Section 10Well Development
The well screen (section 7) is the "heart of a well" and the filter pack (section 8) acts as the "lungs" passing water to the screen! However, after drilling a borehole and installing a casing and filter pack, it is necessary to get the "heart pumping" and the "lungs breathing" since the drilling fluid forms a thin layer of mud on the sand grains of the borehole wall and is forced into the pore spaces and cracks in the aquifer. This plugging effect decreases the flow of water into the well.
By ensuring that wells are developed to the best possible technical standards, fewer boreholes will be required to meet the total demand and the wells will be less likely to fail within a few years (Moffat, 198?).
Development should continue until the discharge water is clear and all fine material from the well and adjacent aquifer have been removed. The time required for development depends on the nature of the water bearing layer, the thickness of screen slots relative to aquifer particle size, the amount of material rinsed from the well prior to placing the filter pack, and the type of equipment and degree of development desired. Large amounts of development energy are required to remove drilling fluid containing clay additives (Driscoll, 1986); well development may be completed in 1 hour, but up to 10 hours may be required (Brush, 197?).
The simplest but least effective development method is pumping a well at 2-3 times the designed discharge rate for a prolonged period. This does not really agitate the soil enough to create a real filter around the screen and it tends to develop only a short section of the length of screen (Anderson, 1993)(2). However, it is useful because if the well can be pumped sand free at a high rate, it can be pumped sand free at a lower rate (Driscoll, 1986).
If the water level is within 3.05 to 4.57 m (10 to 15 ft) of ground surface, it is sometimes possible to use the mud pump as a suction pump to pump water from the well for 2 to 3 hours. If this can be done, do not pump continuously: start-stop cycle pumping is best for developing a well.
If this is not possible, install the bush pump and use a separate cylinder for the development process since particulate matter removed during development can cause an abnormally high rate of wear on the pump resulting in early pump failure. Using a larger pump cylinder than planned for the final installation will enhance the effectiveness of the well development.
The effectiveness of overpumping can also be enhanced by attaching a rubber gasket around the top of the pump cylinder and lowering it into the well until it is adjacent to the top of the well screen. Start developing the well at the top of the screen so that fine material around the screen can gradually loosen and be pumped out of the well without jamming the pump! When pumping no longer produces sediment, the pump can be lowered several feet using specially made half length connecting rod and quarter length sections of rising main (also know as "drop pipe", "draw pipe" or "pump column"). The cycle of pumping until the water clears and lowering the pump further into the screened interval should continue until the entire screen has been developed. Attaching a second gasket 0.5 - 1 metres below the bottom of the pump cylinder would greatly increase the suction effect on the isolated sections of screen.
Backwashing: This too is a relatively simple method of development which requires a water lifting device and a container in which water can be stored and then from which it will be allowed to flow easily back into the well. Water is pumped to the surface until the container is full; it is then rapidly dumped back into the well. Repeating this motion many times can provide some development of the surrounding water bearing formation.
It is crucial that the water which is pumped to surface be allowed to sit until the suspended material has settled. The clear water should then be decanted into a second container and from there dumped back into the well. This will ensure that fine particulate is not inadvertently re-introduced into the well.
If a gasket has not been attached to the top of the pump cylinder, it may be possible to combine overpumping with backwashing by collecting water from the overpumping process, allowing it to settle and then rapidly pouring the decanted water back into the well.
Surging: Surging is the most common method of well development. It involves forcefully moving water into and out of the well screen using one of the following techniques:
Compressed Air: Compressed air can be injected into the well to lift the water; As it reaches the top of the casing, the air supply is shut off, allowing the aerated water column to fall (process called "rawhiding"). The air supply should be periodically run without stopping to pump sediment from the well(3). This equipment is usually not available in remote areas and often only opens a small portion of the screen.
A bailors up-and-down motion causes a surging action which will develop the area around the screen. The heavier and wider the bailor is, the better it will function because it will have more force to push water through the screen (Brush, 197?). Be prepared to bail and bail and bail and bail ... it is hard work and can take all day!
Placing a surge block on the end of Waterra tubing equipped with a one way valve has the advantage of the down stroke being milder than the upstroke because some water passes up the tubing. This is advantageous because it ensures that fines are not driven further into the formation and it helps to remove sediment which is loosened by the surging action. This prevents the screen from becoming totally blocked with accumulated fines.
