In this March 29, 2013 photo, workers tend to a well head during a hydraulic fracturing operation at an Encana Oil & Gas (USA) Inc. gas well outside Rifle, in western Colorado. The first experimental hydraulic fracturing occurred in 1947. More than 1 million U.S. oil and gas wells have been fracked since, according to the American Petroleum Institute.
In this March 29, 2013 photo, workers tend to a well head during a hydraulic fracturing operation at an Encana Oil & Gas (USA) Inc. gas well outside Rifle, in western Colorado. The first experimental hydraulic fracturing occurred in 1947. More than 1 million U.S. oil and gas wells have been fracked since, according to the American Petroleum Institute. Brennan Linsley AP
In this March 29, 2013 photo, workers tend to a well head during a hydraulic fracturing operation at an Encana Oil & Gas (USA) Inc. gas well outside Rifle, in western Colorado. The first experimental hydraulic fracturing occurred in 1947. More than 1 million U.S. oil and gas wells have been fracked since, according to the American Petroleum Institute. Brennan Linsley AP