Part Two: Pumping methods and removing fine sediments.
By Thom Hanna, PG
We are covering the process of well development in two parts.
In part one in the May 2023 issue of Water Well Journal we discussed methods that generally are used to put energy into the aquifer to break down borehole damage. Now in part two we are going to focus more on methods that involve pumping in combination with methods to remove fines from the well.
As a brief review for those who have not read part one yet, multiple techniques greatly enhance development. One of the development methods should move energy alternatively in and out of the aquifer, and the second should be a method (such as airlift pumping, bailing, or pumping) that removes fine-grained sediment from the well.
A good development program consists of the following steps:
- Break down and remove drilling fluids that are in the aquifer, filter pack, and well.
- Mechanically surge or jet the aquifer and filter pack to rearrange the filter pack and aquifer grains so that fines are brought into the well.
- Pump the well to remove fine sediment and establish high rates of water flow into the well.
Water Jetting Combined with Simultaneous Pumping
Although water-jetting procedures are effective in dislodging material from aquifers, optimal development efficiency is achieved when water-jetting procedures are combined with simultaneous airlift pumping or another pumping method.
This combination is especially effective for wells completed with wire-wrap screens. Typically, the well is pumped using either airlifting techniques or a small submersible pump. The volume of water extracted should always exceed the volume injected during jetting and is recommended to be two to three times the jetting rate.
As development proceeds, the water level in the well should be kept below the static level to maintain continuous flow of water from the aquifer into the well. The steady inward movement removes suspended material loosened by the jetting operation before the material can settle.
The pumping can be accomplished by either airlift pumping (Figure 1) or a submersible pump can be used. Typically, the pump is placed far above the jetting tool to minimize sand movement through the pump; this placement allows more sediment to fall to the bottom of the well. Sediment must be removed periodically during the jetting operation.
Developing by Surging and Pumping with Air
The practice of alternately surging and pumping with air is a common development technique.
Surging occurs when air is injected into the well to lift water toward the surface. The air source is shut off as water reaches the top of the casing, which allows the aerated water column to fall back into the well. This creates an outward surge of hydraulic energy through the well screen that aids development. This type of development is more effective in wells with shorter screen sections.
Installing an airline inside an eductor pipe enables airlift pumping to be used periodically during development to remove sediment from the screen. Eductors generally are required for large-diameter wells when limited volumes of air are available or when the static water level is low in relation to the well depth.
Most rotary rigs, however, have sufficient air capacity to use the casing as the eductor for 6-inch to 12-inch-diameter wells. Figure 2 shows the basic layout of an airlift system.
Uphole velocities in the range of 1000 feet/minute to 2500 feet/minute are required to achieve reasonable discharge rates and sand removal.
Generally, predicting actual uphole-velocity requirements is difficult due to submergence factors, total pumping lift requirements, and the unpredictable way that water enters a borehole. If the air volume needed to lift the water adequately is achieved, however, then the uphole velocities are not an issue.
The type of discharge produced from a well during air development depends on the air volume available, total lift, submergence, and annular area. These factors result in the existence of various types of flow regimes of multiphase flow.
Figure 3 presents four common flow regimes that occur when airlift pumping is performed. The percent submergence, total lift, and capacity of the compressor all control the relative proportion of air and water for a particular well.
Some of the difficulties faced when making a rigorous analysis of systems include the following:
- Introduction of a small volume of air under a high head causes little change in the water level in the well. In the present case, the air pressure available is sufficient to overcome the initial head exerted by the water column.
- As air volume increases, the water column becomes partly aerated. Displacement of the water by the air causes the water column to rise in the casing. Drawdown does not change because no pumping is occurring.
- Further increases in air volume cause aerated slugs of water to be lifted irregularly out the top of the casing. Between surges, the water level in the casing falls to near the static level.
