The Marcellus Shale is an organic-rich, sedimentary rock formation in the Appalachian Basin of the northeastern United States that contains significant quantities of natural gas. Published estimates of the amount of gas that may be recoverable from the Marcellus Shale have been higher than 1.42 trillion cubic meters, or 50 trillion cubic feet [Engelder and Lash, 2008]. The recovery of commercial quantities of gas from a low-permeability rock like the Marcellus became economically possible with the application of directional drilling technology, which allows horizontal boreholes to penetrate kilometers of rock, combined with staged hydraulic fracturing to create permeable flow paths into the shale. Each hydraulic fracturing treatment may use more than 11 million liters of water (3 million gallons), which must then be recovered from the ground to allow gas flow [Harper, 2008].
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