The Devil’s in the Details

Subtle details are often critical to the long-term success of a water well installation.

By Marvin F. Glotfelty, RG

We’ve all heard the idiom “The devil is in the details” to describe a situation or process that may seem straightforward at first glance, but upon closer inspection is recognized to be influenced by subtleties and nuances that will govern the success or failure of that endeavor.

Figure 1. Example of drilled cuttings being equal parts clay, silt, and sand. Multiple distributions of the sediment are possible.

My son, Sam Glotfelty, is now a fine young man of 27 who is navigating his own path through life and learning from his experiences every day. When Sam was growing up, I would often preach to him about the importance of “paying attention to the details.”

I often advised him to heed the specifics and minutia of his surroundings, whether we were doing a complex task around the house or just taking a hike through the forest. As with any father’s advice, my words sometimes fell on deaf ears. When I recently asked him about his memory of my advice, though, Sam told me paying attention to the details was one of his most prominent “dad lessons.” My son has grown to be an impressive young man who is becoming skilled at facing life’s challenges with an appreciation of the need to consider the details of every task.

The importance of addressing the subtle details of a project also extends to those of us in the groundwater and water well drilling industry. There is a strong connection between the success or failure of a water well design/installation project and the hundreds of tiny details associated with that effort.

Here are a few—but definitely not all—of the details that we groundwater professionals need to heed. If we don’t, we may expose ourselves to the wrath of the devil who dwells within those fine points.

Drilling Fluid Properties

Maintenance of proper drilling fluid is paramount to any good mud rotary drilling program. Proper drilling mud characteristics are critical to stabilize the borehole and adjacent formation, and to enable the drilling fluid to completely and efficiently clear the borehole of cuttings.

We sometimes see inexperienced groundwater professionals concern themselves only with the viscosity of the drilling fluid, which is an important fluid property, but by no means the only fluid characteristic that requires our attention.

Filtrate: Allowing the filtrate (also called water loss) of drilling mud to get too high seems like a small error that can result from an effort to avoid the cost of the expensive drilling fluid additives required to keep the filtrate low (around 10 cubic centimeters is a good goal).

At the land surface, we don’t see a difference between low-filtrate and high-filtrate drilling mud. However, down in the borehole, the difference between low-filtrate and high-filtrate fluid has enormous ramifications.

Drilling fluid with elevated filtrate properties will allow a large quantity of water to be released from the fluid out into the adjacent formation. That introduced water may cause native clays in the formation to swell, which may lead to increased torque or difficulty with making connections while drilling.

Figure 2. Well with an annular sounding tube (left) and annular gravel feed tube (right).

In many cases, the introduced water is also the culprit that undermines the stability of the formation and causes it to slough off into the borehole and cause penetration problems or even stuck drill pipe.

The relatively large volumes of percolating water from a high-filtrate drilling mud will leave a thick and fluffy wall cake behind as bentonite platelets accumulate on the borehole wall, and that thick and fluffy wall cake will do a poor job of stabilizing the borehole.

The seemingly small adjustment to maintain a low-filtrate drilling mud, including the additional expense of maintaining that fluid property, will definitely pay dividends during the course of the overall well drilling and construction process.

Fluid weight: Drilling mud is a little heavier than water (which weighs 8.3 lb/gallon), so typical drilling fluid weights are in the range of 8.5 to 9 lb/gallon due to the bentonite and other additives, along with the suspended solids (generally, fine silt and clay particles) that are carried with the fluid.

Although there are cases where a flowing artesian water well requires weighted drilling mud, we normally want to see drilling fluid weigh no more than about 9 lb/gallon for typical water well drilling projects.

Drilling mud weights greater than 9 lb/gal are considered an indication that suspended solids are being recirculated in the fluid due to an ineffective solids control system (mud pit design and size, shale shaker, desander, etc.) or due to poor fluid properties (such as gel strength or fluid rheology) that are not removing the fine silt and clay particles from the fluid.

