Researchers led by gas-industry critic Anthony Ingraffea looked at about 75,500 compliance reports from more than 41,000 oil and gas wells drilled in Pennsylvania between 2000 and 2012 to determine whether casing was impaired.
About 9.1% of the active wells examined had a casing defect, according to the study.
About 1% of wells drilled before 2009 had a faulty casing. Most of those were vertical wells.
