Filter Pack Differences
Comparing wells drilled near each other where the major difference is their filter pack.
By Thom Hanna, PG
We will compare two wells near each other in this column and look at the differences in the wells due to the use of different filter packs.
Figure 1. Comparison of Shur Pack 12-16 and No. 1 Northern filter packs.
One well was completed with a naturally derived filter pack used locally and the other had a glass bead filter pack. Glass beads are increasingly being used as filter pack material in place of natural gravels due to their uniform size, smooth surface, and chemical inertness.
Unlike natural gravel, which can vary in shape and mineral content, glass beads are manufactured to precise specifications, offering consistent porosity and improved hydraulic conductivity. This uniformity enhances the efficiency of wells by allowing more effective water flow to the well screen and reducing the risk of clogging or biofouling.
As a result, wells constructed with glass bead filter packs often exhibit higher yields and require less maintenance over time. Additionally, natural gravels are becoming harder to source due to environmental restrictions and depletion of high-quality deposits. In contrast, glass beads can be reliably produced to meet specific filtration needs, making them a technically superior and more readily available option in many areas.
The wells being compared were completed near one another with Shur Pack glass beads and No. 1 Northern gravel to determine differences, with the main difference being the filter media used in the wells (Figure 1).
The wells were completed in the Quaternary High Plains aquifer near Lancaster, Nebraska, for the Lancaster County Rural Water District near Lincoln for municipal and domestic water supply.
The distance between the wells is approximately 600 feet. The wells were constructed by Sargent Irrigation Co.
Well RWD 2004-6 was completed in 2004 using a No. 1 Northern filter pack and well RWD 2024-6 was drilled using the same methodology and completion materials, but the main difference being the filter pack in the wells.
Figure 2. Generalized geology and hydrogeology of the High Plains aquifer in Nebraska (after Korus et al. 2013).
Paleovalleys and Alluvial Aquifers
During the Pliocene and early Pleistocene Epochs, broad alluvial valleys—similar in form to those seen today—were formed and subsequently filled with alluvium. Although these ancient valleys occupied different positions than modern ones, they were rich in sand and gravel deposits, making them excellent aquifers. These buried ancient channels are referred to as paleovalleys (Korus et al. 2013).
Over time, younger geologic deposits covered the paleovalleys. Some were buried beneath glacial till, rendering them completely hidden beneath a hilly, dissected landscape.
Figure 3. Schematic diagram of wells RWD 2004-6 and RWD 2024-6.
In eastern Nebraska, paleovalley aquifers serve as primary groundwater sources. At least seven major paleovalleys trending eastward and southeastward have been identified. While most are only a few miles wide, they can extend up to 70 miles in length.
These are typically confined aquifers, though some are unconfined or partially confined in areas where overlying sediments are thin. Their presence is now known only through the drilling of wells and test holes that intersect them.
The Quaternary Paleo Valley and Glacial aquifers in Lancaster County, Nebraska, play a vital role in the region’s water supply, both for agricultural and municipal use. This aquifer system consists mainly of unconsolidated sands, gravels, silts, and clays (Figure 2). In Lancaster County, it generally lies beneath layers of loess and glacial till, which can affect recharge rates and water movement.
The aquifer is unconfined in many areas, making it more susceptible to surface contamination but also easier to recharge under favorable conditions. Water is stored and transmitted primarily through intergranular pore spaces, and the aquifer’s thickness and productivity vary depending on the local stratigraphy and lithology.
Well Site Hydrogeology
Figure 4. Graph of aquifer sand and filter pack gradation.
Geologic logs from the wells show the upper 80 feet to 150 feet of the aquifer consisting of silts and clays consistent with loess and glacial sediments. Beneath the fine-grained materials are fine to coarse-grained sands of the Palio Vally aquifer that are deposited on top of shale. The thickness of the sands is from
approximately 80 feet to 266 feet below ground surface (ft bgs) in RWD 2004-6 and from 149 to 248 ft bgs in RWD 2024-6. The static water level in the wells was 70 to 80 ft bgs.
Well Design and Construction
Both wells were completed in similar fashion (Figure 3). Test holes were first drilled to evaluate the locations for a suitable high production well. The production boreholes were drilled 28 inches in diameter (Table 1) using the reverse circulation mud rotary drilling method. The drilling fluids were bentonite and polymer, and the final viscosity was 35 to 37 seconds as measured with a Marsh funnel at the end of drilling and prior to well development.
