Wells And Well Drilling - Water Encyclopedia

Casing, Screen, and Seals.

The sanitary seal is the top covering of a well. Its function is to keep insects, small animals, dirt and debris from entering the well and contaminating the aquifer . This cap usually has an access port that allows periodic water-level measurements, and may have a screened breathing tube to allow air pressure to equalize as water levels rise and fall in the well.

The casing has several important functions. It is a conduit used to hold the wellhole open in soft or unconsolidated geologic formations. It commonly serves as the framework needed to support any pumping mechanism. It is an integral part of the casing seal, which is constructed to prevent surface water and shallow groundwater from entering the well and degrading deeper waters. To prevent surface runoff from flowing into the well, the casing usually extends 30 centimeters (1 foot) or more above the land surface. In formations prone to collapsing, such as in sands and gravel, the casing may extend the entire depth of the well. Where the well is constructed in hard formations such as shale or basalt, only the very top portion of the well may be cased. Even in ideal circumstances, a 5.5-meter-long (18-footlong) long casing generally is considered the minimum.

The casing seal is an impervious barrier, usually of cement grout or clay, placed between the well casing and the native material. It prevents surface water and undesirable groundwater from other aquifers from entering the wellbore or from flowing down the outside of the casing and degrading deeper water. Where wells penetrate confining layers at greater depths, seals are commonly constructed at least 1.5 meters (5 feet) into a confining layer. Occasionally, seals are also constructed at greater depths between different aquifers. These seals prevent one aquifer from draining into another, potentially resulting in resource loss or degradation.

Screens in the casing are used to filter out sand and gravel while allowing maximum water flow into the well. Their construction varies from highly engineered screens over 15 meters (50 feet) long, to a few holes punched through the casing. The formations encountered in drilling and the intended use of the well indicate the type of screen, if any, that is needed.