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"Oil
or gas wells may range in depth from a few hundred
to more than 20,000 feet. A well is made
by drilling a hole, called a "well
bore", into the earth. Metal pipe,
called "casing", is placed in
the well bore and cement is pumped
through it. When the cement reaches the
bottom of the casing, it is forced out
around the end, and pushed to the surface
between the outside of the casing and the
well bore. This critical layer of cement
bonds the casing to the well bore. It
protects oil, gas and underground water
resources, keeping them from moving
freely into and out of the well to mix
with -- and contaminate -- each other." |