When the well is abandoned, it is sealed with cement and thought to be fine unless it begins to leak. Some believe that greater regulation of these wells is necessary, including expansion of no-build zones:
Similar rules are lacking in most of the U.S., even as new development is encroaching on old oil and gas fields in places like the Front Range of Colorado.
State oil and gas regulators say the situation is out of their hands.
"We really try to discourage the construction on top of old wells," says Stuart Ellsworth, engineering manager for the Colorado Oil and Gas Conservation Commission.