To effectively surge a well, apply an up and down motion, repeatedly raising and dropping the plunger 2 to 3 feet. The plunger should drop rapidly on the downstroke in order that turbid water will be lifted out of the connecting tubing. While the plunger can be forced down on each stroke, adding weight just above the surge block will make it easier to work for a longer period of time.
Surging should start above the screen to reduce the possibility of "sand-locking" the surge block (Anderson, 1993). Initial surging should be with a long stroke and at a slow rate (20 to 25 strokes per minute); after surging above the screen, the hole should be cleaned and surging started at the lower end of the screen - gradually working upward until the entire screen has been developed (Anderson, 1993).
When the amount of fine material drawn into the well begins to decrease, the process should be repeated, beginning at the bottom of the screen, but with a faster stroke (30 to 35 strokes per minute). The final surging should be as rapid as possible for as long as possible.
Well yield is the volume of water that can be pumped during a specific period of time (it is expressed as litres or gallons per minute). Sometimes the yield of existing wells will be tested to determine if it is worthwhile to drill in the same area. If a submersible pump is installed, a full pump test can be done(4). If a handpump is installed, try to measure the water level before and after pumping. Pump at a steady rate for as long as possible (1-4 hours if new wells will be heavily used). This pumping rate is sustainable if the water level returns to pre-pumping levels within 6-12 hours. The shorter the time, the better the aquifer. Section 10.4 - Capacity
If the yield of a newly drilled well is questionable, it is often a good idea to test it to determine whether or not it is worthwhile to pour a concrete pad and install a bush pump. In general, a well which is capable of reliably supporting a heavily used bush pump should be able to yield at least 0.2 L/s (3 gpm) and have a specific capacity of at least 0.01 L/s for every meter of drawdown(5). Rough estimates of the yield of new Lifewater wells can be obtained using an air compressor, Waterra tubing equipped with a foot valve or a bailer.
If available, use an air compressor to inject large volumes of air into the well. This will cause the water to spill over the top of the well casing. A trench should be prepared ahead of time to carry this water away so that it does not pond around the well. After 30 minutes, the amount of water still flowing over the top of the well casing will provide a rough estimate of how much water the well can produce. This should be confirmed by turning off the pump and measuring how long it takes for the water in the well to return to the pre-pumping level. Measure the water level every minute for 10 minutes, then every 5 minutes for half an hour, then every 15 minutes for an hour and then every half hour until recovery is complete. These readings can be used by hydrogeologists to analyze the aquifer.
Finally, an inertia-lift system (Waterra) or a bailer can be used to test the yield of a newly constructed well. If the well can be pumped dry using these devices and the yield does not improve with development, the well will not have sufficient yield to support a hand pump.
If the well yield is too low to support a hand pump, the well should be abandoned by removing as much casing as possible and filling the well with clay or silty sand and filling the top 2 meters with concrete. If this is not done, future well supplies may be jeopardized since the well may allow contaminants to pass into groundwater.
Footnotes & References
1 In sands and gravels, filter packs are often created by developing the well so that 30 to 60 percent of the aquifer material adjacent to the screen passes into the well leaving a hydraulically graded filter of coarse sand and gravel around the screen (Anderson, 1993).
2 For a given pumping rate, the longer the screen, the less development will take place in the lower part of the screen. After fine material has been removed from the permeable zones near the top of the screen, water entering the screen moves preferentially through these developed zones, leaving the rest of the well poorly developed and contributing little to well yield (Driscoll, 1986).
3 If limited volumes of air are available, put a small diameter air hose down a larger pipe (such as the rising main, Waterra tubing or drill pipe); blowing air through the small air hose will cause water to lift out through the larger pipe (Anderson, 1993). A useful rule of thumb for determining the proper compressor capacity for air-lift pumping is to provide about 0.35 L/s (3/4 cfm) of air for each 0.06 L/s (1 gpm) of water at the anticipated pumping rate (Driscoll, 1986). In general, a compressor producing 861.8 kPa (125 psi) and 94.4 L/s (200 cfm) is required. Submerse the air line about 60 percent of its length during pumping.
5 Note that if tests are conducted immediately after a well is constructed and before it is put into full use, incomplete development will often cause the calculated yield to be 10-30 percent less than the yield after 2-4 weeks of continuous use.