Part 4: Rehabilitation techniques and improving efficiency in wells
By Ed Butts, PE, CPI
We’re going to explore an important aspect of continuing operation of water wells. Namely, outlining the well rehabilitation techniques that stabilize or lower the pumping lift component of total head and improve overall well performance—and some of the various methods used to conduct this process.
If anyone has ever wondered if regular well maintenance or periodic rehabilitation programs were worthwhile investments, consider studies have shown an annual or biannual well maintenance program usually costs between 10%-20% of the cost required for a full-scale rehabilitation procedure. But a typical full-scale rehabilitation process can cost up to 20%-30% of the cost for a new replacement well with the same size and depth.
A well should generally receive some type of maintenance if the relative yield or specific capacity falls to or is more than 2%-5% from its new or previously tested state. It should be rehabilitated if the relative yield or specific capacity declines more than 15%-20% from its previous condition. Any drop more than the values just stated can result in a “situation of no return” where the best and most aggressive rehabilitation techniques may not be capable of recovering more than 75%-80% of the original yield.
During the previous three columns, we have reviewed and defined many basic concepts of well water chemistry and various electrochemical, biological, chemical, and physical causes of declining well and pump efficiency. Over the next three columns, we will continue examining this topic by detailing specific areas where we can evaluate the individual losses occurring in wells and pumping systems to determine the overall efficiency of each component along with the system in its entirety, and various ways to improve the efficiency of new and existing wells and pumping plants. In this discussion, we will review the specific elements comprising and defining a well and aquifer’s efficiency and the many methods used to improve these two elements for new and existing wells.
Aquifer and Well Efficiency Defined
The efficiency of an aquifer or well as separate values is not only difficult to define, but difficult to effectively control or remediate once the well has been constructed and in service for some time. With an aquifer, generally what you find is what you get, and the so-called “efficiency” of an aquifer is often defined as the loss or drop in the static water level between the immediate exterior region of a pumping well vs. another non-pumping production or observation well at a fixed radius from the pumped well.
On the other hand, the efficiency of a well is generally considered as the vertical difference in the dynamic water levels occurring between the interior of the well casing (pumping water level) vs. the associated water level demonstrated within the immediate exterior region of the well or filter pack, both during operating conditions.
Figure 1. Ideal water well at variable flows (Q).
A slight difference between these points during static and pumping conditions is often an accepted and natural occurrence due to the hydraulic gradient of the underground water surface. In many cases, a significant increase or change in this gradient during pumping conditions may not be due to any real loss of actual well efficiency. It may be the individual or combined result from a higher well pumping rate (Q2 vs. Q1, Figure 1), local aquifer dewatering, well to well interference, regional overpumping, changes in the aquifer head or storage coefficient, the relative capacity and recharge rate of the aquifer, or the water flow (transmissivity) qualities of the materials comprising the specific type of aquifer in use.
As an example: All aquifers have a limitation as to the daily, seasonal, and yearly volume and the instantaneous flow rate that can be transmitted to a given well. This is largely based on the aquifer thickness and well depth and the available interface to the aquifer (the type, thickness, shape, gradation, and binding of the aquifer’s granular material vs. the inlet screening area that defines an unconsolidated aquifer). For a semi- or fully consolidated formation, the variables include: the type and thickness of the formation, degree and type of exposure between the aquifer and well, as well as the location, number, and size of faults, fractured area, fissures, and “seams” (open area) of the formation adjacent to or within a reasonable transmission distance from the pumping well to ensure flow continuity during pumping conditions.
Even though we cannot generally or easily control or modify many of the aspects or characteristics of the various types of aquifers we encounter and pump from, we have a definite control on the development and efficiency of the well structure itself. Defining a precise determination of well efficiency often means different things to different people. But most would agree a fundamental definition should be something like “the efficient transmission of whatever water is available from the aquifer into a pumping well at the lowest practical loss of head.” That’s as easy as it gets in my opinion.
Figure 2. Encrusted minerals on a totally plugged well screen.
