The location at which you choose to install a well site can affect the safety and performance of your well. As you examine various potential sites it is important to consider any future development you may be planning for your farm or acreage, such as barns, storage sheds, bulk fuel tanks or other improvements. Provincial regulations that dictate where wells can be located must also be taken into consideration.
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Well design and construction details are determined after a test hole has been completed and the geological zones have been logged. There are many components to well design the driller must take into account. Decisions will be made about:
Wells that require licensing cannot be constructed with a multi-aquifer completion.
During the test hole drilling, the drilling contractor will complete a formation log. Soil and rock samples are taken at various depths and the type of geologic material is recorded. This allows the driller to identify aquifers with the best potential for water supply. Some drillers also run an electric or gamma-ray log in the test hole to further define the geology. This gives them more accurate information about aquifer location.
Generally, a well is completed to the bottom of the aquifer. This allows more of the aquifer to be utilized and ensures the highest possible production from the well.
There are 2 main types of wells: bored wells and drilled wells; each is distinguished by the diameter of the bore hole.
A bored well is constructed when low yielding groundwater sources are found relatively close to the surface, usually less than 30m (100 ft.). Bored wells are constructed using a rotary bucket auger. They are usually completed by perforating the casing or using a sand screen with continuous slot openings (see Figure 1, Bored Well).
One advantage of bored wells is the large diameter of the casing, from 45-90 cm (18-36 in.). It provides a water storage reservoir for use during peak demand periods. A disadvantage of utilizing a shallow groundwater aquifer is that it relies on annual precipitation for recharge. Water shortages may occur following long dry periods in summer and periods of extended freeze-up during winter months.
Drilled wells are smaller in diameter, usually ranging from 10-20 cm (4-8 in.), and completed to much greater depths than bored wells, up to several hundred metres. The producing aquifer is generally less susceptible to pollution from surface sources because of the depth. Also, the water supply tends to be more reliable since it is less affected by seasonal weather patterns.
There are 2 primary methods of drilling:
Rotary drilled wells are constructed using a drill bit on the end of a rotating drill stem. Drilling fluid or air is circulated down through the drill stem in the hole and back to the surface to remove cuttings. Rotary drilling rigs operate quickly and can reach depths of over 300m (1000 ft.), with casing diameters of 10-45 cm (4-18 in.).
Cable tool drilled wells are constructed by lifting and dropping a heavy drill bit in the bore hole. The resulting loose material is mixed with water and removed using a bailer or sand pump. This method, also called percussion drilling, reaches depths up to 300m (1000 ft.). Well diameters can range from 10-45 cm (4-18 in.). The drilling rate is typically much slower than for a rotary rig, but low yielding aquifers may be more easily identified using this method.
There are 3 types of possible well completions for both drilling methods (See Figure 2, Well Completions):
Casing Size & Type
Decisions about the diameter and type of well casing are made after the driller considers the following:
The casing must be large enough to house the pump and allow sufficient clearance for installation and efficient operation.
Materials used in the drilling and construction of water wells must be new and uncontaminated. Provincial regulations provide detailed specifications for casing diameters and wall thicknesses. All casing must meet or exceed standards set by the Canadian Standards Association or the American Society of Testing and Materials (see Appendix 1, Water Well Casing Specifications.)
Water moves from the aquifer into the well through either a manufactured screen or a mechanically slotted or perforated casing. Screens are manufactured with regularly shaped and sized openings. They are engineered to allow the maximum amount of water in with minimal entry of formation sediments. Stainless steel screens are the most widely used because they are strong and relatively able to withstand corrosive water. Screens are manufactured with various slot sizes and shapes to match the characteristics of the aquifer.
A slotted or perforated casing or liner is made by creating openings using a cutting tool or drill. Pre-slotted plastic pipe is also available.
Slot openings and perforations are spaced further apart than screen openings. This reduces the amount of open area to allow water into the well. The openings tend to vary in size and may have rough edges, depending on how they were made. This impedes the flow of water into the well and may not hold back the formation sediments.
The drilling contractor examines the cuttings from the borehole and makes a judgement whether to use a screen, or slotted or perforated casing/liner. While a screen is the more expensive alternative, it is necessary if the aquifer is composed of loose material, such as fine sand, gravel or soft sandstone. A slotted or perforated casing/liner can be used when the aquifer formation is more consolidated, such as hard sandstone or fractured shale.
After choosing a screened, slotted or perforated casing/liner, other decisions will be made regarding:
Ensure that the pumping water level in the well never goes below the top of the slot openings or perforations. This will prevent oxygen exposure to the aquifer, which would enhance bacterial growth and reduce well yield.
Slot Size Openings
The slot openings must be small enough to permit easy entry of water into the well while keeping out sediment. The slot size chosen will depend on the particle size of the earth materials in the producing aquifer.
Typically a drilling contractor will select a slot size that allows 60 percent of the aquifer material to pass through during the well development phase of drilling. The remaining 40 percent, comprising the coarsest materials, will form a natural filter pack around the perforations or screen.
Total Open Area of Screen
The total area of the slot openings is dependent on the length and diameter of the screen. While the length of the screen is variable, the diameter of the screen is determined by the diameter of the well casing. The yield from a well increases with an increase in screen diameter, but not proportionately so. Doubling the screen diameter raises the well capacity only 20 percent.
The amount of open area of the screen or slotted or perforated casing/liner must be calculated to ensure the water from the aquifer does not enter the well too quickly. A larger amount of open area allows the water to enter the well at a slower rate, causing a lower drop in pressure in the water as it moves into the well. If the water flows too quickly, there will be problems with incrustation.
Incrustation is a buildup that occurs when dissolved minerals in the groundwater come out of solution and deposit on the screen or casing. As a result, the perforations get plugged, water cannot enter the well at the same rate, and the yield from the well will be reduced. The pore spaces in the aquifer immediately adjacent to the perforations may also get plugged with fine material, which could result in yield reduction.
Placement in the Aquifer
The screen or perforations on the casing/liner must be placed adjacent to the aquifer. If improperly placed, the well may produce fine sediment which will plug plumbing fixtures and cause excessive wear on the pump. If the driller uses geophysical logging equipment to accurately identify the boundaries of the aquifer, the exact placement will be easier.
Sealing the well protects the well’s producing zone from contamination. The diameter of the bore hole is usually slightly larger than the casing being installed. The space between the bore hole and the casing is called the annulus of the well. It must be sealed to prevent any surface contamination from migrating downward and contaminating the water supply. It also prevents any mixing of poor quality aquifers with the producing aquifer of the well (see Figure 3, Annulus Seal).
Provincial regulations require the annulus be filled with impervious material, such as cement or bentonite. To isolate the producing zone of the well, the annulus is filled from immediately above the perforated zone to the ground surface.