This is installed to support the individual casing strings in the well.

Thus, each casing string will have a subsequently smaller diameter.

The top of the casing string and annulus is usually sealed.

Also, the inside diameter of the first casing string must be large enough to fit the second bit that will continue drilling.

A float collar is installed near the bottom of the casing string.

This is usually welded or screwed to the top of the surface casing string.

This refers to the integrity of the casing strings, which bound the annuli.

The wellhead usually is installed on top of the first casing string after it has been cemented in place.

April 19 - Halliburton completes cementing of the final production casing string.

Because casing strings are cemented in place, this is significantly more difficult and expensive than replacing the completion string.

Although less exposed to wellbore fluids, casing strings too have been known to lose integrity.

The 'A' annulus is the void between the production tubing and the smallest casing string.

Provides a means of pressure sealing and isolation between casing at surface when many casing strings are used.

The 18,300 ft long 400-ton casing string had a 5,800 ft long lower portion with 7-inch diameter.

Concrete is circulated down the casing string so that it pushes up between the outside of the casing and the well bore.

A normal well will also have a 'B' and frequently a 'C' annulus, between the different casing strings.

However, each bit must be smaller than the previous casing string, which in turn has to be smaller than the previous hole.

When running long, heavy string or casing, buoyancy possible to run casing strings whose weight exceed a rig's hook load capacity.

Liner hangers enable drillers to lengthen their casing strings without having the liner pipe extend all the way to the surface.

Casing strings are supported by casing hangers that are set in the wellhead, which later will be topped with the Christmas tree.

Typically, the lower completion is set across the productive zone using a liner hanger system, which anchors the lower completion to the production casing string.

This is the pressure containing equipment at the surface of the well where casing strings are suspended and the blowout preventer or Christmas tree is connected.

Therefore, the well is drilled in sections by running casing strings to cover depth ranges between which the required mud densities are suitable for the entire range.

In drilling technology, casing string is a long section of connected oilfield pipe that is lowered into a wellbore and cemented.

Depending on the conditions encountered (e.g., zones of differing formation pressure gradients), three or four casing strings may be required to reach the target depth.