An Introduction to Engineered Glass Beads as a Filter Pack Media

The fundamentals and beyond.

By Gary M. Gin, RG

As the water well industry continues to advance well design methods and construction approaches, it must
look beyond traditional natural sources for filter pack media. For more than a decade, engineered glass beads have been utilized as a filter pack media with improved well hydraulics, resiliency, and longevity.

This article focuses on the physical properties of glass beads and the hydraulic improvements at the Phoenix, Arizona, Cave Creek Aquifer Storage and Recovery (ASR) Well No. 1, which is where glass beads were first introduced as a filter pack media for recharge operations.

Reduced injection rates and recharge durations can occur in ASR wells when clogging due to the mobility and reorientation of fine-grained particulates within the aquifer and filter pack media filling pore spaces. The pore-size diameter (aperture) and interconnected nature of pore spaces within the filter pack is one of the primary pathways for water to flow through the media.

To mitigate high-frequency clogging observed after successive recharge cycles, ASR wells are pumped to remove these reoriented particulates, then returned into recharge mode. Well hydraulics and the type of filter pack medium installed can play important roles during ASR backwashing events. The goal of backwashing is to restore well efficiency, injection rate, and duration by removing clogging particulates with the least water volume pumped to waste.

Effective backwashing results in lowering operational and maintenance costs. The use of engineered glass beads can yield substantial hydraulic improvements by removing clogging particulates.

The Purpose of Filter Pack

First, let’s examine the purpose of filter pack media installed between the borehole wall and well screen. According to Groundwater & Wells, Second Edition (Driscoll 1986), filter pack is installed to:

• Retain most of the coarser-grained formational material behind the well screen
• Reduce the amount of fine-to-medium-grained sand entering the well (sand pumping)
• Improve well hydraulics.

According to Basic Principles of Water Well Design (Johnson 1963), filter pack materials should ideally be clean, siliceous, in composition, well rounded, smooth, and uniform in grain-size. These physical properties tend to increase the porosity, and hence, the permeability of the filter pack.

Traditional filter pack materials tend to be quartz and feldspar grains that are rounded by weathering processes. Grains should not be composed of angular, multimineral rock fragments, or chemically reactive materials like calcium carbonate.

Occasionally, wells are poorly built that pumping large volumes of sand will destroy pump components such as bowls and impellers (Figure 1). While designers are focused on optimizing well hydraulics by selecting the best coarse-grained filter pack, the appropriate filter pack media should not just exclude fine-grained sands, but the media should also limit fine-to-medium-grained formation sands being pumped into the well.

Generally, the pumping of sand is due to one or more factors: poor installation of filter pack, poor well screen design, poor selection of filter pack media, or a combination of these factors.

But what are our options for increased efficiency without increasing the pumping of sand, even if the well screen was properly designed and the filter pack was installed appropriately? Is there a filter pack that can yield better hydraulics and reduce the potential of pumping sand into the well?

Filter Pack Selection

Filter pack media selection is particularly critical for ASR wells since over time they tend to clog more frequently, resulting in declining injection rates and reduced duration of recharge periods.

To answer the question above prior to installing Cave Creek ASR Well No. 1, a series of tests were conducted by a geotechnical laboratory that measured the degree of roundness (sphericity) and collapse strength of glass beads (2.4–2.9 mm) and silica sand (6 × 9 mesh, 2.2–3.4 mm) (Figure 2 and Figure 3).

The study noted that the degree of roundness and sorting was higher in the glass beads versus the silica sand filter pack. The collapse strength of the glass beads was 7.4 times greater than naturally occurring silica sand with minor feldspar grains, which means glass beads can withstand more abrasion and maintain consistent pore space when stressed under dynamic hydraulic conditions.

Maintaining consistent pore space diameters for glass beads is important in sustaining consistent well hydraulics for both pumping and recharge operations. Beard and Weyl (1973) and Nagtegaal (1978) demonstrated the greater collapse strength and roundness of grains (such as glass beads) coupled with a higher degree of sorting results in a greater uniformity of pore spaces (higher porosity and permeability).

Both also demonstrated that with the well sorted, rounded, unconsolidated quartz-dominated sands, the resultant porosity and permeability will be higher than that of poorly sorted, angular media composed of lithic fragments.

Specific yield is a hydrologic physical property that is not typically considered in the design of filter pack media, but the concept of this hydraulic property can be used to compare different filter pack media.

Specific yield is a measurement of how water drains from sediments (Johnson 1967). From operational experience with glass beads, the narrow gradation of bead diameters coupled with higher degree of sphericity provides a specific yield similar to medium-to-coarse-grain sands.

Table 1 shows that medium to coarse sands have a higher specific yield than medium-to-coarse-grained gravels. Snieder (1987) demonstrated that there is a direct relationship between sorting and grain-size to porosity and permeability. The relationship between sorting and grain-size would infer the highest degree of sorting and larger grain-size would yield the highest porosity and permeability.

Resorting to larger grain-sizes and higher degree of sorting for filter pack media will be problematic in some cases because if the pore throat apertures between grains are too large, the chance of pumping formational sands increases. Instead, select the optimal filter pack media, then calculate the maximum pore throat aperture between grains to assess and ensure formational sands will not flow through the pore throat apertures (Figure 4).

