Type Of Wells

Wells

Wells are extremely important to all societies. In many places wells provide a reliable and ample supply of water for home uses, irrigation, and industries. Where surface water is scarce, such as in deserts, people couldn't survive and thrive without groundwater. Well drilling is also used for the extraction of a natural resource such as, brine, natural gas, or petroleum, for the injection of a fluid from surface to a subsurface reservoir or for subsurface formations evaluation or monitoring.

Types of wells

Hand Dug wells

Digging a well by hand is becoming outdated today .Modern wells are more often drilled by a truck-mounted drill rig. Hand-dug wells are excavations with diameters large enough to accommodate one or more people with shovels digging down to below the water table. They can be lined with laid stones or brick; extending this lining upwards above the ground surface to form a wall around the well serves to reduce both contamination and injuries by falling into the well. A more modern method called caissoning uses reinforced concrete or plain concrete pre-cast well rings that are lowered into the hole. A well-digging team digs under a cutting ring and the well column slowly sinks into the aquifer, whilst protecting the team from collapse of the well bore. Hand dug wells provide a cheap and low-tech solution to accessing groundwater in rural locations in developing countries, and may be built with a high degree of community participation, or by local entrepreneurs who specialize in hand-dug wells. They have been successfully excavated to 60 metres (200 ft). They have low operational and maintenance costs, in part because water can be extracted by hand bailing, without a pump. The water is often coming from an aquifer or groundwater, and can be easily deepened, which may be necessary if the ground water level drops, by telescoping the lining further down into the aquifer. The yield of existing hand dug wells may be improved by deepening or introducing vertical tunnels or perforated pipes.

Figure 1: showing open hand dug well Figure 2 :showing hand dug well

Page | 1

Driven wells

Driven wells may be very simply created in unconsolidated material with a well hole structure, which consists of a hardened drive point and a screen (perforated pipe). The point is simply hammered into the ground, usually with a tripod and driver, with pipe sections added as needed. A driver is a weighted pipe that slides over the pipe being driven and is repeatedly dropped on it. When groundwater is encountered, the well is washed of sediment and a pump installed.

Figure 3: showing bored well with driven point

Drilled wells

Drilled wells are typically created using either top-head rotary style, table rotary, or cable tool drilling machines, all of which use drilling stems that are turned to create a cutting action in the formation, hence the term drilling.Drilled wells can be excavated by simple hand drilling methods (augering, sludging, jetting, driving, hand percussion) or machine drilling (rotary, percussion, down the hole hammer). Deeprock rotary drilling method is most common. Rotary

Page | 2

can be used in 90% for formation types.Drilled wells can get water from a much deeper level than dug wells can—often up to several hundred metres. Rotary drilling machines use a segmented steel drilling string, typically made up of 6 metres (20 ft) sections of galvanized steel tubing that are threaded together, with a bit or other drilling device at the bottom end. Some rotary drilling machines are designed to install (by driving or drilling) a steel casing into the well in conjunction with the drilling of the actual bore hole. Air and/or water is used as a circulation fluid to displace cuttings and cool bits during the drilling. Another form of rotary style drilling, termed mud rotary, makes use of a specially made mud, or drilling fluid, which is constantly being altered during the drill so that it can consistently create enough hydraulic pressure to hold the side walls of the bore hole open, regardless of the presence of a casing in the well. Typically, boreholes drilled into solid rock are not cased until after the drilling process is completed, regardless of the machinery used

Figure 4 : showing componenets of a driven well

Page | 3

Water Wells Design And Construction

Determining A Well Location (Siting)

Groundwater exploration is not a hit or miss (or random) proposition. It is true that any well penetrating an aquifer will yield water but the amount of water produced from a randomly sited well may be very small. Such low producing wells often provide adequate water for domestic or farmstead uses. If a well is to provide irrigation water a more highly productive well will usually be needed. Scientific methods have been developed for locating wells where they will penetrate into zones of fractured rock buried beneath the soil surface. Wells located on a fractured rock zone will produce much larger quantities of water than wells drilled into zones where the rock is not fractured. Finding the fractured rock zones, or better yet, finding the intersection of two fractured rock zones can be a time consuming and expensive procedure. Only geologists and engineers with training in aerial photo interpretation and hydrogeology are qualified to locate wells by the fracture-trace technique. If a high producing well is desired, however, the consultants fee for siting the well is worth the extra benefit.

