While natural gas production in the Marcellus Shale has increased the volumes of wastewater produced in the region nearly sixfold, shale wells in the play produce about one-third the wastewater per unit of gas recovered than do conventional wells, a new study has found.
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"Despite producing less wastewater per unit of gas, developing the Marcellus shale has increased the total wastewater generated in the region by [about] 570% since 2004, overwhelming current wastewater disposal infrastructure capacity," the study released this week by researchers at Kent State and Duke universities said.
The study, which its authors said is the first comprehensive characterization of wastewater volumes generated by Marcellus wells, analyzed data from 2,189 active Marcellus wells in Pennsylvania and compared gas production and wastewater volumes with conventional wells.
"We found the average shale gas well is producing 10 times the amount of waste water as an average conventional well, which really isn't that surprising," Brian Lutz, Kent State professor and an author of the study, said in an interview Friday.
"What did surprise us is when we actually looked at the amount of wastewater that's being produced relative to the amount of gas that's being produced we found that shale gas wells are actually producing about three times less wastewater per unit of gas recovered than conventional wells," Lutz said.
He added this finding goes against the common public perception that shale gas development is inordinately more water intensive than conventional gas production. "It seems to be exactly opposite reality," he said.
The study estimated that the average Marcellus well generated 5.211 million liters (about 1.38 million gallons) of total wastewater and [about 1 Bcf] of gas over the first four years of operation.
In addition, the study found that most of the wastewater generated by Marcellus gas wells was not associated with hydraulic fracturing, but was instead water produced from the natural formation.
Well operators classified only 32.3% of wastewater from Marcellus wells as flowback from hydraulic fracturing, which returns to the surface during the initial four weeks of production. "Most wastewater was classified as brine, generated over multiple years of the well's productive life," the researchers said.
With the exception of a small percentage of wastewater that is trucked to Ohio for disposal in underground injection wells, the "majority of wastewater produced in Pennsylvania has stayed in Pennsylvania," where it is reused.
"Wastewater that's produced by one well is used to hydraulically fracture a subsequent well," Lutz said.
Nationally, disposal in deep injection wells is the most commonly used and cost-effective method of disposal of wastewater, he said. However, the geology of Pennsylvania does not lend itself to the construction of disposal wells.
There are about 150,000 injection wells in the US, but "only maybe six or seven in Pennsylvania and about 185 to 190 in Ohio," Lutz said.
"The primary means of disposal of waste remains underground injection wells, but there's not enough disposal capacity in some regions where it's needed. That's particularly true in the Marcellus and that's what's motivated the increase in recycling," Lutz said.
Recycling of wastewater also is becoming more common in more arid energy-producing states in the West, but for a different reason, Lutz said.