When energy companies extract shale gas, they emit only a fraction more methane into the atmosphere than companies doing conventional gas drilling, according to a new study. That fraction -- about 216 gigagrams of methane in 2010 -- was due to hydraulic fracturing, a technique in which drillers inject pressurized water, sand and chemicals to fracture shale rock and release trapped gas. Fracking accounted for 3.6 percent of the 6,002 gigagrams of methane emitted overall by natural gas operations in 2010.