The borehole casing is keeping the keep the cylindrical shape of hole with external and internal force.
The purpose of the screen is to sustain sand and gravel from the gravel pack out of the well while providing ample water flow to enter the casing. The screen should be designed by the size and the type of well.
The annular space is filled with gravel or coarse sand and it is called the gravel pack or filter pack. The gravel pack prevents sand and fine sand particles from moving from the aquifer formation into the well. The uppermost section of the annulus is normally sealed with a bentonite clay and cement grout to ensure that no water or contamination can enter the annulus from the surface.
At the surface of the well, a surface casing is commonly installed to facilitate the installation of the well seal. The surface casing and well seal protect the well against contamination of the gravel pack and keep shallow materials from caving into the well. Surface casing and well seals are particularly important in hardrock wells to protect the open, uncased borehole serving as a well.