In cases where the well efficiency is impacted due to frictional losses within the well screen or perforations, the loss is usually associated with an inadequate inlet area for the capacity required or a restriction within the well’s inlet areas. This is shown in Figure 2, which demonstrates a severely plugged well screen. Either situation leads to an increase in the entrance velocity.
As with most definitions of efficiency, well and aquifer efficiency are separate values, each defined as a lowered percentage from a theoretical base of idealized conditions, usually 100% (1.0) as a “gross” value. Losses of efficiency in each are individual, but cumulative, until a final decimal result is obtained for each.
For example, assuming a net (or corrected) well efficiency of 60%, the efficiency used in calculations would be likely stated and entered as a ratio or: 0.60 (60%/100%) with the base ideal value of 100% equal to the mathematical value of 1.0.
Although we outlined various methods commonly used for well rehabilitation in Part 3, I reserved a more detailed discussion for now when I had the space to adequately cover the topic. When scheduling rehabilitation of a specific well is initially considered, selecting one of two fundamental potential choices must be addressed and acted on:
(1) Is the well rehabilitation to be planned with the well pump in place, limiting access to the well screen? (2) Will the well pump be fully removed from the well, providing full access to all internal well areas?
Obviously, this “either/or” decision is paramount as to the process and time allotted for well rehabilitation for more than one reason—as removing the well pump also generally means the pump will undergo a parallel rehabilitation or replacement process. This will usually restore or, at the very least, partially elevate the efficiency, head, and capacity potentials of the well pump.
In addition, unless a loaner or rental unit is temporarily installed, removing the well pump usually indicates the well is planned to be out of service for some time. This is true for most irrigation wells operating seasonally and many municipal wells where system redundancy from alternate sources is usually available and the rehabilitation and pump repair processes are concurrently planned for the lower water consumption months of fall to spring.
This scenario typically indicates a permissible interval of up to one to three months is available for a combined well rehabilitation and pump repair procedure.
In many cases, repair of the well pump is deferred to the end of the well’s rehabilitation and retesting. This allows the well contractor, and in turn, the owner or client to fully consider all aspects of the reclaimed or adjusted well yield before proceeding with an expensive and possibly unnecessary pump repair or replacement, where downsizing may be indicated due to a permanent loss of well yield.
Generally, this process involves an additional two to three months to execute, which when combined with a well rehabilitation in advance will total around three to five months of downtime. This is well within the allowable time span for most irrigation or municipal systems (with source redundancy). If a replacement well is warranted, an additional five or six months of downtime should be assumed in most cases. This may create a total downtime (or loss of source) period for up to 12 months.
Figures 3a and 3b. Typical well and pump installation (vertical turbine pump) on the left and typical well and pump installation (submersible pump) on the right.
Rehabilitation Procedures with Pump in Well
In addition to the scheduling aspects of rehabilitating a water well or well pump, deciding if the well pump will remain in place or removed from the well has a tremendous impact on selecting the methods, chemicals, and time for the actual process.
If the plan for well rehabilitation or maintenance assumes leaving the well pump in the well throughout the process, the methods and materials that can be used by their nature and space constraints must be able to be introduced either down the interior of the wellhead, pump column/riser pipe, and bowl assembly (Figure 3a—for those types of pumps without a check or foot valve) or fit within the annulus between the pump column or riser pipe, check valve, and bowl assembly (and motor, for submersible pump installations).
In some cases a PVC or other material water observation (sensing) tube has been placed in the well during the original pump installation. This can often be used in introducing certain chemicals into the wellbore and toward the screened area (Figure 3b). Always verify any chemicals planned for use will not be injurious to the material comprising the sensing tube.
However, this type of installation greatly limits direct physical access to the well screen and casing and forces chemical treatment to be considered as the primary or sole treatment method along with using the well pump for well surging, pumping, chemical dilution and removal, and final flow testing.
In some cases, particularly with those using submersible or vertical turbine pumps with check or foot valves, well surging or backflushing through the column pipe and bowl will be severely limited or not possible. In situations where the well pump is expected to be left in the well during service or rehabilitation, it is also important to only use chemicals and methods that will not be injurious to the components or surfaces of the well pump and well itself.