Water Well Design And Installation

Once the well location has been determined, a preliminary well design is completed. For many large production wells, a test hole will be drilled before well drilling to obtain more detailed information about the depth of water-producing zones, confining beds, well production capabilities, water levels, and groundwater quality. The final design is subject to site-specific observations made in the test hole or during the well drilling. The overall objective of the design is to create a structurally stable, long-lasting, efficient well that has enough space to house pumps or other extraction devices, allows ground water to move effortlessly and sediment-free from the aquifer into the well at the desired volume and quality, and prevents bacterial growth and material decay in the well

Figure 5 Showing Compontents of a well

Page | 4

Well Drilling

Drilling a well is more than boring a hole into the earth. A finished well will consist of a borehole cut into the aquifer at a diameter large enough to accept the well casing which will receive the pump. The decision on how large the pump must be to meet your intended demand must, therefore, be made before drilling starts. Table 1 relates the necessary well casing size to the size of pump needed to pump various quantities of water. The borehole itself can be drilled using any one of several types of drill rigs including impact, rotary, or various combinations. After the borehole has been drilled into or through the water bearing aquifer, the well screen should be installed in the producing zone. The zones above the producing aquifer must be cased to prevent cave-ins, and the annulus between the borehole and casing must be filled with grout to keep surface contaminates from entering the well.

Table 1. Well casing and borehole diameter for desired pumping rate

Page | 5

Borehole Size (In)

Casing Size (in)

Pumping rate (gpm)

6 4 less than 20

8 6 20 to 100

10 8 75 to 175

12 10 150 to 400

14 12 350 to 600

20 16 600 to 1300

24 20 1300 to 1800

28 24 1800 to 3

Well Development

Developing a well is the process of clearing the well of fines left by the drilling operation, and flushing these fine particles out of the gravel and aquifer between the well screen and the first few feet of the aquifer. Development is accomplished by surge pumping, bailing or any operation which will force water through the development zone at high velocities. Developing a well is the responsibility of the well driller. Properly developed wells will yield more water than poorly developed wells.

Pump Testing

Once the well is completed and developed, it is a good practice to conduct an aquifer test (or pump test). For an aquifer test, the well is pumped at a constant rate or with stepwise increased rates, typically for 12 hours to 7 days, while the water levels in the well are checked and recorded frequently as they decline from their standing water level to their pumping level. Aquifers test are used to determine the efficiency and capacity of the well and to provide information about the permeability of the aquifer

Page | 6

Evaporation pan

Types of Evaporation Pans

Two types of pans are commonly used. Class A pan is the standard one used by research facilities and climate stations where a standard method of measurement is essential. Galvanized washtubs are often used in situations where a simpler and less expensive method of collecting evapotranspiration data is needed.

Class 'A' Pan

The Class A evaporation pan is a universally used standard-sized pan with a diameter of 1.2 m and a depth of 250 mm. When installed, it is elevated 150 mm off the ground. The operating water level is 175 – 200 mm deep; therefore, the water level in the pan is kept 50 – 75 mm from the rim. A stilling well located on the side of a Class A pan has a level sensor which is used to record water depths. The measurements can be taken automatically.

Figure 6: showing a Class A evaporation pan

Page | 7

Galvanized Washtub

The galvanized washtub is approximately 0.50 m in diameter and 0.25 – 0.30 m in depth. A tub with this depth is most desired. Since the tub is located in the field, a wire cage is placed over the tub to keep away birds and animals. A ruler is attached to the tub to measure the water level. It is important to measure the water depth at the same place in the tub every time. This is to ensure the differences in water depths in the tub are due to evaporation and rainfall, not irregularities at the bottom of the tub.

Figure 7: showing a Galvanized washtub

Page | 8

References

http://groundwater.ucdavis.edu/files/156563.pdf

http://extension.psu.edu/natural-resources/water/drinking-water/wells/before-you-drill-a-well

http://en.wikipedia.org/wiki/Pan_evaporation

http://www.kean.edu/~csmart/Hydrology/Lectures/Evaporation_pan.pdf

http://www.agf.gov.bc.ca/resmgmt/publist/500Series/577100-6.pdf

Page | 9