Water Technology Plastic Industries(WTPI)

A guide to water well

casing and screen selec on Water has played a vital role in the advancement of mankind. The earliest traces of civilization reveal that those cultures which flourished were able to employ water sources for domestic, and agricultural purposes. A c h a ra c t e r i s t i c o f a l l d e v e l o p e d countries today is their ability to use productively the water made available to them.

Regardless of purpose, almost all water wells must be provided with a means of protecting the borehole and provision made for the entrance of water from surrounding aquifers. While well casing is simply defined as the material that cases or lines a borehole to prevent formation collapse, there are varied interpretations of the meaning of well screen.

This is due to the existence of many designs throughout the world. In this guide, screen refers to that structure in a well, which protects the borehole, but allows the entrance of water. In this sense, screen is a filter. The durability and efficiency of a well depend to a large degree on its design, construction procedures and selection of casing and well screen. However, casings and screens alone comprise the principal substance of a completed water well.

Strength and Durability Regardless of construction method, water well casings and screens have some common requirements. Strength must be adequate to withstand not only the stresses of installation, but

also other forces which may be applied during well completion, development and use.

The forces of installation which tend to pull the casing and screen apart must be exceeded by the tensile strength of the material. The resistance of the casing to collapse must be greater than the external hydrostatic forces calculated.

Handling and Maintenance E a s e o f t ra n s p o r t , h a n d l i n g a n d installation are important considerations.

These not only influence cost but are also relevant to the selection of the type of casing and screen field connections.

Casing and screen should be designed to facilitate future rehabilitation including cleaning, incrustation removal, redevelopment and repair. Another

Mud drilling tri-cones

consideration is the possibility of future well deepening. Casing and screen diameter as well as type and material selected are influenced by the parameters.

A Hydrological survey is essential when extracting water Water Technology Plastic Industries (WTPI) a company specialises in abstracting water from sand deposits using mud drilling techniques and specialized screening methods.

The company is one of the largest manufacturers of PVC Borehole Casings and Screens in the Southern Hemisphere. Founded in 1997 and situated in Randburg South Africa, WTPI supplies PVC Products in all forms, extensively covering all aspects of water supply and the movement of water.

According to the company a thorough geophysical survey by qualified hydrologists should be regarded as an essential prerequisite to drilling for water on any farm.

From this survey, the farmer will know where his best source of water is located on the farm, and how best to tap the source.

For more than 20 years, this company has specialised in working on farms and sites where ground water is flowing through sand and gravel, such as an existing or an ancient river bed, and a survey would show this up but not necessarily result in a call for their services.

“But we strongly urge farmers looking for water to have their land surveyed,” says Mr Billy Shield.

Johnson wedge wire screen

Well configuration drawing

“To date we have been involved in several major water extraction projects, including the massive Sabie River at Chisambanji in Zimbabwe in 80's where a 12 well point system delivers some 999 240 litres/h, says Mr Shield.

It is important, says Mr Shield, that the rate at which the water flows into the screen be kept as low as possible, with a maximum rate of 0,0305 m/sec (0.1ft/sec). In this way, movement of silt is kept to a minimum, and the gravel envelope around the screen should remain permeable for a long period A major advantage of this system, says Mr Shield, is that if a farmer finds this well producing less water, it is easy to service the well by re-developing it, or even treating it with acid or other chemicals to clear algae or mineral deposits.

Well production and capacity depend on the amount of water in a given formation, what the available head of water is above the screen, and how rapidly this water can permeate through the formation.

Where there is too little drawdown (i.e. the level of water above the screen is to shallow), or the rate of replenishment is too slow for a given pump, the well will begin sucking air and cavitation will take place, and the motor could burn out.

A thorough knowledge of the

hydrology of the area is essential in order to design a system which will best suit that farmers needs and the nature of the supply.

From this survey, the farmer will know where his best source of water is located on the farm, and how best to tap the source.

“For a farmer or land owner who is serious about getting high-quality clean water from the drilling operation this is a worth-while investment”, concludes Mr Shield Contact: Billy Shield Managing Director Water Technology Plastic Industries pty Tel: +27 11 708 3691 +27 11 026 4251 +27 11 708 3695 Fax: +27 83 654 6771 Email: billy@wtpi.co.za www.wtpi.co.za

illustration - Mud drilling

The drill string is turned at surface, which turns the bit at the bottom of the hole

The teeth on the drill bit grind the rock into fragments, or “cuttings”

Drilling mud is pumped down the inside of the drill pipe, through jet nozzles in the bit, and into the ‘annulus”. This is the space between the sides of the hole and the drill pipe.

The mud lifts the cuttings and circulate them back to surface where they are removed.