Elevated mud weight can cause or exacerbate the potential for lost circulation conditions, because the increased hydraulic pressure of a heavy drilling mud column can “knock the bottom out” of the borehole and cause drilling mud to migrate into formation pores or factures.

A fluid weight adjustment of less than one pound per gallon can sometimes make the difference between the drill crew spending the time and expense to fight lost circulation conditions versus the drill crew continuing to advance the borehole at a good penetration rate.

Equipment Maintenance

There are many detailed items on and around a drilling rig that require maintenance and attention, and bypassing just about any of those details can result in a significant equipment failure.

Accordingly, it behooves drillers to periodically inspect and provide maintenance to their equipment. Experienced drillers know the frequency with which their equipment requires maintenance, and which parts of their particular drilling rig may be especially finicky or problematic.

Figure 3. Dielectric coupling.

In general, an appropriate equipment maintenance program will include consideration of fluid leaks and fill levels (diesel fuel, hydraulic fluid, brake fluid, etc.); wear and tear on brakes and clutches; electrical and lighting systems; wear on hoses or cables; and wear and tear on all moving parts such as drive lines, swivels, bushings, pipe handling equipment, etc.

This is just a partial list, but we should be mindful of the fact that failure of almost any single piece of equipment due to poor maintenance or inattention can ultimately result in a breakdown of the entire drilling operation or cause an unsafe working condition.

Hydrologic Data Analysis

There is definitely a devil in the details of collecting and analyzing hydrogeologic data. We rely on our interpretation of drilled cuttings, geophysical logs, and hydrologic data (water levels, flow rates, aquifer test analysis results, etc.) to characterize groundwater systems, and we make important infrastructure and groundwater policy decisions based on that information. However, much of the raw geologic and hydrologic data we collect is somewhat indirect or provisional.

For example, the drilled cuttings that were collected from a 10-foot interval of a borehole may indicate that the drilled sediment is composed of equal parts of clay, silt, and sand (Figure 1). The drilled cuttings are definitely representative of the borehole interval, but those cuttings arrive in a mixed pile at the land surface. Thus, the three sediment types could be distributed throughout that borehole interval in a variety of ways without altering the proportions we observe in the cuttings sample.

As groundwater professionals, it is part of our business to deal with incomplete data and uncertainty. Problems tend to arise when a groundwater professional fails to heed our reliance on just bits and pieces of measured data and forgets to also consider the larger body of information that remains unmeasured and unknown, for which we can only speculate or make informed interpretations.

Consider another example where the water table was measured at a depth of 50 feet in a well. Is the water table 50 feet below the land surface, or 50 feet from the top of the well casing, or 50 feet below the rig floor or kelly bushing? All those measuring points are commonly used, and if the field technician who recorded the measurement leaves out that information, it can lead to data errors and analytical misinterpretations.

Figure 4. Impact of annular width on conveying well development energy to the borehole face.

The vocabulary we use to describe hydrologic data can also introduce errors into our analyses. Commonly used nomenclature for hydrogeologic discussions include multiple units of measure (such as gallons per minute, cubic feet per second, acre-feet per year, and million gallons per day—not to mention the full suite of metric units).

Groundwater professionals also have the habit of adding confusion to our hydrologic analyses by describing various distances and depths on the basis of different datum points (such as feet below land surface versus feet above mean sea level versus feet of aquifer thickness). The small details of these description varieties lead to large differences in the actual values we’re referencing.

Well Design Details

It is also critical to consider every detail that impacts the progress of drilling, constructing, and subsequently, developing and operating a water well. There are many considerations incorporated into each water well design, and every single detail is capable of obstructing the project goals if left unresolved.

Water well design requires consideration of the forces of nature and physics, which may be exerted for only a moment in time, such as when an instantaneous pressure surge exerts force against a well casing. If such forces become excessive—even for that instant in time—then the ramifications can be disastrous.