- If sufficient air is available, then aerated water continually flows out the top of the well. With average submergence and total lift, the volume of air versus water is about 10 to 1. Higher air volumes can increase the pumping rate somewhat, but additional increases actually can reduce the flow rate because flow into the well is impeded by excessive air volume.
Air pressure and volume both are important in initiating and maintaining an air-surging or airlift pumping operation. For typical head conditions found in wells 300 feet to 400 feet deep, the compressor used for the air supply should be capable of developing a minimum pressure of 125 psi.
This is sufficient to overcome the initial head created by the submergence of the airline and is called the starting submergence. After the pressure initiates flow, the air capacity (volume) becomes the most important factor in successful airlift pumping. A useful general guideline for determining the proper compressor capacity for airlift pumping is to provide about 0.75 cfm of air for each 1 gpm of water at the anticipated pumping rate.
The volume of air required to operate an airlift efficiently depends on the total pumping lift, the pumping submergence, and the area of the annulus between the eductor and casing. Acceptable results can be obtained with a pumping submergence as low as 30%.
The volume of air needed to lift water also depends on whether an intermittent flow or a steady flow is required. For development work, it is not necessary to maintain a steady discharge, and in fact, some surging of the air is beneficial. If steady flow must be maintained (e.g., in pumping tests), then the air-volume requirements are the minimum of those given in Table 1.
Table 1 lists the recommended sizes of eductor pipe and airline for airlift pumping. Some variation from these sizes might be necessary for practical reasons, but the combinations shown generally provide acceptable results.
Air development procedures should begin with the establishment of water flowing freely into the screen. When the aquifer is clogged, the application of too much air volume can result in a collapsed screen due to significant fluid pressure differentials between the aquifer and inside the well. To prevent accidental overload, the airline and the eductor (if used) can be placed at a rather shallow submergence (approximately one half of the available drawdown or less).
If this setting is used, then even the introduction of large air volumes produces only moderate differential pressures on the well screen initially. Introduction of small air volumes at greater submergence also produces low yields.
Due to the uneven nature of compressed air development—and the potential danger of personnel standing too close to an air-discharge pipe—the air-water discharge should be directed to a mud pit or cuttings pit in which a discharge channel or pipe has been installed. If possible, the discharge can be run through a flume to measure the discharge rate. This enables estimation of the discharge rate and sediment content.
When flow has been established, the eductor pipe can be lowered to within 5 feet of the bottom of the screen, or if preferred, development can start near the top of the screen. Air is released and the well is pumped until the water produced is relatively free of sand. Quickly opening the valve drives water outward through the screen openings, and occasionally water blows from the casing and eductor pipe at the surface.
Retracting the airline into the eductor pipe resumes airlift pumping, completing the surging cycle. Cycles are repeated until sand-free water is produced.
Double Surge Block with Airlift
Some drillers use a double surge block or isolation tool to remove sediment from the aquifer in conjunction with airlift pumping. Flanged gaskets are mounted on the top and bottom of the isolation tool. The gaskets should be sized so that they touch the sides of the screen but allow some sediment to move around them so that sand locking does not occur.
After the screen’s initial cleaning, the double surge block is lowered to the top of the screen and the airline is set at the proper depth. After each zone is developed by surging and airlift pumping, the tool is lowered to the next section.
The double surge block assembly that is attached to the drill pipe is raised and lowered in a screen section to produce turbulence. Thus, the aquifer is developed in separate stages. This tool also can be used to introduce dispersants into the aquifer and surge the dispersant into the appropriate interval.
The double surge block with airlift method is especially effective in long screens because it can concentrate the development energy on short sections of the aquifer. It is important to start the procedure at the top of the screen to avoid sand locking the assembly.
The double surge block is also a cost-effective tool for use in deeper wells because multiple development techniques can be employed without removing the tools from the well; and it is excellent for the injection of chemicals to disperse clays.
Double Surge Block with High-Pressure Jetting
Combination tools—such as a double surge block with high-pressure jetting tool—are used in deeper wells because the time to install and remove tooling for development can be significant (Figure 4).
