"A study of tainted drinking water in areas where natural gas is produced from shale shows that the contamination is most likely caused by leaky wells rather than the process of hydraulic fracturing used to release the gas from the rock.
The study looked at seven cases in Pennsylvania and one in Texas where water wells had been contaminated by methane and other hydrocarbon gases. Both states have extensive deposits of gas-bearing shale that have been exploited in recent years as part of a surge in domestic energy production. Some environmental groups have suggested that hydraulic fracturing, or fracking, could cause the gas to migrate into drinking water aquifers.
Shale-gas producers commonly drill a deep vertical well that is then extended horizontally in several directions into the rock, like spokes from a hub. In fracking, water and chemicals are injected at high pressures into these spokes, creating fissures and releasing the natural gas trapped within.
But in their analysis, published Monday in The Proceedings of the National Academy of Sciences, the researchers found no evidence that fractured shale led to water contamination. Instead, they said cement used to seal the outside of the vertical wells, or steel tubing used to line them, was at fault, leading to gas leaking up the wells and into aquifers.
“In all cases, it basically showed well integrity was the problem,” said Thomas H. Darrah, a researcher at Ohio State University and the study’s lead author. The gas that leaked, he added, most likely came from shallower gas-rich pockets that the vertical wells were drilled through on their way to the shale formations, rather than from the shale itself.
“The good news is, improvements in well integrity can probably eliminate most of the environmental problems with gas leaks,” Dr. Darrah said.
Richard J. Davies, a professor at the Newcastle University in Britain and a petroleum geologist not involved in the study, said it confirmed what he and others had shown in earlier research, that the fissures created by fracking were generally not long enough to affect aquifers."...
8/12/14, "Noble gases identify the mechanisms of fugitive gas contamination in drinking-water wells overlying the Marcellus and Barnett Shales," PNAS.org
"Abstract:" "Noble gas data appear to rule out gas contamination by upward migration from depth through overlying geological strata triggered by horizontal drilling or hydraulic fracturing." (last sentence)
9/15/14, "Weak wells not fracking caused US gas leaks into water," BBC, Matt McGrath
"Researchers in the US analysed the gas content in 130 water wells in Pennsylvania and Texas....In none of the investigated wells was there a direct link to fracking.
"These results appear to rule out the possibility that methane has migrated up into drinking water aquifers because of horizontal drilling or hydraulic fracturing, as some people feared," said Prof Avner Vengosh, from Duke University."...