WATER RESOURCES AND NATURAL GAS PRODUCTION FROM THE MARCELLUS SHALE

The Marcellus Shale occurs in the Appalachian Basin of the eastern United States in New York, Pennsylvania, West Virginia, western Maryland, and eastern Ohio. This shale contains significant natural gas resources; published estimates suggest that up to 363 trillion cubic feet (TCF) of gas ultimately may be recoverable. Recent advances in drilling technology, combined with higher wellhead prices have made the Marcellus Shale an attractive exploration target.

The Marcellus Shale was deposited as an organic-rich mud across the Appalachian Basin in the Middle Devonian. The production of commercial quantities of gas from such a fine-grained rock requires the creation of high permeability flowpaths within the formation to allow the gas to flow to a well. In the Marcellus, this is generally done by hydraulic fracturing, where water or other liquids under high pressure are used to create fractures in the rock. Along with pre-existing natural fractures, the hydraulic fractures act to provide a network of flowpaths for transporting gas to a well. Using directional drilling to penetrate long horizontal distances through the shale allows a series of hydraulic fractures to be generated from the horizontal borehole, contacting a very large volume of rock.

Several million gallons of water are required to drill and hydraulically fracture a typical horizontal Marcellus Shale gas well. Flowback fluids recovered from these wells, including the proprietary liquids used for the fracturing and any produced formation brines, must be handled and disposed of properly. The formation brines may contain high concentrations of sodium, chloride, bromide, and other inorganic constituents, such as arsenic, barium, various heavy metals, and radionuclides that significantly exceed drinking-water standards.

Large-scale production of gas from the Marcellus Shale may impact local water supplies, and also result in large quantities of potentially contaminated fluids that require disposal. New and ongoing investigations focused on improving the predictability of the gas resource, along with a better understanding of the geochemistry and evolution of formation brines may help to resolve these water resource concerns.