Using this tool, wells are developed by alternating high-pressure jetting and airlift pumping. During high-pressure jetting, the check valve in the tool remains closed, forcing all the water through the nozzles. After jetting approximately 20 feet to 30 feet of screen, the same interval is swabbed and pumped by airlifting without changing tooling.
When airlifting, the check valve opens, allowing water and sediment to move through the screens and up the drill pipe. The packers on the tool isolate a, 6- to 10-foot section of the screen so that the full force of airlifting is focused on a smaller section to remove any sediment.
Overpumping is pumping a well at a rate greater than will be pumped when the well is put into service. This is an appropriate final step-in well development to ensure sand-free pumping and to confirm the proper sizing of the permanent pumping equipment.
Overpumping, like airlift pumping, alone seldom produces an appropriately developed well, full stabilization of the aquifer, or acceptable development throughout the screened zones, particularly in unconsolidated aquifers.
Most of the development action from overpumping takes place in the most permeable zones of the aquifer closest to the pump intake (which often is near the top of the screen). The longer the screen, the less effective the development in the lower portion of the screen at a given pumping rate.
After fine sediments are removed from the upper permeable zones, water enters the screen, moving preferentially through the developed zones. The rest of the well is developed insufficiently, contributing only small volumes of water to the well’s total yield.
In some cases, overpumping can compact finer sediments around the borehole and restrict flow into the screen. If more powerful agitation is not performed, then an inefficient well can result. Overpumping might be effective in filter-packed wells in competent, relatively non-stratified sandstone aquifers because flow toward the wellbore is somewhat uniform.
A drawback to using only overpumping is that water flows in only one direction—into the screen—and some sand grains could be left in a bridged condition, resulting in a partially stabilized aquifer. After the well is operational, the aquifer is agitated during normal pump cycles and if sand bridges become unstable and collapse, then sediment can enter the well.
During overpumping a surging action can be achieved by rawhiding, which is alternately lifting a column of water a significant distance above the pumping water level and then shutting off the pump and letting the water fall back into the well through the pump column.
The pump should be started at reduced capacity and gradually increased to full capacity to minimize the danger of sand locking. This cycle is repeated as rapidly as the power unit and starting equipment permit. The permanent pump should never be used for rawhiding because the high sediment content accelerates pump wear.
Development of Open-Borehole Wells
To create open-borehole wells that perform at maximum efficiency, the wells must be developed to repair borehole damage and remove drilling fluids that have invaded the aquifer. The combination of water jetting and airlift pumping is recommended for open-borehole or bedrock wells.
Inflatable packers can be used to isolate productive zones so that development efforts can be focused on certain portions of the borehole. It has been shown that much of the water entering an open borehole in bedrock enters through fracture zones.
One technique for cleaning wells completed in sandstone aquifers combines airlift pumping, air jetting, and rawhiding, which overcomes development difficulties in stratified aquifers having layers cemented by silica, calcium, iron, or fine material such as clay.
Ledges form where the sandstone is most resistant and well-cemented, and borehole enlargement occurs where more friable layers erode. In some portions of the country explosives are used in friable sandstones to create a cavity in the aquifer. The formation is blasted, and the sand is removed. Upon removing a significant amount of the formation, the velocities will be slow, and the cavity is stable under the low velocity conditions.
If the well is put into production after bailing or air development, it often continues to produce sediment. This problem stems from the inability to remove all the loose material in the well because the development procedures do not extend far enough from the wellbore.
Borehole camera surveys have shown that this loose sediment is removed well away from the borehole when airlift pumping, air jetting, and rawhiding are used in combination.
Allowable Sediment Concentration in Well Water
Sediment in water supplies can be destructive to pumps and to water-discharge fittings such as valves and irrigation nozzles. One of the requirements of a production well is that it does not produce sediment; however, occasionally sediment production occurs.
