It’s important to understand as it will impact the well’s performance throughout its life.
By Marvin F. Glotfelty, RG
Water well development can be described in several ways, but a good definition was provided by Thom Hanna, PG, in his recent Water Well Journal column titled “Goals of Well Development.”
Thom described well development as “repairing aquifer damage near the borehole that was caused by the drilling operation.”
The importance of water well development is universally recognized as an important step in water well construction, but the minutia and details of the process are sometimes lost on even seasoned groundwater professionals.
We carefully monitor the results of our well development—including the discharge flow rate, turbidity, sand content, etc.—but we cannot actually go into the wellbore and observe what is occurring during the well development activities. Therefore, it’s worthwhile for us to get into the weeds on just what happens during well development. A good approach is to consider the influence of physics as it affects this process.
Purpose for Proper Well Development
The objective of water well development is to break down and remove the residual drilling fluid, fine sediment, chemicals, and polymers from the well.
As stated in Thom’s description of well development, the borehole damage that results from the drilling operation must be repaired during well development, and there is always some degree of borehole damage. Sediment and drilled cuttings must be removed in order to advance the drilling of a borehole—and to remove those cuttings, we rely on either hydraulic forces (circulated fluid), pneumatic forces (airlifting),
or mechanical forces (auger flights or bailed cuttings).
All these processes involve a positive pressure head within the borehole or abrasion of metal surfaces against the borehole wall. Thus, borehole damage (wall cake development) will always occur to varying degrees, depending on the drilling method and local geology.
If a hypothetical well was perfectly developed to the condition where every bit of wall cake was removed, then groundwater flow into the well would be completely unimpeded by the well structure itself. Such an imaginary condition cannot be practically achieved, but it is certainly our goal. This imaginary perfectly developed well would mirror the groundwater flow characteristics of the local aquifer, so the well would be 100% efficient with identical pumping water levels inside the well and in the surrounding aquifer.
Measures can be taken to improve the porosity and fracture connectedness of the aquifer itself, such as hydrofracturing or acidizing the well, but those activities constitute aquifer stimulation, which is not the same as well development.
Groundwater flow into a water well must travel through four pathways:
- The screen slots, which have an effective permeability much greater than the formation itself
- The filter pack material, which also has a permeability much greater than the aquifer material
- The wall cake that has accumulated on the borehole wall
- The surrounding formation that constitutes the local aquifer matrix.
Of these pathways, the bottleneck to groundwater production in undeveloped wells will be the wall cake (Figure 1). Thus, removal of that wall cake is our focus during well development.
The wall cake will be generally composed of fine-grained sediment, residual drilling fluid, and polymers. If the wall cake is not removed during well development, it will not only impede water production in the short term but will also degrade the well’s water production over time, as chemical precipitation and biological growth increase the disruption of groundwater flow into the well.
Impact of Fluid Velocity During Well Development
Well development can be achieved through several different techniques, such as swab-and-airlift, high-velocity horizontal jetting, and pump-and-surge methods (to name only a few). All these mechanical well development techniques can also be strengthened with the use of chemical treatments, such as surfactants, acids, or chlorine.
Each of these approaches to well development are appropriate as long as they’re consistent with the drilling method and the local geology.
A common feature of almost every well development method is that hydraulic forces are directed outward and inward between the well screen and the borehole wall by fast-moving water. The water movement could be initiated by mechanical action, as is the case with swabbing or surge block development, or the water movement may be initiated by pumping, as is the case with jetting or pump-and-surge development.
Even airlift development involves the flow of fast-moving water in and around the wellbore, which greatly increases the pneumatic energy from the airlift process. So, to understand the physics of well development, we can focus on the velocity of flowing water that achieves the work.
The effective velocity of water flowing through a sedimentary media is described by the Darcy velocity equation, which is illustrated in Figure 2 and from the 1978 book, Groundwater Hydrology by Herman Bouwer, Ph.D.
The Darcy velocity (v) is a function of the hydraulic gradient that is shown in Figure 2 to be the difference between the up-gradient water level (h1 + z1) and the down-gradient water level (h2 + z2). The hydraulic gradient is divided by the length of the flow path (L) and that ratio is multiplied by the hydraulic conductivity (K).
The direction of hydraulic gradients alternates between inward and outward flow during well development, and the magnitude of those gradients depends on the specific development technique being applied. The K value of the filter pack media will vary in accordance with several well design attributes that are within our control.
Filter pack K values are primarily determined by the grain size of the filter pack sand with increased surface areas (greater hydraulic friction losses) occurring in finer filter packs. Other attributes of filter pack that influence its hydraulic conductivity include sorting, roundness, and rugosity (surface roughness) of the filter pack material.
If we consider the Darcy velocity relationship in Figure 2 as it applies to the scenario of well development, we can illustrate the Darcy velocity variables as shown in Figure 3. If we hold the hydraulic conductivity and hydraulic gradient values as unchanged constants, we see that the annular thickness (L) has an inverse relationship to the Darcy velocity during well development. This is to say as the width of the well annulus gets larger, the velocity of well development water will be reduced when it arrives at the borehole face.
We should keep in mind that the Darcy velocity represents the time required to move from point A to point B. As water travels through the pore spaces of the filter pack media, the actual velocity of the water must be a bit greater than the Darcy velocity since it cannot take a direct flow path. To accommodate for this, we divide the Darcy velocity by the sediment porosity (ƞ) to estimate the actual speed of the flowing water (Figure 4).