For example, this is especially critical when multiple metals or alloys are used in the well. The mixed use of bronze, brass, cast iron, and black or galvanized steel components is common in most well pumps and wells. When acids are used, this typically means the acid must be blended or provided with an inhibitor specifically designed not to attack the metals in the well. Always verify the inhibitor is not designed or intended to protect one type of metal only and not all the metals present in the well.
Rehabilitation Procedures with Pump Out of Well
If the well pump and all related equipment is removed from the well during rehabilitation, the available options for conducting the rehabilitation and the likelihood of success are both greater than with the well pump left in the well. This is due to the relatively larger working area and access to the full depth of the well afforded by removing the well pump and the ability to get into and work over the entire well screen or perforated section.
Before starting any meaningful major well rehabilitation, it is vital a careful and complete analysis of the groundwater’s scaling or corrosive potential and water chemistry is conducted along with a downhole video inspection.
A water quality examination—by sampling water obtained before the well pump is pulled—and a video inspection can provide needed guidance to determine if the well problem is most likely due to physical, biological, corrosion, or chemical issues.
This will be invaluable when selecting a rehabilitation method. Even the costs necessary to bring a firm in from another city to conduct the video inspection will pay dividends far beyond the video cost itself, both in the well contractor’s time and effort and the client’s money.
For one thing, conducting a video inspection before and after a well rehabilitation and recording the process provides a permanent visual record of the current condition of the well including depth, casing, screen, or perforation sizes and intervals, static water level, and the relative condition of the casing, welds, well screen or perforations, and other internal well components. This type of record will also be invaluable to the future generations of well owners and contractors contemplating any future work on this well.
The amount of information usually gained from a simple one or two-hour video examination cannot be overstated. In addition to a direct observation and confirmation of the physical conditions and any possible existing damage to the well, a video inspection usually permits a visual examination of possible isolated regions of high entrance velocity occurring through the well screen or perforations (often indicated from the appearance of shiny or very clean surfaces), mechanically blocked or severely plugged screens or perforations, and the likely areas of water transmission and where to best concentrate rehabilitation efforts.
In my opinion, there is virtually no excuse or negative reason not to conduct both a water quality examination and downhole video inspection of the entire wellbore and well screen before embarking on a formal well repair on a high value well.
Methods of well rehabilitation when the well pump remains in the well typically means the use of chlorine, acids, surfactants, dispersants, and other chemicals. Chlorine and various acids have been used for decades, often with mixed success due to the uncertainty of solution strength, mixture uniformity and distribution, and penetration into the screen and filter pack.
When properly used, however, a success rate of recovering up to 80%-90% of the well’s original specific capacity is not uncommon. It should be noted well plugging due to mineral encrustation alone is relatively rare when not accompanied by some form of bacterial or biofilm growth. Therefore, the proper use of an appropriate biocide along with an acid is recommended for complete and effective dispersion, especially in alluvial and many semi-consolidated or marine formations. Chlorine works best when used as part of a preventive maintenance program and for bacterial control.
Acids lower the water’s pH sufficiently to dissolve scale, and when combined with a biocide, biological growth on well screen and casing surfaces. They also work when properly used and distributed within and into the filter pack. There are several powerful acids that can and have been used in water well rehabilitation for decades. These include muriatic (high-strength hydrochloric acid), sulfamic, hydroxyacetic (glycolic), and sulfuric acids. Each acid has advantages and disadvantages in use and application and contractors are advised to determine the best type of acid to use only after an initial water quality examination has been completed for the specific well conditions, material, and construction.
All acids also have potential safety issues associated with their use and combining with other chemical agents. Therefore, proper caution and care should always be applied when mixing or combining any chemicals or when using chemicals at all and all safety precautions related to handling, mixing, injection, and disposal must be observed.
I have also used another unique method of rehabilitation chemical injection over the past years with excellent results. While I was working in Alaska in 1978, I was introduced to a treatment method the local drillers had used for years that works for applications with the well pump either in or out of the well.
The method uses a heated chlorine solution injected into the well, either directly down the borehole or within the annulus between the well pump and casing. A 65%-70% strength of dry calcium hypochlorite (swimming pool chlorine) is used and mixed into a 55-gallon drum with preheated water developed from a steam cleaner to create a chlorine solution of about 500 mg/L strength (using nearly 6 ounces of 65% strength dry chlorine per 55 gallons of water) at 170°-180°F degrees, 30°-40°F below boiling.
