Well development is the most important part of well design and construction.
By Thom Hanna, PG
It does not matter what you use for well construction materials. You can make the casing out of gold and use diamonds for filter pack, but if you do not properly develop the well it will be inefficient, have maintenance issues over its life, and be expensive to own.
The process of well development starts during the design phase. This includes borehole sizes, filter pack size, slot selection, drilling fluids, and methods of development.
Well development includes procedures designed to maximize well yield by repairing aquifer damage near the borehole that was caused by the drilling operation. This is accomplished by removal of fines and drilling fluids near the borehole wall and filter pack so that the natural hydraulic properties of the aquifer are restored and eliminate the pumping of sand that can destroy pumps and enter the distribution system.
All drilling operations alter the geologic characteristics of the aquifer materials in the vicinity of the borehole. Such alterations often result in a significant reduction of the aquifer’s hydraulic conductivity near the wellbore—even that of a well drilled without using drilling fluids.
Some common causes of borehole and aquifer damage include the following:
- A casing driven through clay or fine-grained sediments can entrain some of the sediments and carry them down the borehole, coating the borehole in the area adjacent to the aquifer.
- In wells drilled using drilling fluids (especially bentonite), the aquifer can become sealed due to the invasion of fine-grained particles. Such invasion minimizes water movement from the aquifer (Figure 1).
- Not understanding what the drilling fluids contain and how to break them back.
- In consolidated or crystalline aquifers, fine-grained cuttings can plug interstices or fracture openings.
- In wells drilled with freshwater, naturally occurring clays can become incorporated into the drilling fluid and plug the pore space of aquifers.
Aquifer damage to some degree is unavoidable regardless of which drilling method is used.
Development Objectives
The objectives of development procedures are to:
- Reduce the compaction and intermixing of grain sizes produced during drilling by removing fine sediment from pore spaces adjacent to the borehole.
- Increase the hydraulic conductivity of the previously undisturbed aquifer near the borehole by selectively removing the finer fraction of aquifer material.
- Remove the filter cake or drilling fluid that coats the borehole.
- Remove the drilling fluid and solids that have invaded the aquifer.
- Form a graded zone of sediment around the screen in a naturally developed well, and thereby stabilizing the aquifer so that the well yields water attaining a suspended sediment load criterion.
- Create an environment that reduces the potential for bacteria growth, thereby increasing the life of the well.
- Remove fine-grained cuttings from the borehole wall and fracture openings.
Studies conducted by the State of Michigan concluded that wells that were properly developed had fewer problems with positive coliform tests. Further work by Schnieders (2003) confirmed the findings of the Michigan study, and showed noflow areas (e.g., zones clogged with drilling fluids) near the screen are locations for enhanced bacterial growth.
Thus, wells that are not properly developed tend to have biofouling problems more frequently than do properly developed wells.
Proper well development ensures the production of water of acceptable sediment concentration, maximizes well efficiency, reduces a well’s maintenance costs, and increases the service life of a well. It is covered in great detail in chapter 11 of Groundwater & Wells, Third Edition.
Well Completion Methods
I believe one factor that has resulted in wells not being developed as well as they have been in the past is the rotary drill. Because the cost of the rotary drill is many times more than a cable tool rig, it is expensive to spend a lot of time developing a well with a rotary rig.
This type of equipment is making money when drilling and completing wells. Cable tool rigs are not as expensive to operate, so more time was devoted to developing a well.
The most common well completion methods used are natural development, filter packing, use of pre-packed screens, and open borehole (no screen is used). The particular completion method is selected based on the geologic character of the aquifer. To some degree, the completion method determines the effectiveness of specific development methods.
Natural Development
The goal of natural development is to create a more-permeable zone or area of enhanced hydraulic conductivity around the screen in the aquifers. During development in the zone just outside the well screen, development removes most particles smaller than the screen openings, leaving only the coarsest sediment in place.
Farther out from the screen, some medium-sized grains remain mixed with the coarse sediment; beyond that zone, the material gradually grades back to the original character of the aquifer (Figure 2).
Finer particles brought into the screen during natural development are removed by bailing or pumping, and development continues until there is negligible movement of fine sediments from the aquifer into the well.
Filter Packing
In the filter-packing process, a specially graded sand or gravel—having high porosity and hydraulic conductivity—is placed in the annulus between the screen and the aquifer.
During the development of a filter-packed well, mechanical energy is directed through the screen and filter pack. The energy impinges on the walls of the borehole and removes the fine sediments from the aquifer and from the finer fraction (5% to 10%) of the filter pack.
As the sediments migrate into the well, the filter pack settles and bridges are removed, leaving material that grades from fine to coarser from the aquifer to the screen. It is important to note that development of the disturbed aquifer outside the pack still is required to achieve maximum specific capacity.
Wells completed using a pre-packed screen have the benefit of a properly sized filter pack and have the advantage derived from having the aquifer collapse around the screen (Figure 3). This type of completion and subsequent development rectifies the borehole-related damage as the aquifer collapses, creating an efficient well (Hanna 2012).
Open Area and Slot Configuration
All development methods work best in wells equipped with screens having both maximum open area and a slot (opening) configuration that permits the greatest hydraulic forces exerted inside the well screen to be directed into the aquifer.
Screen open area typically varies from 1% (for perforated pipe) to more than 40% (for continuous-slot, wire-wrap screens). Screens with maximum open area can be developed more effectively because more of the development energy can reach the aquifer.
