DESIGN AND CONSTRUCTION INTAKE WELL OR WELL SCREEN

• A reliable and sustainable water supply consistent with your needs and the capability of the aquifer
• Good quality water that is free of sediment and contaminants
• Increased life expectancy of the well
• Reduced operating and maintenance costs
• Ease of monitoring well performance.
Although you need to hire a drilling contractor to design, drill and construct the well and choose the appropriate materials, it is important for you to know what is going on. You can then work with the drilling contractor to ensure you get the well design you need.
 

AS A Drilling Contractor

We can be a drilling contractor with an experience in this area. We complete a survey of existing wells in the existing area before drilling work starts. It Provides Usefull Information
• Typical yields and water quality
• Which aquifer to tap into
• Trends in well construction methods
• Prior drilling success rates.
Surveys of existing wells are available for a nominal fee from the Groundwater Information Centre.

Choosing a Well Site

Choice of well site will affect the safety and performance of your well. We can examine various sites.WE take care while we choose a site for intake well design that no contaminants enter the well either through the top or around the outside of the casing. Sewage or other contaminants that may percolate down through the upper layers of the ground surface to the aquifer.

Well Design Considerations

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:
• Well depth
• Type of well
• Casing material, size and wall thickness
• Intake design
• Formation seal
• Monitoring and preventive maintenance provisions.

Well depth
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.

Casing size and type
Decisions about the diameter and type of well casing are made after the driller considers the following:

• Aquifer characteristics
• Hydraulic factors that influence well performance
• Drilling method
• Well depth
• Cost (in discussion with the well owner).
The casing must be large enough to house the pump and allow sufficient clearance for installation and efficient operation.

It is recommended that the casing be at least one nominal size larger than the outside diameter of the pump. The more space there is between the pump and the casing, the easier it will be to service and repair the pump in the future.

There are two common materials used for casing: steel and plastic. Steel casing is the strongest but is susceptible to corrosion. Plastic casing is becoming more popular because of its resistance to corrosion.
Intake design
Water moves from the aquifer into the well through either a manufactured screen or 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.

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 a choice is made between a screen, or slotted or perforated casing/liner other decisions is  made regarding:
• Size of slot opening
• Total area of screen or perforation that is exposed to the aquifer
• Placement of the screen or perforations within the aquifer.

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.

As a drilling contractor we 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 is 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.

Placement in the aquifer
The screen or perforations on the casing/liner is  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. To drill we use  geophysical logging equipment to accurately identify the boundaries of the aquifer, the exact placement will be easier.


Sealing The Well
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).


Well development
Well development is the process of removing fine sediment and drilling fluid from the area immediately surrounding the perforations. This increases the well’s ability to produce water and maximize production from the aquifer.

Jetting, surging, backwashing and over pumping are methods used to develop a well. Water or air is surged back and forth through the perforations. Any fine materials that are in the formation become dislodged and are pumped or bailed from the well. This procedure is continued until no fine particles remain and the water is clear. Coarser particles are left behind to form a natural filter pack around the screen, slot openings or perforations.

If the aquifer formation does not naturally have any relatively coarse particles to form a filter, it may be necessary to install an artificial filter pack. The pack is placed around the screen or perforations so the well can be developed. For example, this procedure is necessary when the aquifer is composed of fine sand and the individual grains are uniform in size.

It is important to match the grain size of the filter material with the size of the slot openings of the screen to attain maximum yield from the well. Typically the slot size of the screen is selected so that 85 percent of the artificial pack material remains outside of the screen.

Yield test
A yield test, often called a pump test, is important because the information gathered during the test assists the drilling contractor to determine the:
• Rate at which to pump the well
• Depth at which to place the pump.
Provincial regulations outline the minimum yield test for all new wells. After drilling and developing a well, the drilling contractor must remove water from the well for at least 2 hours. If a pump is used to remove the water, then water level measurements can be recorded as the water level draws down. After 2 hours, water removal stops and the recovery of the water level is monitored and recorded. Measurements must be taken at specific time intervals for a 2 hour period or until the water level returns to 90 percent of its original level.

Once the yield test is complete, the drilling contractor will decide at what rate the aquifer can be pumped without lowering the water level below the top boundary of the aquifer, the top of the perforations or below the pump intake.
The pump that is installed in the well should have a capacity equal to, or less than, the rate at which the well can supply water for an extended period of time without lowering the level below the pump intake. That rate is considered the safe pumping rate for the well.

Disinfecting the well

Provincial regulations require the drilling contractor to disinfect new wells with chlorine. The concentration is calculated on the volume of water that is in the well. The concentration must be at least 200 milligrams of chlorine per litre of water present throughout the water in the well and must be left in the well for at least 12 hours to ensure any bacteria present are destroyed. Chlorination is done after the pumping equipment is installed and before the well is put into production. The yield test provides a benchmark of your well's performance. Repeating this test at a later date can be used to assess any changing conditions of the well and determine when maintenance is required.

SOME OF THE PROJECT SUCCESSFULLY IMPLEMENT FOR INTAKE WELLS AND WATER SUPPLY SYSTEM