Water Well Casing Selection
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Water Well Casing and Screen Selection

General Casing and Screen Considerations

Water has played a vital role in the advancement of mankind. The earliest traces of civilization reveal that those cultures that flourished were able to employ water sources for domestic and agricultural purposes. A characteristic of all developed countries today is their ability to productively use the water made available to them.

While ground water use began in arid and semi-arid regions thousands of years ago, more recently, development has occurred in areas with ample surface supplies. This has provided supplemental sources for use by agriculture at the most beneficial time – during the growing season – and availability during drought conditions. Thus, a world ground water development industry has grown, focusing attention on water well construction techniques, design and operating methods.

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 simply is 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 article series, 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. While representing a fraction of total investment, they are critical to the productive life of a well and pump. In addition, yield and operating expenses are influenced. The latter has grown more important due to rapidly escalating energy costs.

Another matter attracting increased attention today concerns contamination of ground water supplies from harmful materials originating at the surface. Protection of a well requires controlling the pollution source. In many cases, the use of sealed casings and more durable materials is helpful.

This article discusses casing and screen materials in common use in the word today. It also sets forth the factors to be considered in their choice. These factors include methods of well construction and their relationship to well design. While a complete answer to every planning requirement cannot be provided here, the information following may be used as a general guide by those concerned with ground water development.

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 that 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.

Radial stresses of the wall of a small diameter hole in a consolidated formation are negligible. However, it is impossible to calculate the load on casings and screens in unconsolidated formations. Unknown are forces from formation sloughing, caving and subsidence, or the sudden downward movement of filter pack material. These stresses can rupture casings and screens.

Another requirement related to strength is durability. Small increases in the wall thickness of ordinary low-carbon steel casings not only improve strength, but also, under most conditions, extend well life, from a corrosion standpoint, by a factor greater than the percentage thickness difference. Corrosive environments may require the use of special corrosion-resistant material.