Selection of the correct slot size for well screens also is essential for successful well development. Slot openings are chosen to permit maximum removal of the fine material from the aquifer.
For naturally developed wells, it is common practice to select a slot width that retains approximately 40% of the sediment in the aquifer adjacent to the screen. For filter-packed wells, the slot openings are selected to retain approximately 90% to 100% of the filter-pack material based on the goals of the well design. We will cover this in a future column.
It is important to note that removal of too much sediment can cause the overlying geologic materials to settle. This can have undesirable effects on the well and can create dangerous conditions (e.g., land subsidence) for the drilling rig. There have been drill rigs that have either tipped over or fallen in a depression caused by excessive sand removed during development.
Conversely, when well screen openings are too small, adequate development might not be possible and the well yield will be below the potential of the aquifer. Greater entrance flow velocities caused by inadequate development or slot size (open area) and the corresponding pressure drop near the wellbore can lead to the formation of mineral precipitates in the screen, filter pack, or adjacent aquifer.
I have seen wells completed with 6-9 filter pack (approximately 0.08- to 0.16-in) and 0.040-in slot screens when the only purpose of the screen is to hold the filter pack in place. In this case, a 0.080 slot screen would retain almost 100% of the filter pack and it would be far easier to develop the well and get energy through the screen back to the borehole wall.
Slot configuration also controls both how much development energy reaches the aquifer and the area of the aquifer that the energy can affect. More fine-grained material can be removed faster if the available energy can be directed at most or all of the surrounding aquifer.
Larger slot sizes and greater open area create wells that both minimize head losses through the screen for a given pumping rate and ultimately are more efficient. For a given pumping rate, large open areas help to maintain entrance velocities at the optimal 0.1 ft/sec (3 cm/sec), thus sustaining laminar flow to the well.
If larger slot sizes are used, then—in the attempt to achieve maximum specific capacity and well efficiency—longer development time should be anticipated (Wendling et al. 1997). See Figure 4.
People often ask how much time is required to develop a well. There are many variables but based on the work completed by Wendling et al. my first thought is you need to spend at least five hours working over the screen.
So, for a well that has 100 feet of screen and your tooling is capable of developing or working on 20 feet at a time, you would anticipate at least 100 hours of development. That’s a good place to start.
Bridge-slotted, slotted pipe (including PVC) and louvered screens require significantly more time to develop because of their low percentages of open area, and because the shape of their openings causes dissipation of more of the development energy before the energy can reach the filter pack and aquifer. The slot configuration of some screens can impede transfer of energy from the well to the aquifer (Figure 5).
Drilling Fluid Type
Bentonite and polymers are the two primary drilling fluid additives used. After a well is drilled, all drilling fluid should be removed from both the borehole walls and the aquifer via either physical or chemical means.
Although some polymeric drilling fluid additives are designed to break down naturally over time, it is recommended that they be broken down chemically and removed from the well at the time that the well is completed.
The rate and effectiveness of drilling fluid removal depends not only on the type of additive used but also on the physical character of the aquifer, the depth of the well, the length of time the drilling fluid has been in the borehole, and the weight and viscosity of the drilling fluid.
Results from an experimental wellfield show that a large amount of development energy is required to remove drilling fluid containing clay additives. Significantly less development energy is required to achieve maximum specific capacities for wells drilled using polymeric drilling fluid additives that have been broken down.
Filter-Pack Thickness
I often get questions as to how thick the filter pack should be . In general, you want to keep it relatively thin—but still be able to place it in the annulus without bridging or having the screen rest against the borehole wall, leaving an area without proper filtration.
The thickness of the filter pack has considerable effect on development efficiency, and the filter pack reduces the amount of energy reaching the aquifer. The thinner the filter pack, the greater the amount of energy transferred to the aquifer to remove undesirable sediment during development.
As such, a filter pack generally is designed to be more permeable than the aquifer materials. If the development energy is not properly focused through the filter pack to the borehole wall, then the energy might disperse and flow vertically in the filter pack rather than moving directly into or out of the aquifer.
To facilitate the maximum transfer of development energy to the aquifer, filter packs normally should be about 2 inches to 5 inches thick.
To a large degree, the annulus size is determined by the minimum borehole size needed to properly complete the well, centralize the casing and screen, and install the filter pack. I have seen wells that have filter packs that are thicker than 6 inches that were difficult to develop. These wells were eventually developed but at a significant cost due to excessive development time.
Summary
The development of the well starts at the time of the well design. If you think through how the well will be completed, the type of drilling fluids, the hydrogeology of the aquifer, and consider how the well will be drilled—you will be able to better plan how to develop the well.
In our next column, we will discuss the process of well development. See you then.
References
Hanna, T.M. 2012. Design and Completion of Mine Water Wells and the Application of Wire-Wrap Pre-Pack Screens. Paper at 3rd International Conference on Water Management in the Mining Industry, Santiago, Chile.
Schnieders, J.H. 2003. Chemical Cleaning, Disinfection and Decontamination of Water Wells, New Brighton, MN; Johnson Screens.
Sterrett, R.J. 2007. Groundwater and Wells, Johnson Screens, New Brighton, MN.
Wendling, G., R.P. Chapuis, and D.E. Gill. 1997. Quantifying the Effects of Well Development in Unconsolidated Materials. Ground Water 35, No. 3: 387-393.
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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.