The solution is created and thoroughly mixed in the drum while the chlorine is slowly introduced and mixed with the hot water. Upon complete mixing, the solution is then introduced into the desired screen or casing region needing the most treatment, preferably using a steel tremie pipe. A contact time of at least four to six hours along with periodic agitation of the water/solution mixture is recommended before diluting and pumping the solution out of the well. The process may have to be repeated several times and concentrated in the specific region with the thickest encrustation for best results.
I have used this process numerous times over the past 40 years on wells with severe biofilm growth as well as in wells with residual old lineshaft lubricating oil floating on the water surface from oil-lubricated pumps with excellent results—often superior than when acids were used.
We conducted downhole video inspections on several of these wells before and after treatment and the difference was usually remarkable. In many cases the area of treatment previously plugged or layered with biofilm or scale was now open and “clean as a new rifle barrel” (to quote one happy well owner).
As with all treatment methods, safety procedures must be observed and adhered to when using this technique or any other process using chemicals of any type. For one, never mix or prepare a chemical solution inside an enclosed building or environment, without adequate ventilation, and without full knowledge of the impact the chemical mixing may have. The vapors generated during mixing and preparation of a chemical solution are usually potentially hazardous to health and will cause breathing problems in an unventilated or enclosed space. Appropriate safety goggles, gloves, and equipment must also be used when handling, mixing, and injecting any chemical solution.
Space does not permit a full treatment of every available chemical option and the procedures to use. As always, I recommend enlisting and heeding the advice of knowledgeable and experienced individuals and well contractors who have had previous success with situations similar or exact to your application and the use of the best chemicals for the specific well.
Figure 4a. Well brush detail.
Most mechanical methods are most effective when used for those wells where the well pump has been removed from the well. Once the well pump has been pulled and a downhole video inspection performed, I generally recommend most wells receive an initial brushing of the well screen and casing/perforated interior surfaces. As seen in Figure 4a, the brush can be easily shop-built using unwound wire rope strands for the bristles. The unwound strands are placed between two packer plates or flanges to create a brush for the full diameter of the well screen or casing and compressed to hold them in place.
A cable-tool machine or pump hoist with a pitman arm works
Figure 4b. Well redevelopment by mechanical swabbing/surging with a surge block.
best for this type of well work. The brush is lowered on the drill or pipe string and alternately raised and lowered to dislodge built-up growth and sediments on the
internal well casing and screen surfaces. Jars are sometimes advised to be used to permit dislodging of the brush in tight or misaligned wells. Most of the scraped material will dislodge and settle, so any accumulated material can then be bailed or airlifted from the bottom of the well. Typically, an initial brushing of the well casing and screen will remove enough material to save at least one chemical treatment and can also provide better access to the filter pack and any potential plugging existing on the outside of the screen or within perforations for treatment.
Another word of caution. Verify any metal present in the well
Figure 5. Well redevelopment using high-velocity jetting.
screen or casing is not severely corroded or a PVC screen was not used where an aggressive brushing could possibly make things worse from the brush slapping against, damaging, or even destroying plastics and already weakened materials.
In these situations consider using a smaller diameter or alternate, weaker material for the brush, such as PVC or copper wire bristles, or scouring using high pressure water or air.
Beyond brushing, other mechanical methods, especially
Figure 6. Common airlift method.
swabbing, can also be very helpful with distributing a chemical solution throughout all surfaces and within the region of the filter pack surrounding the well screen or perforations.
The use of either well swabbing or surging (Figure 4b) or jetting (Figure 5) can be advantageous with a well screen, especially for chemical distribution and mixing and well redevelopment. Well surging using a surge block, airlift (Figure 6), or alternate cycles of starting and stopping well pumping seems to work best for perforations as most of the energy imparted from jetting is generally a hit-or-miss proposition.
