Scientists have linked the underground injection of oil-drilling wastewater to a magnitude-5.7 earthquake in 2011 that struck the US state of Oklahoma.

BBC: Oklahoma earthquake linked to oil extraction wastewater

In its review of incidents of drinking water well contamination believed to be associated with hydraulic fracturing, EPA found no confirmed cases that are linked to fracturing fluid injection into CBM wells or subsequent underground movement of fracturing fluids.

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With the usual steam injection methods, only a quarter or so of the oil trapped underground can be recovered.

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