%.

Using an even smaller slot size, retaining 50% is a wise choice in corrosive water and with low-carbon steel screens because slot enlargement of only a few thousandths of an inch could allow the well to pump sand. Slot enlargement from corrosion generally is not a problem for stainless steel or PVC screens.

Slotted PVC and steel or louvered screens can be difficult to develop due to the low percentage of open area. Other situations dictating a conservative slot opening (retaining 40% to 50%) can be used if the formation consists of calcium carbonate, which dissolves readily with acid treatments, and if the aquifer is thin and overlaid by fine-grained loose material and formation is well sorted.

In the example presented in Figure 4, the formation is plotted on a graph where on the X-axis is the cumulative percent retained and on the Y-axis is the grain size in thousandths of an inch. In a typical application where the sample is of good quality, you go to the 40% retained line and move to the right to where it crosses the sieve data. This is the point for selecting the slot size; for this example it is a 0.050-inch slot.

Often well designers are reluctant to open up the slot size and allow 60% of the finer material to enter the well. Allowing 60% of the formation into the well during development at times, depending on the formation, can speed up the development process in a formation that has a significant variation of grain sizes.

For example, in incrusting water, longer service life results before plugging reduces the well’s yield. Larger slot size permits extending the permeable zone around the screen, which generally increases specific capacity and efficiency, thereby lowering operating costs.

Artificial Filter Packed Wells

Figure 5. Comparison of glass bead filter pack and silica sand filter pack.

Artificial filter packed wells initially were used because of their low cost and the need to reduce sand pumping by using wide-slot screen devices made from torch-cut perforations, louvers, mill slots, and punched openings.

In filter packed wells, the zone immediately around the well screen is replaced with a graded material consisting of sand, gravel, or glass bead filter pack. A screen slot size is then selected to retain 90% or more of the filter pack. Most commercial filter packs have uniformity coefficients of approximately 2.5 or smaller.

Desired characteristics of filter pack materials are presented in Table 1. Glass bead filter packs such as Shur-PakTM and Sili BeadsTM have uniformity of 1.5 (Figure 5). The uniformity coefficient is defined as the 40% retained size of the sediment divided by the 90% retained size (the lower the value, the more uniform).

Filter packing can be advantageous in situations where:

  • Sediments are highly uniform and fine grained.
  • Sediments are highly laminated, making it difficult to determine precise layer locations. The grading of the filter pack should be based on the grain size of the finest layer to be screened.
  • Materials to be used in construction must be on site before drilling begins.
  • Small slot size required for natural development limits the transmitting capacity of the screen; so the desired yield could not be obtained.
  • The water is extremely incrusting.
  • Poorly cemented sandstone aquifers provide little or no lateral support for the screen.

Figure 6. Grain-size curves for aquifer sand and corresponding curve for properly selected filter pack material (Sterrett 2007).

Installation of a filter pack and screen can reduce the specific capacity of a well in an open borehole completion, but the reduction in yield is usually preferable to a sand-pumping well.

Filter pack material should consist of clean, well-rounded grains of a uniform size, composed mostly of silica-containing particles. Table 1 lists the desirable physical and chemical characteristics for a filter pack and the advantages of using these materials.

The use of glass beads reduces many of the uncertainties of using silica sand filter packs. All filter pack material that is delivered to the well site should be protected from contamination at the well site prior to installation. Changes in filter pack materials being mined and processed can result in changes in filter consistency. Water well professionals should contact their filter pack suppliers for current material information.

