Researching to determine the best filter pack for well performance.
By Michael Schnieders, PG, PH-GW
The groundwater industry has faced a growing challenge over the last decade with the declining availability of good quality, natural filter pack material.
This issue stems from a dual problem: resource loss and an escalating demand within associated markets.
The scarcity of high-quality filtration material, such as silica sand, has been exacerbated by reduced deposits and environmental regulations limiting extraction and increasing cost.
Concurrently, the domestic oil and gas sector demands efficient and reliable filtration material for both new well completion and hydraulic fracking. As a result, our industry has struggled to secure quality filter pack material leading to longer delays, higher costs, and an acceptance of lower quality materials.
Due to the availability of good quality natural material, most articles and research on the subject has historically focused on the most efficient sizing and balance with screen selection or determining the right thickness of pack based on the aquifer and desired flow profiles.
As we strive to advance well design and construction technology, we must look beyond traditional sources for filter pack material. Enter the availability of an engineered media, a filter pack material designed with resiliency and longevity in mind.
Gravel pack or filter pack is intended to make the area around the well more permeable by replacing formation materials with more uniform and resilient material. Filter pack is defined in Groundwater & Wells, Second Edition as “sand or gravel that is smooth, uniform, clean, well-rounded, and siliceous.” Those are simple requirements with major impacts on well design.
Filter pack materials were recently evaluated using the same core characteristics of design as well as implications of the material on well operation and longevity. Three different regional suppliers of natural filter pack materials and two sources of engineered filter media were evaluated as part of the research.
High Silica Content
High silica content in filter pack material is crucial for ensuring the longevity and effectiveness of well completions. Silica-rich materials, such as quartz sand, have long been preferred due to their exceptional durability and impact on permeability. Silica is chemically inert and does not react with groundwater or the well structure, preventing any unwanted changes in water quality over time.
Additionally, the high silica content adds structural stability to the well screen, preventing collapse and the intrusion of fine particles into the well.
The degradation of silicate minerals over time can have a significant impact on water quality as it can release unwanted constituents into the water. Feldspars, a common silicate component of natural sands, often contain secondary mineralogy which can release ions and compounds into the groundwater as they weather.
This process can result in the deterioration of water quality with increases in iron, aluminum, and manganese. This deterioration can occur as part of natural weathering over time, or during rehabilitation, especially if chemicals or mechanical energy is employed.
A silica content of 90% or greater with a Moh’s hardness of 7 is desired. Natural filter pack media is available in a silica range of 75% to 90%. Engineered media, known to many in the industry as glass beads, is available with a silica content of 96% or greater.
The high concentration of silica is beneficial in several areas. First, with almost pure silica mineralogy, there is an absence of secondary constituents that can degrade over time and impact water quality or compromise the well design. Engineered silica material maintains its structural integrity even under varying flow rates, pH levels, rehabilitation efforts, and other challenging conditions.
Sphericity and uniformity are critical factors in the design and construction of well filter packs. Sphericity refers to the roundness of the individual filter media particles, while uniformity pertains to the consistency in size and shape of these particles.
These characteristics play an important role in ensuring the effectiveness of a filter pack in a well. High sphericity and uniformity are essential because they promote optimal porosity and permeability, allowing water to flow through the filter pack without turbulence while providing filtration from fine sediment.
Irregularly shaped or poorly sorted particles can lead to channeling, clogging, and mechanical fouling, which can impact well hydraulics, water quality, and efficiency.
The uniformity coefficient, in the context of filter pack selection, quantifies the range of particle sizes within the filter material. A lower uniformity coefficient indicates a narrower range of particle sizes, which is desirable for filter packs as it ensures a more consistent and efficient packing of filter material, minimizing the risk of clogging and maintaining a uniform flow of water through the well screen.
