ENVIRONMENTAL EFFECTS OF NATURAL GAS PRODUCTION FROM THE MARCELLUS SHALE

The Marcellus Shale occurs in the Appalachian Basin of the eastern United States throughout large areas of Pennsylvania and West Virginia, and in parts of New York, Maryland, Virginia, and Ohio. The formation underlies nearly 50,000 square miles of land and may contain more than 400 trillion cubic feet of recoverable natural gas. Recent advances in directional drilling and reservoir stimulation technology have made the Marcellus Shale an attractive exploration target.

The Marcellus is a Middle Devonian age, organic-rich black shale. Hydraulic fracturing is required to produce economical quantities of gas from such a fine-grained rock. Water or other liquids under high pressure are used to create fractures in the Marcellus Shale. These hydraulic fractures often intersect pre-existing natural fractures, providing a network of permeable flowpaths for natural gas transport to a well. Directional drilling that penetrates long horizontal distances through the shale allows multiple stages of vertical hydraulic fractures to be generated from the lateral borehole, creating permeable pathways into a very large volume of rock.

Several million gallons of water are needed to hydraulically fracture a typical horizontal Marcellus Shale well. Obtaining the required volume of water resources, and transporting these to often remote, rural well sites can have significant environmental impacts. Fluids recovered from the well after the treatment must be handled and disposed of properly. These fluids commonly contain a mix of the proprietary chemicals used for the fracturing, plus any produced formation brines. The formation brines often have high TDS, including sodium, chloride, barium, strontium, and other metals, plus unknown organic constituents and radionuclides that may exceed drinking-water standards.

Although natural gas as a fuel generally has less of an environmental impact than other fossil hydrocarbons, large-scale production of gas from the Marcellus Shale may affect watersheds, water supplies, and require the disposal of large quantities of potentially contaminated fluids. Investigations focused on improving the predictability of the shale gas resource, along with a better understanding of the geochemistry and evolution of formation brines may help resolve these environmental concerns.