Although the impacts of short-term details cannot be overlooked, we also need to consider long-term details of each well design. The enduring properties of a water well will be revealed during the years and decades of its use.

Too often, the long-term well design details are discounted by well designers who think they will be “finished and gone” during those future years of well operation.

Figure 5. Well design with telescoping borehole.

My experience with designing water wells for the past several decades supports my belief that the long-term details of a well design should never be overlooked. Consideration of long-term well design features is an ethical responsibility, and it is also a defining aspect of the well designer’s professional reputation from which nobody can hide.

There are many long-term well design details to be considered, but one of the most common design mistakes I’ve encountered is inappropriate annular sizes. The annulus of a well is the area between the outside of the well casing or screen, and the inner face of the borehole wall. Well designers must wrestle with the dimensionally skewed situation that we need to account for well depths of many hundreds of feet and also for annular widths of just a few inches.

Some well owners prefer annular sounding tubes and gravel feed tubes (Figure 2) to be included in their well design so that the well owner can easily monitor water levels, and if necessary, add additional filter pack sand to the annulus at some point. These tubing strings require the annulus to be wide enough to accommodate the tubing string, and the annular fill or sealing material that will surround that tubing string.

Even if an annular tubing string is not included in the well design, there should be an adequate annular width (typically, about 2 inches) to accommodate a tremie pipe so that filter pack sand and other annular materials can be properly emplaced.

The sizing of a well’s annulus may conflict somewhat with other well design elements in some cases. Some well designs include different steel types in the upper and lower portions of the well so that mild steel can provide its best attributes (low cost and strength) in portions of the well where those are most useful, and stainless steel can provide its best attributes (corrosion resistance and minimal scale growth) in portions of the well where those are most beneficial. When dissimilar metals are to be connected in a well, a dielectric coupling (Figure 3) must be used to prevent galvanic corrosion at the connection point of the two different metals. Such couplings enable us to utilize different metal types in a single well so as to optimize the well’s performance, while minimizing overall material costs.

Dielectric couplings (and some other well design features such as compression sections) present an annular width problem because they add an extra inch or so to each side of the well casing, so the outside diameter is increased by around 2 inches. That increase must be accommodated with a larger-diameter borehole to provide the required annular width. Unfortunately, while that larger annular width provides a solution to address the well design detail of the dielectric coupling, the larger annulus becomes a problem in other portions of the well.

Adjacent to the well screen, the annulus will be filled with an envelope of filter pack sand that will prevent native sands from invading the well while it’s being pumped during its years of operation. The filter pack envelope plays an important role to prevent sand production while the well is being pumped, thus preventing the sand from abrading and damaging the pump equipment.

A larger annular width assists the filter pack envelope with filtration but undermines another important attribute of the filter pack—its ability to convey energy during well development. During the drilling and construction of the well, a wall cake composed of clay platelets and (typically) organic polymer will accumulate on the borehole wall. The wall cake helps to stabilize the borehole during the drilling process, but it must be removed during well development to achieve maximum water productivity and well efficiency.

If the development process fails to remove the wall cake, organic polymers in the wall cake may become a food source for naturally occurring bacteria, which may lead to biofouling and scale accumulation problems in the future.

The well development effectiveness is determined by the ability of energy from the development tool (jetting tool, swab block, etc.) to be conveyed through the filter pack to the borehole face. For the development energy to be capable of breaking down and removing the wall cake, a thinner annular width is preferred (Figure 4) so that the energy has a shorter pathway to travel between the well screen and borehole wall. To accommodate both the need for a larger annulus in some portions of the well and a smaller annulus in other portions of the well, many well designers include telescoping borehole diameters to address all the detail considerations of the well design (Figure 5).


The multiple chain reactions that may arise from well design details sometimes conflict with one another, but those details will make or break the success of the well construction program. Thus, it is the well designer’s responsibility to address each and every detail to prevent any one of them from potentially introducing the devil to the future functionality and value of the well.

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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