One Achilles' heel of the shale gas boom is the potential failure of a vital well construction operation known simply as "the cement job." Every state with oil and gas operations requires drillers to pump cement into the spaces between the well hole and the outside of the steel pipe casing at a number of critical points hundreds of feet down. The cement sheaths are installed in order to form impenetrable barriers blocking methane, brackish water or drilling chemicals from migrating up the space around the drill pipe to contaminate drinking water aquifers higher up. But errors, mishaps or geological traps can leave channels running through the cement.