Steps in Designing a Filter Pack

  1. Choose the layers to be screened and construct sieve analysis curves for these formations.
  2. Base the filter pack grading on an analysis of the layer composed of the finest material. Figure 6 shows the grading of two samples; the finest material lies between 75 feet and 90 feet.
  3. Multiply the 70% retained size of the sediment by a factor of between 3 and 8 (see the selection criteria below). Place the result of the multiplication on the graph as the 70% size of the filter material.
    a. Use a 3 to 6 multiplier if the formation is uniform and the 40% retained size is 0.010-inch or smaller.
    b. Use a 6 to 8 multiplier for semi-consolidated or unconsolidated aquifers when formation sediment is highly non-uniform and includes silt or thin clay stringers to aid in complete development.
    c. Using multipliers greater than 8 could result in creating a sand-pumping well.
  4. In Figure 5, 0.005-inch is the 70% size of the sand between 75 feet and 90 feet. Multiplying by 5 produces a 70% size of the filter material as 0.025-inch. This is the first point on the curve that represents the grading for the filter pack material.
  5. Through this initial point, draw a smooth curve representing material with a uniformity coefficient of approximately 2.5 or less. (In Figure 5, the solid line curve has a uniformity coefficient of 1.8, the dashed-line curve a coefficient of 2.5.) Always draw the filter pack curve as uniform as practical. Thus, in this example the material indicated by the solid line curve is more desirable.
  6. As a final step, select a screen slot size that will retain 90% or more of the filter pack material. In the example, the correct slot size is 0.018-inch. In completions using glass beads, it is best to retain 100% of the filter pack as the glass beads are uniform.
  7. The pack should extend well above the screen to compensate for any settling occurring during development. Using a caliper log could reveal the presence of washouts in the borehole, necessitating additional filter pack. It is good practice to have extra filter pack on the site, especially if the size and stability of the borehole is questionable.

All too often water well professionals do not do enough work to provide a good well completion that is the most economical for the well owner. In naturally developed wells, the slot size will be too small, resulting in increased development time and a well that is not as efficient as it could have been. In filter packed wells, the same combination of filter pack and slot size will be used as a standard construction.

I have head well designers say, “I always use a 10-20 filter pack and a 0.030-inch slot screen in all my wells and I never have a problem.” Yes, these folks typically do not have sand-pumping issues, but quite often the wells could have been designed with larger filter pack and slot size.

The end results are wells that are not as efficient as they could be and have more maintenance issues over the life of the well. It is our job as water well professionals to provide the best well for the well owner. That means an efficient well that is sand-free.

Thickness of Filter Pack

Theoretically, a filter pack need be only two or three grains thick to retain and control a formation. Laboratory tests conducted by Johnson Screens have demonstrated that a thickness of less than 0.5-inch of properly sized material successfully retains formation particles, regardless of the velocity of the water passing through the filter pack.

In practice, it is impossible to place filter pack that is only 0.5-inch thick and expect to completely surround the well screen. To ensure that filter material surrounds the entire screen, the minimum design annulus should be at least 3 inches.

Filter pack thickness does little to reduce the pumping of sand because the controlling factor is the grain-size ratio of the pack material to the formation material. Ideally, filter packs should not exceed a thickness of 5 inches because a pack that is too thick makes development difficult. The filter pack should be 2 inches to 5 inches thick, with 3 inches being optimal.

The use of centering guides (centralizers) is important to ensure complete and even distribution of filter pack along the screen length. Fine-grained filter packs also tend to be more difficult to place around well screens than are coarser-grained materials, which settle more readily.

Summary

If the proper procedures are followed in designing a well in unconsolidated materials, we can construct wells that are a good value for the well owner. I have helped several water well professionals who are not as familiar with the design of wells in finer-grained unconsolidated aquifers and they are typically surprised at how much water you can get from thin, fine-grained aquifers if the well is designed properly.

Reference

Sterrett, R.J. 2007. Groundwater & Wells, Third Edition. Johnson Screens: New Brighton, Minnesota.

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Thomas M. Hanna, PG, is a technical director of water well products/hydrogeologist for Johnson Screens where he works in areas of well design, development, and well rehabilitation. He is a registered professional geologist in Arizona, Kentucky, and Wyoming and has worked for several groundwater consulting firms. Hanna can be reached at thom.hanna@johnsonscreens.com.

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