These findings indicate a coarser-grained, poorly sorted, gravel filter pack may not always be preferred. We believe that selecting a filter pack with an optimal specific yield may unlock new opportunities in advancing well hydraulics and that coarser-grained gravels may have limited potential because there is a higher probability for finer-grained particulates to clog larger pore spaces and creating pathways for pumping sands.

Testing Hydraulics of Glass Beads vs. Silica Sand

Let’s explore whether these initial findings are valid and measurable within an ASR well. To test the difference in performance between these filter pack media, an ASR well was constructed with both glass beads (2.4–2.9 mm) and naturally occurring silica sand (6 × 9 mesh) (Figure 5).

Twenty (20)-foot-thick intervals of glass beads and silica sand were selected based on similar hydraulic conductivities of the aquifer (36 feet/day adjacent to the glass beads and 28 feet/day adjacent to the silica sand). Hydraulic conductivity of the aquifer was determined by conducting multiple slug injection tests during depth-specific zonal sampling in the open borehole (i.e., falling head tests).

The goal of the evaluation between the two media was to assess whether glass beads have better hydraulic properties than naturally occurring silica sand. Multiple spinner log analyses were conducted over the first year of recharge and recovery operations. The spinner logging tool measures the vertical changes in productivity in a cased well and measures variability in injection rates during recharge. In other words, how much water is flowing out of and into the glass beads and silica sand filter pack?

Figures 6 and 7 represent the results in specific capacity (gallons per minute per foot of drawdown [gpm/feet]) during recovery and recharge operations.

Conclusions

The benefits of utilizing engineered glass beads over natural sand filter packs are clear and the operational and maintenance cost savings are being recognized by water providers. The hydraulic and physical properties of glass beads such as roundness, sorting, and collapse strength yields stable porosity/permeability and limits the possibility of mechanical clogging.

The high collapse strength of glass beads is an unrecognized hydraulic benefit because when these beads are subjected to hydraulic stresses, the grain-on-grain contacts will not break down, retaining primary porosities (i.e., pore spaces) to be open and allow water to efficiently flow through the media.

Specific yield is a hydraulic physical property (i.e., surface tension and grain-on-grain contacts) that should be considered when designing filter pack media. As we advance our knowledge of glass beads as a filter pack, we should be reminded of the fundamentals that higher degrees of sorting, grain-size, and roundness will enhance and maintain consistent hydraulic performance for well systems.

The following conclusions were derived from the yearlong ASR well performance test:

1. During recharge, the specific capacity of the glass beads (Zone 3) was three to nine times higher than the
silica sand (Zone 2). This means that the injected water prefers to flow through the glass beads than the silica sand, which results in a higher volume of water flowing through the glass beads (Figure 6).

2. The performance of the silica sand (Zone 2) slowly degraded from 5.9 to 2.3 gpm/feet, despite backwashing efforts, which suggest pore spaces are either being clogged or reducing in diameter due to grain-on-grain abrasion processes. The glass beads (Zone 3) retain their initial specific capacity of 10 gpm/feet or better after backwashing efforts (Figure 6).

3. During recovery, the specific capacity of the glass beads (Zone 3) was about 1.8 to 2.4 gpm/feet higher than silica sand (Zone 2). This means that water was being pumped out of the aquifer at a higher rate through the glass beads. Higher specific capacity results in shallower pumping water levels, which extends the life of the pump equipment and lowers pumping and maintenance costs over time.

4. For both recharge and recovery operations over one year, the glass beads showed no decline in performance after backwashing events (unclogging events) over the testing period, indicating longevity and consistent well hydraulic performance.

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For those who manage wellfields and well recharge systems, these test results show that engineered glass beads are a viable material alternative to sand filter pack media and provide comparably better well efficiency. It can be inferred that they would provide tangible savings in the form of both operational and maintenance costs and program capital costs.

References

Beard, D.C., and Weyl, P.K. 1973. “Influence of Texture on Porosity and Permeability of Unconsolidated Sand.” The American Association of Petroleum Geologist Bulletin 57, No. 2: 349-369.

Driscoll, F. 1986. Groundwater & Wells, Second Edition. Johnson Screens: St. Paul, Minnesota.

Johnson, E.E. 1963. “Basic Principles of Water Well Design, Part 3.” Johnson Drillers Journal 35, no. 6: 4-8.

Johnson, A.I. 1967. “Specific Yield-Compilation of Specific Yields for Various Materials.” Geological Survey Water-Supply Paper 1662-D. Prepared in cooperation with the California Department of Water Resources.

Nagtegaal, P.J. 1978. “Sandstone Framework Instability as a Function of Burial Diagenesis.” Geological Society of London 135: 101-105.

Schnieders, M. 2023. “Engineering a Better Filter Pack.” Water Well Journal 77, issue 12: 23-25.

Sneider, R.M. 1987 “Practical Petrophysics for Exploration and Development.” AAPG Education Department Short Course Notes.

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Gary M. Gin, RG, is the vice president and ASR program leader for LRE Water. He has 26 years of extensive experience in water resources planning and economic studies associated with recharge systems. With more than 8000 operational hours of ASR wells, Gin was instrumental in the initial testing of glass beads as a filter pack media to optimize bi-directional flow for these direct injection systems. He can be reached at gary.gin@lrewater.com.