Corrosive Zone

The Corrosive Zone is the name of the first string of casing in a hydrocarbon well.

It may be lowered into a hole and drilled into the formations near the surface hole and cemented into place, or it may be driven into the ground by a pile driver.

Its purpose is to stop soft formations near the surface from caving into the well.

For a very good explanation of the fracking process see this.