Additional Rehabilitation Methods
There are various other well rehabilitation methods that don’t fit neatly into either conventional chemical or mechanical methods. These include down-well methods imparting sonic waves (sonar-jetting) onto screen surfaces which loosen and dislodge scale, silt, and other impinged material. There are also other methods using small explosive charges, combinations of chemical and mechanical methods, and various proprietary and many home-grown methods tailored to work within a specific marketplace and well and aquifer type.
Figure 7 a, b, and c. Standard jetting assembly with packers (left). Isolated well rehabilitation using vertical turbine pump method (center). Submersible pump option (right).
One specific method I have had good success with is concentrating the chemical and rehabilitation efforts in shorter screen or perforated regions by jetting with or without the aid of a well pump. This is performed using inflatable rubber packers that isolate the process to shorter and separate intervals from as short as 2 feet up to 20 feet or more at a time (Figure 7a). Although this takes more time and work than most alternate efforts, the higher cost is generally returned due to better and more precise results.
Depending on the application and degree of screen/perforation blockage, a pump can also be placed within the two packer intervals, allowing isolated treatment of specific screened or perforated zones using either a vertical turbine pump (Figure 7b) or submersible pump (Figure 7c). This process, when used with a well pump, provides the water and pressure necessary for jetting and also permits test pumping of isolated screen or well sections, which I have found to be invaluable when deciding where to concentrate rehabilitation and redevelopment as well as gauge the effectiveness of our efforts.
Another method uses water treatment injected to and within the aquifer itself (in situ) from an upgradient well during pumping and conditions the water before traveling to and impacting the problem well. This method, called Vyredox, provides oxidation to iron and manganese ions in the aquifer, resulting in precipitation of these elements well before they reach the pumping well. This is an expensive and potentially long-term solution to a well problem and requires consideration of specific features to work and access to an upgradient site for oxygen creation and injection.
A method well received in many regions of the United States is the use of carbon dioxide (CO2) injection. This process has reportedly been used on wells throughout the country with favorable results, although the process at the present time is site specific and somewhat geographically limited and therefore more expensive for many situations. I haven’t used this method so I cannot provide any input as to its effectiveness, but I look forward to becoming better acquainted with it.
Most methods of well rehabilitation are capable of working “somewhere.” The key is recognizing the specific groundwater chemistry and precise chemical processes and exchanges unique to and occurring in the aquifer affecting the particular well. You must identify the abilities, limitations, and available and potential qualities of the well in question, and the balance between expending a reasonable cost for the chemicals, equipment, and manpower resources required for an effective rehabilitation process with the likelihood of success. However, there are four fundamental tenets I can state with reasonable certainty.
- A well rehabilitation process must be tailored to and designed around the precise characteristics and chemistry of the water in the aquifer along with the specific construction and use patterns of the well for the greatest chances of success. Conventional and alternate chemicals, methods, and procedures should be evaluated with the ultimate method selected based on: safety considerations, the specific well and aquifer type and construction, local availability, past experience and degree of success, cost, recommendations from peers, and the background groundwater quality.
- When properly performed, a regular well monitoring and maintenance program will usually delay the greater expense and downtime associated with a major well rehabilitation, and in some cases may offset the need indefinitely, and should be conducted for all wells at the required or recommended frequencies
- Most wells should receive a redevelopment process immediately following or together with the rehabilitation procedure. This will aid in redistributing the natural or artificial filter pack material surrounding the well and improve flow distribution and uniformity throughout the well screen or perforations, lower pumping head, and prevent sand pumping. Well redevelopment is almost always most effective when conducted in a two-way fashion, where water is alternately moved back and forth through the well’s inlet openings and filter pack. This greatly assists in the loosening, dispersion, breakdown, and ultimate removal of scale, sand, gravel, silt, and other fine material as well as redistributing and redeveloping the filter pack.
- Always remember for our purposes, efficiency is related to how much energy is actually used to move a given volume of water against a given value of resistance or head vs. the amount of energy required for this purpose. Optimizing this fundamental tenet throughout the entire process may allow you to gain what appears to be incremental changes, that in totality may end up a significant improvement.
Next month, we will start to review the procedures and testing methods for determining the efficiency of a pumping plant and its individual parts.
Until then, work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at firstname.lastname@example.org.