Casing pack:

A means of cementing casing in a well so that the casing may, if necessary, be retrieved with minimum difficulty.

A special mud, usually an oil mud, is placed in the well ahead of the cement after the casing has been set.

Non-solidifying mud is used so that it does not bind or stick to the casing in the hole in the area above the cement.

Since the mud does not gel for a long time, the casing can be cut above the cemented section and retrieved.

Casing packs are used in wells of doubtful or limited production to permit reuse of valuable lengths of casing.