Well casing
A well casing is a lining that surrounds drilled wells. The purpose of a casing is to isolate the contents of the well from the surrounding rock and soil, to prevent loss of product as well as contamination.
The well casing is constructed in multiple parts. The interior of the casing is made from steel, and is smaller than the diameter of the well. The exterior of the casing is made of concrete, poured into the gap between well and steel casing. This creates a further barrier and prevents the interior casing from shifting. Furthermore, the thickness of the casing varies depending on the depth, with the casing being thicker near the top of the well, where it is surrounded by soil and permeable rock, and thinner lower down in the well, where it is surrounded by solid rock.
The proper casing of a well is critical, as it prevents any chemicals used inside the well from leaching into the surrounding soil and rock. In theory, a well that is properly cased should prevent any contamination of groundwater. This is a concern when drilling either for natural gas or petroleum.