• Surface casing is then inserted into the well hole to isolate the freshwater zone the
bore has passed through, and to act as a foundation for the blow out preventer (a
safety device which connects the rig to the well bore). Cement is pumped down the
casing and out through the bottom of the casing, and forced up between the casing
and the bore hole sealing off the well from the freshwater zone preventing
contamination of the water supplies.