An estimated 56 million barrels of water are produced as a result of onshore oil and natural gas development everyday, and while the quality is poor, most of the contaminants occur naturally, according to a Government Accountability Office (GAO) report. Only some of the contaminants are added through drilling, hydraulic fracturing (fracking) and pumping oil and gas.
The estimate is based on an Argonne National Laboratory study of produced water volumes generated during 2007, the most recent year for which such data were collected, and was derived from information collected from state agencies in 31 oil- and gas-producing states, said the GAO report, which was presented to the House Science, Space and Technology Committee in January, but only just made public. The volume could be even more because data from the states is incomplete.
"The reporting time frame for the study largely occurred prior to the recent, dramatic increase in shale gas production in the United States, which had an average annual growth rate of 48% from 2006 to 2010, according to the EIA [Energy Information Administration]," the report said.
The volume of water generated by a specific oil and and natural gas well varies significantly based on three key factors: the type of hydrocarbon being produced, the geographic location of the wells, and the method of production used, the GAO report said. "Coalbed methane wells produce large volumes of water in the early stages of production because coal beds are essentially aquifers that contain coal rock and gas bound together from the pressure of the water present in the aquifer. By pumping out water, the resulting drop in pressure allows the gas to detach from the coal and flow to the surface.
"In contrast, one producer noted that their conventional gas wells produce much less water than their coalbed methane wells because the formations from which conventional gas is drawn contain much less water."
As for the location differences, "stakeholders noted that the Barnett Shale formation in Texas is generally known to be a 'wetter' formation than the Marcellus Shale formation in the Northeast, with shale gas wells in the Barnett typically producing three to four times more water than shale gas wells in the Marcellus. Similarly, USGS [U.S. Geological Survey] reported that coalbed methane wells in the Powder River Basin in Wyoming and Montana produce, on average, 16 times more water than coalbed methane wells in the San Juan Basin in Colorado and New Mexico," it said.
Because of the different geology, the GAO report said the Marcellus Shale formation has a higher level of radionuclides (naturally occurring radioactive materials) than water from gas wells in the Barnett Shale. And produced water from coalbed methane wells in the Raton Basin in Colorado and New Mexico has a salt content, on average, that is roughly two and a half times higher than produced water from the Powder River Basin in Wyoming and Montana.
According to the GAO report, methods of production that rely on the injection of water and other fluids into the formation to stimulate oil and gas production generate more produced water than in cases in which oil and gas comes to the surface under existing pressure.
"The use of hydraulic fracturing to produce oil or gas can result in larger volumes of produced water than production in more porous formations, although the larger volumes associated with hydraulic fracturing are limited to the initial flowback of water and fracturing fluids. For example, with shale gas production, stakeholders reported that flowback volumes can range from approximately 10,000 to 60,000 barrels per well for each hydraulic fracture. However, once the initial flowback ceases, the volume of water produced by shale gas production may be relatively small, sometimes decreasing to just a few barrels per day," the GAO said.
According to Environmental Protection Agency records, in 2010 there were 150,855 injection wells in the U.S. authorized for the injection of fluids brought to the surface during oil and gas production, including produced water. In most cases underground injection is the lowest cost option ($0.07 to $1.60) for producers to dispose of the liquid. Trucking water via truck or pipeline can significantly increase the costs, the GAO said.