Perspectives on Scale and Time

Intangible factors need to be considered in well design because they too can impact a water system.

By Marvin F. Glotfelty, RG

As we ruminate about the considerations for water well design, there are many tangible well design elements that we must consider.

Tangible design elements include a variety of things that are incorporated into the well’s structure—things that can be touched, measured, or mathematically evaluated to determine the proper dimensions (length, width, wall thickness, etc.) for the well being designed. These well design elements include many common objects used in well construction such as the well casing, well screen, filter pack sand, and cement grout material.

This column is focused on a different category of well design elements, though. There are also intangible well design elements that cannot be directly measured or generally observed. The characteristics of an intangible well design element cannot be readily perceived or monitored, so it can only be visualized and indirectly considered by the well designer.

For example, the wall cake that accumulates on the borehole face during drilling is physically located down the borehole, so it cannot be directly observed or measured. However, we can estimate the characteristics of this intangible feature by testing the drilling fluid at the land surface.

Intangible well design features are sometimes difficult to fully understand, but they will impact the constructability and ultimate value of the well.

Two overarching considerations with intangible well design elements are time and scale.

The time required for an activity to occur during the installation of a well and the operation period of a water well may vary from an almost instantaneous moment in time up to multiple years or decades.

The scale of items to be considered during the design of a water well are also far-ranging, with the sizes of well design elements stretching from millimeters to miles. The intangible well design considerations of time and scale may seem somewhat esoteric, but they are nonetheless critical to the success or failure of a water well’s installation and performance.

Consideration of the Scale of Water Well Design Elements

A water well interacts with and generally becomes a part of the surrounding groundwater environment. Many physical, chemical, biological, and hydrological interactions occur throughout every aquifer as the groundwater makes its way from the point of recharge to the point of discharge (the well).

Figure 1. Different drawdown cone shapes and sizes with high and low values of transmissivity (T) and storage coefficient (S).

We can consider these relationships at various scales. They range from the megascopic level (aquifer-wide or regional) to the microscopic level (at the square inch or even molecular scale). Consideration of hydrogeologic realities at
different scales will enable a well designer to account for the different but interrelated well design influences that arise in both large and small sizes.

To consider potential impacts to water well design from an aquifer-wide perspective, we must contemplate the nature of the overall aquifer. The water that will be produced by a well will be withdrawn from the local aquifer system, which may be of limited aerial extent (a square mile or less in some cases) or may be part of a large regional groundwater system.

One of the largest and most important aquifers in the United States is the Ogallala Aquifer, which covers a vast area (approximately 174,000 square miles) beneath the Great Plains of South Dakota, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas. As a continuous aquifer, the groundwater within the Ogallala has potential pathways to migrate, commingle, and chemically interact at various locations and with various results.

Currently, some portions of the Ogallala Aquifer have extensive water-level declines, whereas the water table in other areas is generally stable. The location of a well within the Ogallala Aquifer is certainly a primary factor in the determination of the well’s design, but since the well will be withdrawing water from a regional aquifer, the well design should also be consistent with potential influences of other more distant areas that will be interconnected with the local groundwater system.

Figure 2. Schematic cross-section of the well screen, filter pack sand, and formation.

Another important aquifer is the Salt River Valley Aquifer that underlies the Phoenix, Arizona, metropolitan area. The SRV Aquifer occupies a large valley within the Basin and Range Geologic Province, covering about 1500 square miles and extending to depths of more than 6000 feet. The SRV Aquifer is divided into the East SRV and West SRV subbasins by a bedrock ridge near the center of the valley.

Shallow groundwater levels in the SRV Aquifer during pre-development times were mapped as a continuous groundwater system that was interconnected across the entire basin. But increases in the area’s population and water demand over the past several decades have dropped the water table and split the SRV groundwater system into two distinct and disconnected sub-basin aquifers in the west and east portions of the valley. Nowadays, well designers in this area must make a judgment call as to whether their well design should account only for current water table depths versus projected groundwater levels that may occur in future years.

The shape and size of the drawdown cone that forms around a pumping well depends on the well’s pumping rate and duration, but also on the local aquifer characteristics. The anticipated size and shape of the drawdown cone is an important well design consideration to prevent excessive drawdown that may result from dewatering of well screen intervals that can cause cascading water.

Horizontally extensive drawdown cones are also a concern because if neighboring wells are operated simultaneously, their drawdown cones may extend horizontally to the point of intersection. Interconnected drawdown cones are additive, so the resulting water-level drawdown in each well will reflect the sum of the individual drawdown cones.

This means the water-level drawdown in each well would be greater than what was predicted by single-well pumping tests. Consideration of the drawdown in well design is straightforward but can be challenging because we usually have not yet determined the site-specific aquifer characteristics when the well is being drilled.

As shown in Figure 1, the drawdown cone shapes and sizes can vary substantially, and we can only measure the water level inside the well. This means at the time of well installation, we won’t actually know the shape and horizontal extent of the predicted drawdown cone.

If we shift our perspective from macroscopic well design considerations to microscopic well design considerations, we’re still dealing with intangible items that cannot be directly observed or measured. We can only evaluate the clues left behind by microscopic elements that impact the well’s performance.

At the microscopic scale, much of the well designer’s focus is on the availability of flow paths for groundwater to travel into the well from the formation through the filter pack envelope and well screen, and on into the well (Figure 2).

