Circulation of cementin g fluids will now be begun, being usually preceded by mud or other fluid which is separated 4from the body of cement by a first closure plug P1, shown .in broken lines in FIG. 1B, of a generally conven- As the pre-cementing fluids flow downwardly ahead of the body of cement, they will ow through the interior of the mandrel and its plug extension thence out through lower d c