The reason we concern ourselves with the velocity of well development water is that the work required to achieve our goal of breaking down and removing the wall cake can be described in terms of kinetic energy (KE). The relationship between kinetic energy and velocity is:
KE = mv²/2
KE =kinetic energy
m = mass
v = velocity
This means that doubling the velocity will cause the kinetic energy to quadruple. Similarly, reducing the velocity by half will cause the kinetic energy to drop to only one-quarter of its previous magnitude.
This is why a small adjustment of a well’s annular thickness will have a considerable impact on the effectiveness of well development. Of course, the filter pack envelope must be adequately thick to prevent sand invasion during pumping, so a balance is needed to design wells that can achieve both goals and prevent sand invasion while also facilitating adequate well development.
Although we may seek to avoid oversized annular widths to allow for good well development, a larger annulus may be necessary in some cases to address other well construction considerations.
For example, two versions of an aquifer storage and recovery (ASR) well design are shown in Figure 5. To accommodate both injection and withdrawal of water from this well, it is designed with a large-diameter upper well casing that is composed of mild steel and a lower well casing/screen composed of stainless steel. The two steel types must be connected with a dissimilar metal coupling to prevent galvanic corrosion, and that coupling extends outward from the casing by about an inch on each side.
To accommodate large pump bowls, the well also has a telescoping casing diameter. The original design for the well was envisioned to be within a 30-inch-diameter borehole (Figure 5, left diagram). The 30-inch-diameter borehole provides adequate room for a tremie pipe to install annular materials, but it leaves an annulus width adjacent to the well screen of almost 6 inches. Thus, complete well development of the well may be challenging.
The goal of this well design is to accommodate both an adequately large annulus for tremie pipe installation and also an adequately small annulus adjacent to the well screen.
To address both of these goals, the borehole diameter was reduced from 30 inches to 26 inches in the lower portion of the well (Figure 5, right diagram), with all other dimensions of the well remaining unchanged. This modification of the well design accommodated the required annular widths in both portions of the well, with the lower annulus being reduced to only about 4 inches.
Radius of Impact of Well Development
Another aspect of well development physics is consideration of just how far from the well the development energy reaches. I’ve heard claims from some hydrologists that certain well development techniques cause movement of sediment at distances of more than 10 feet from the well.
Since the radial extent of effective well development can only be assessed indirectly from our observations at the land surface, the radial extent of development energy is subjective and somewhat uncertain since it cannot be directly measured.
There are formations such as porous gravels or fractured karst limestones that could facilitate drilling fluid migration many feet away from the borehole, but in the great preponderance of situations, unconsolidated alluvial aquifers or even fractured rock aquifers will have borehole damage and wall cake accumulation only along the borehole face, extending no more than an inch or two into the adjacent formation.
Therefore, well development generally does not apply adequate energy to break down and remove fine material from deep portions (several feet) of the aquifer, and such energy is unnecessary to achieve the goal of well development.
As water moves outward from the well screen to the borehole face and beyond, it passes through radially increasing circles that define the cross-sectional area through which the water is flowing. If water is flowing outward from the well screen at a particular velocity, then that velocity will be increased if the cross-sectional area becomes smaller (like putting your thumb over the tip of a garden hose), and similarly, the velocity will be reduced if the cross-sectional area becomes larger.
It can be instructive to consider a somewhat extreme scenario when assessing concepts like this, so let’s envision a well for which we are conducting pump-and-surge development. Assume the well has an 18-inch casing diameter and a 6-inch-thick annulus. We’re inducing a flow rate of 2000 gpm inward and outward from the well screen, but the screened length is only 10 feet long (Figure 6).
To focus our consideration on the impact of radially increasing cross-sectional areas, we will disregard hydraulic head losses that result from the water flowing through the filter pack and formation sediments (even though we know those losses are significant). We will, however, increase the water’s velocity as it moves through constrained areas such as the well screen (we’ll assume slotted screen with 3.4% open area), filter pack sand (assuming 20% porosity), and the adjacent formation (also assuming 20% porosity).
As 2000 gpm flows downward through the 18-inch well casing, it will pass through the casing’s cross-sectional area of 1.77 square feet, so the water velocity is calculated to be about 2.52 feet per second (ft/s). As the water passes through the well screen, the cross-sectional area is reduced to 1.60 square feet, so the water will speed up to 2.78 ft/s as it leaves the screen.
Since we are considering the porosity of the filter pack (actual velocity instead of Darcy velocity), the water’s flow velocity will increase somewhat as it moves out to the borehole face. But by the time the water travels that 6-inch distance from the screen to the borehole face, the cross-sectional area will have increased to 15.71 square feet, so the flow velocity at the borehole face will drop to only 0.28 ft/s (Figure 6, left image).
At this point, the water has traveled outward only to the borehole face but has lost more than 90% of its initial velocity. If we consider a continuation of the water’s outward flow path to a location 2 feet away from the well screen (Figure 6, right image), we calculate the cross-sectional area to have become 34.56 square feet, so the flow velocity will drop to only 0.13 ft/s. That is an additional 45% drop in velocity compared to the water speed at the borehole face.
Some well development methods apply more focused energy in localized portions of the well screen, so the velocities would extend farther outward. However, it is generally the case that well development energy does not extend far into the adjacent formation, nor does it need to. If we can break down and remove the wall cake at the borehole face, the well development operation will have been successful.
These physical attributes of well development are only mathematical estimations, but we cannot directly observe well development from our perspective at the land surface, so it’s worthwhile for us to be mindful of the physical principles that are at play during the well development process.
We should also be mindful of the chemical and biological aspects that impact the water well environment, and how those aspects interact with one another to influence the well’s performance throughout its operational life.
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 email@example.com.