USING GAS CHEMISTRY TO ASSESS CHANGES IN HYDROLOGIC CONNECTIONS ASSOCIATED WITH MARCELLUS SHALE GAS DRILLING

One of the major concerns in areas of accelerating Shale gas development is that stimulation of hydraulic fractures during drilling or leaky well casings can allow methane to escape into shallow underground sources of drinking water (USDW). Areas of Marcellus Shale development in the Appalachians have a long history of coal mining and oil/gas development. Thousands of abandoned wells and surface/underground coal mines in these areas along with natural faults/fractures can serve as additional pathways of methane leaks and migration. Methane can also occur in shallow aquifers underlying sewage plants, landfills, and coalbeds where pH, temperature and redox conditions are conducive for biogenic methanogenesis. Molecular and isotopic composition of dissolved gases can be used to fingerprint different methane sources as different pathways and degree of methanogenesis can impart unique signatures to gases in different aquifers. However, it is important to note that post genetic processes like mixing, migration and oxidation can overprint some of these signatures. Therefore, additional geochemical parameters should be used in conjunction with isotopic and molecular composition of gas to better understand and assess changes in hydrologic connections associated with shale gas drilling. Results will be presented from a few ongoing studies in southwest Pennsylvania and northern West Virginia to demonstrate how these geochemical approaches are being used to understand changes in hydrologic connections associated with hydraulic fracturing of Marcellus Shale.