The American Water Works Association suggests that water should contain less than 5 ppm of total suspended solids by volume for municipal supply wells. However, for many applications varying concentrations can be required. Pump manufacturers suggest that less than 1 ppm by volume is acceptable to minimize pump damage and increase service life.
The concentration of suspended sediment usually is estimated using a centrifugal sand sampler or an Imhoff cone.
The Imhoff cone is considered less accurate because of the small and instantaneous sample volume used. Accuracy of the Imhoff cone method can be improved by increasing the frequency of sampling.
The sediment concentration is determined by averaging the results of five samples taken during an aquifer test after the well is installed at the times listed below.
- 15 minutes after the start of the test
- After 25% of the total aquifer-test time has elapsed
- After 50% of the total aquifer-test time has elapsed
- After 75% of the total aquifer-test time has elapsed
- Near the end of the aquifer test.
Such averaging considers the fact that at startup wells can have some sediment pumping that dissipates over time. It is helpful to record and graph this information to help determine whether further development is necessary.
A water sample must be of reasonable volume to obtain accurate measurements. The following is an example of how to collect a representative volume of water.
The amount of sediment collected in each sample can depend on how the water sample is collected. For flow from a straight, horizontal pipe, most of the sediment can be at the bottom of the pipe; if the sample is collected there, then the sediment concentration is higher than the average of the total stream. Conversely, a sample taken at the top might show a lower-than-average concentration. For accurate results, samples of the entire flow should be collected over a specific time interval.
Collection of an integrated sample becomes difficult as the flow volume increases. With large flow rates the use of a centrifugal sand sampler is recommended. When using a centrifugal sand sampler, the sample should be collected through a connection at the midline of the pipe. The sediment in the sample should be allowed to settle out, and then the sample should be weighed and compared with the total volume of water collected. In cases when the control of suspended solids is critical, automatic particle counters are used to measure the amount of sediment. Particle counters commonly are found in municipal water supplies and water-treatment plants.
The acceptable sediment concentration depends on the use of the water. The National Ground Water Association recommends the following limits, most of which are widely accepted in the water well industry.
- Sediment concentration of 1 ppm (by volume) for water to be used directly in contact with, or in the processing of, food and beverages
- Sediment concentration of less than 2 ppm (by volume) for wells discharging directly into municipal water treatment of distribution mains
- Sediment concentration of 5 ppm (by volume) for water for homes, institutions, municipalities, and industries
- Sediment concentration of 10 ppm (by volume) for water for sprinkler irrigation systems, industrial evaporative cooling systems, and any other use where a moderate amount of sediment is not especially harmful
- Sediment concentration of 15 ppm (by volume) for water for flood-type irrigation in applications where the amount of sand pumping will not harm the well and pump.
In many instances, an Imhoff cone or sand tester is not available at the well site. However, a 5-gallon bucket can be used to collect sediment and obtain a rough estimate of sand content. In a bucket that size, 10 ppm (by volume) equals approximately 0.04 teaspoons (tsp) of sediment, which is approximately the amount of sand that can cover a dime (or a circle that has a 0.7-inch diameter).
A successful development program consists of being mindful of how important well development is to the success of a well project. This starts during the design phase of the project in determining what the drilling method will be, the type of drilling fluids, and how the drilling fluids are broken back.
The process used includes the type of development tools to be used and what methods will be the most effective to the given completion.
Sufficient time needs to be taken to properly develop a well along with multiple development techniques that ensure that energy is getting back to the borehole wall to remove the filter cake that is needed to drill the well. Development is not considered done until the well is producing sand-free water at the anticipated pumping rate.
As we discussed in the previous column, it does not matter what you use for well construction materials if you do not properly develop the well. It will be inefficient and have more maintenance issues over its life and be a well that is expensive to own.
Sterrett, R.J. 2007. Groundwater & Wells, Third Edition. Johnson Screens: New Brighton, Minnesota.
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 firstname.lastname@example.org.