An evaluation of three natural filter packs identified a range of sphericity and uniformity. Sphericity ranged from 45%, classified as sub-angular-rounded, to 77%, classified as rounded-well-rounded. The uniformity coefficient for the three options ranged from 1.12 to 1.5. Conversely, the glass beads evaluated ranged from 85% to 95% sphericity, classified as well-rounded, with a uniformity coefficient of 1.09.
Collapse strength is typically measured and reported in newtons (N) by subjecting a structure or material to a controlled force or load until it deforms or fails, with the maximum force applied just before failure being expressed in newtons.
Collapse strength is a critical property of filter pack materials in water wells as it determines the ability of the filter media to withstand external pressure, preventing deformation or collapse as the well ages.
A high collapse strength is desirable for maintaining the integrity of the filter pack and ensuring the long-term reliability and performance of the well. Reported collapse strength for the native filter pack materials evaluated ranged from 105 to 200 N. Engineered filter media has a reported collapse strength of 1466 N, indicating a far superior product with regards to strength.
Imperfections in natural filter pack material such as occlusions and fractures play a crucial role in their effectiveness as filtration media in various applications, including water treatment.
These natural deformities reflect the mineralogy and occur because of weathering. These characteristics enhance filtration by increasing the available surface area in the media. Unfortunately, these areas also become an anchor point for the development of biofilm and the accumulation of precipitating mineral scale.
Glass beads exhibit significant improvements when compared to native materials with regards to mineral scale development and biofilm formation. A biofilm is an accumulation of polysaccharide exopolymer exuded naturally by bacteria for defense, nutrient capture, and communication.
Biofilm production and growth occurs because of the type of bacteria present and changes in the environment. In the filter pack, this can largely reflect operation of the well. As biofilm expands, it can lead to agglomeration of the filter pack, impacting flow and influencing the types of bacteria present.
Mineral scale development in a well is largely driven by the source aquifer and the native water chemistry. As a water becomes saturated with respect to a specific mineralogy, precipitation of scale can occur. Biofilm development offers an excellent point of accumulation for precipitating scale. In the filter pack this can lead to cementing of individual material and the direct impaction of flow.
The rounded shape of the glass beads minimizes occlusions, fractures, and other irregularities, reducing the surface area available for microorganisms to anchor and form biofilms. Not only does this reduce the potential for biofouling, but it decreases the tendency for mineral scale accumulation within the filter pack, leading to less need for more aggressive rehabilitation and improving efficiency over the well’s operational life.
When mineral scale or biofilm development occurs on the glass beads, cleaning is more effective. Native filter pack material and the engineered media were subjected to induced fouling in a laboratory setting. Acid and oxidizing treatments alike proved more successful in removing the fouling from the glass beads. The improved flow profile, uniform roundness, and smooth surface aided in more efficient cleaning in comparison to native material.
Glass Beads Advantageous
Although outside the nature of the laboratory work, it is speculated that similar benefits would be realized during the development of new wells employing glass beads.
Although more expensive than natural filter pack material, the improvements in core filter pack characteristics and attributes offered by glass beads should be considered. The benefits should be viewed in a life cycle analysis that accounts for more even flow, increased structural integrity, and reduced fouling potential.
The benefits of using glass beads over natural filter pack for the operational lifespan of a well are clear. Glass beads offer consistent particle size and shape, ensuring stable permeability and minimizing the risk of flow impaction and mechanical fouling.
Their resistance to mineral scale accumulation and biofilm formation means the well remains efficient and reliable over the long term, reducing maintenance costs and downtime.
These advantages make glass beads an excellent choice for maintaining water well performance throughout its operational life, enhancing its overall sustainability and cost-effectiveness.
Michael J. Schnieders, PG, PH-GW, is a professional geologist serving as the principal hydrogeologist and president of Water Systems Engineering Inc. in Ottawa, Kansas. Schnieders’ primary work involves water resource investigation and management, specializing in the diagnosis and treatment of fouled well systems. Schnieders served as the Groundwater Foundation’s 2017 McEllhiney Distinguished Lecturer in Water Well Technology. He can be reached at firstname.lastname@example.org.