After the boreholes had reached total depth, the drilling fluids were thinned using NW-220 Clay Dispersant, and the wells were completed with 16-inch 304SS screens and PVC riser. Upon installation of the casing and screen, the filter pack was tremied into the wells. RWD 2004-6 had a thicker section of sands and was completed with 70 feet of wire-wrap screen, and RWD 2024-6 had a thinner section of sands and was completed with 40 feet of wire-wrap screen.
The aquifer sands in RWD 2004-6 are coarser and were designed with a larger filter pack and slot. RWD 2004-6 was packed with No. 1 Northern sand and a 0.045-inch slot wire-wrapped screen. RWD 2024-6 was completed with Shur Pack 12-16 and a 0.035-inch wire-wrapped screen.
A representative plot of a composite of the formation sands and the selected filter packs is presented in Figure 4. After the installation of filter material, the wells were grouted from approximately 190 ft bgs to approximately 17 ft bgs using ⅜-inch bentonite chips and topped with neat cement to approximately 7 feet to complete the sanitary seal.
Well Development
Prior to development, the drilling fluids were thinned using NW-220 Clay Dispersant and were circulated to reduce the fluid viscosity. A double surge block/isolation tool was used to develop the wells by surging with 20-foot strokes at a rate of 10 seconds per stroke. Surging water was pumped from between the surge blocks while surging at approximately 400 gallons per minute until the water was clean.
In well RWD 2004-6, which was completed with No. 1 Northern gravel, the time to reduce the water from muddy to cloudy was about two hours for each 20-foot interval being developed. To complete the development for each interval from cloudy to clear, an additional 30 minutes of development was required. There were 10 mL to 15 mL fines present at the end of each development interval.
In well RWD 2024-6, which was completed with 12-16 Shur Pak glass beads, the time to reduce the water from muddy to cloudy was about 45 minutes for each 20-foot interval being developed. To complete the development for each interval from cloudy to clear, an additional 10 minutes of development was required. There were no fines present at the end of each development interval. Table 2 provides a summary of the well development.
Test Pumping
After well development, the wells were test pumped for a period of 24 hours. Table 3 presents a summary of the pumping test data.
Well RWD 2004-6 was pumped at 800 gpm, the drawdown in the well was 15 feet, and the specific capacity was 53.3 gpm/ft. Well RWD 2024-6 was pumped at 750 gpm, the drawdown in the well was 13 feet, and the specific capacity was 57.7 gpm/ft. There was no sand produced by either well at the end of the pumping test. After the pumping, testing of well RWD 2004-6 had 1 foot of fill in the bottom, and RWD 2024-6 had no fill in the bottom of the well.
Thoughts and Conclusions
There are several differences that stand out between the wells. Well RWD 2004-6 is located in a portion of the aquifer that appears to have a larger transmissivity based on the aquifer thickness and grain size. Because the wells were drilled with reverse circulation mud rotary, there could be silts and very fine materials that were lost in the drilling fluids, but the descriptions of the drill cutting and the sieve analysis show that the formation has a greater permeability and thickness.
Figure 5. Jose Gamboa and Donald Page with Sargent Irrigation showing a sample of clear water after developing the RWD 2024-6 that was completed with Shur Pack 12-16 glass beads.
Well RWD 2024-6 was fully developed in less than half the time, which is consistent with most wells completed with glass beads for filter pack. It could be that part of the reduced time was due to the screen interval being shorter, but each 20-foot interval that was developed took 45 minutes for the well completed with glass beads as compared to two hours for the well completed with gravel. It is interesting that the filter pack and slot size were smaller as well, which would typically require more development time to get the energy through the screen and filter pack to remove the damage at the borehole wall due to the formation of a filter cake during drilling.
The time savings in development and enhanced efficiency of well RWD 2024-6 completed with glass beads more than pays for the higher cost of glass beads as compared to a naturally sourced filter pack gravel.
A big thanks to Sargent Irrigation (Figure 4) for providing the well completion, development, and pumping test data, and Lancaster County Rural Water District for its support in providing the information about the well completion and development.
References
Korus, J.T., Howard, L.M., Young, A.R., Divine, D.P., Burbach, M.E., and Hallum, D.R. 2013. Groundwater Atlas of Nebraska. Resource Atlas No. 4b/213.
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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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