An estimated 65.5 billion dollars was invested drilling more than 10,000 shale oil and natural gas wells, according to a survey released by the American Petroleum Institute (API) on Monday.

The investment in shale drilling represents an 87.6 percent increase from 2010 levels, and the shale wells drilled in 2011 is 43.8 percent more than in 2010, concluded the survey.

"The rising numbers illustrate the growing importance of shale drilling in U.S. oil and natural gas development," said API Statistics Director Hazem Arafa. "Shale drilling drove most of the overall increase in drilling and now accounts for an estimated 23 percent of all wells drilled in the United States."

The survey found that shale drilling expenditures were more than half of an estimated 124.8 billion dollars spent on all new wells drilled in 2011, while in 2009 they accounted for barely more than a quarter.

It also showed that the number of estimated shale wells had risen rapidly, from 5,531 in 2009 to 7,077 in 2010 to 10,173 in 2011. Almost twice as many shale gas as shale oil wells were drilled in 2011, the survey estimated, 6,759 versus 3,414.