The flow of groundwater into the well may be restricted by residue or scale that clogs the well screen and filter pack, but the primary impediment to groundwater production is typically neither the screen nor filter pack envelope, but the wall cake that coats the face of the borehole shown in Figure 2. The wall cake is composed of residual drilling fluid and fine-grained native sediment, so it can constitute a physical, chemical, and biological barrier to water production.

The well screen and filter pack material (natural sand or manufactured glass beads) are tangible design components of the well, but they are intimately related to several intangible design attributes.

An intangible design element of well screen and filter pack is their ability to convey energy from inside the well out to the borehole face, break down, and remove the wall cake during well development. The energy emitted during well development activities (such as swabbing, jetting, or surging) will be ineffective if the energy cannot travel all the way through the screen and filter pack envelope and reach the borehole face and break down and remove the wall cake.

Another intangible design element related to the well screen and filter pack is the ability of their surface areas to impede bacterial growth to minimize biological clogging of the well. The prevention of biofilm growth on the well screen seems to be best accommodated by the use of stainless steel screen material, as opposed to mild steel screen.

The prevention of biofilm growth within the pore spaces in the filter pack envelope is aided by selecting filter media that is composed of hard, inert material like quartz or glass, with a smooth surface texture that will not provide a favorable environment for bacterial growth.

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Consideration of the Time Duration of Water Well Design Elements

In my book, The Art of Water Wells, I recommended water well designers envision “every moment in time” during the drilling, installation, development, and testing of a new well.

I shared a story of a well design experience that turned out fine but could have been catastrophic because an instance in time in which well materials may be exposed to excessive forces can cause compressive or tensile failure of the well.

As part of the well design process, we calculate the anticipated forces under which the well casing and other materials will be subjected. However, those calculations are only as accurate as our assumptions of the downhole conditions that we’ve visualized since the subsurface conditions are generally unavailable for direct measurement.

The most common force calculations that are incorporated into the well design are the potential for casing collapse that could result from differential hydraulic head forces; the potential for tensile failure due to excessive string weight; the potential for hose failure during high-pressure pumping events; and the possibility of rapid gas and heat emissions resulting from chemical reactions.

All these forces can result in well failure in a literal blink of the eye. Even with preemptive calculations and safety factors incorporated in the well design, catastrophic events can still sometimes occur when unmeasured and unanticipated forces arise—in a moment in time.

The other end of the time spectrum for water well design is the operational life cycle of a well. The operational life of a well generally ranges from about 25 to 100 years.

During this relatively long time period, changes in both the well and surrounding aquifer may occur. After years of groundwater pumping, the water table may decline by 10 or even hundreds of feet as has been documented in many aquifers.

Water table declines impact the functionality of water wells because even though the water table has dropped, the well structure remains unchanged. Declining water tables can cause reduced water production from the well and even change the quality of water being produced by the well. This is because groundwater quality is stratified in many aquifers, just as the sediment is layered.

The impact of declining water tables on a water well is a fact of life in some areas, and water table declines cannot be prevented by any single well design. Therefore, in areas with an ongoing trend of dropping water levels, well designers should incorporate versatility into the well design to facilitate future modification (rehabilitation) of the well (such as deepening, liner installation, changing the pump setting, etc.) when necessary.

Another long-term time consideration relates to situations when the well being designed is in reasonable proximity to an area with groundwater quality concerns, such as a contaminant plume or other environmentally sensitive area.

In such cases, we can only make approximations of the intangible design element of the time required for the water of concern to reach our well. If we have enough information about the local aquifer, we can, however, reasonably estimate that travel time. The groundwater will move though the aquifer in accordance with Darcy’s equation, a version of which is:

Va= KIηe

where:

Va is the average groundwater velocity (feet/day)

K is the hydraulic conductivity (permeability, in feet/day) of the aquifer

I is the gradient (slope of the water table, in vertical feet/horizontal foot)

ηe is the effective porosity of the aquifer material (usually about 20% for sand formations).

When all the information is plugged into the Darcy velocity formula, we may find that the speed of groundwater movement (under natural gradients and typical conditions) is often just a few feet per year.

Recommendations for Considering Time and Scale in Water Wells

During the design process for each well, a well designer will need to address both tangible and intangible variables. He or she should not fail to consider all the intangible (as well as tangible) design elements that will affect the well from the beginning of its construction to the end of its useful life.

This includes the extremes of short-term and long-term impacts, as well as large-scale and small-scale impacts. Well designs sometimes need to just accommodate an opportunity for future modification of the well to address changed conditions as they come along.

Have a Drilling Question for Glotfelty?
Is there a drilling issue that you have wondered about for a long time? A question you have wanted a second opinion on for a while? Send them to The Art of Water Wells column author Marvin F. Glotfelty, RG, and he will utilize his more than 35 years of experience to tackle the question for you. Email Glotfelty at mglotfelty@geo-logic.com, and the answer will appear in an upcoming NGWA: Industry Connected Video.

Marvin F. Glotfelty, RG, is the principal hydrogeologist for Clear Creek Associates, a Geo-Logic Associates Co. He is a licensed well driller and registered professional geologist in Arizona, where he has practiced water resources consulting for more than 35 years. He is author of The Art of Water Wells (NGWA Press, 2019) and was The Groundwater Foundation’s 2012 McEllhiney Lecturer. Glotfelty can be reached at mglotfelty@geo-logic.com.