T124 Dissolved Gases and Bubbles in Groundwater: Applications and Emerging Topics

Monday, 5 November 2012
1:30 PM - 4:45 PM
213A

UNDERSTANDING NATURAL VARIATIONS OF DISSOLVED METHANE IN AREAS OF ACCELERATING MARCELLUS SHALE GAS DEVELOPMENT

Paper number: T124-11
Program: Technical Sessions
Time: 3:50 PM – 4:05 PM

Authors

Abstract

There is rising concern that hydraulic fracture stimulation during shale gas drilling or leaky well casings can allow dissolved methane (commonly referred to as stray gas) to escape into public drinking water supplies, which can be an explosion and fire hazard. However, in coal mining areas of the Appalachians, methane can also originate from shallow gas bearing strata, coalbeds and gas storage fields. Therefore, an increase in methane concentrations cannot always be attributed to Marcellus Shale drilling activity. To understand any methane leaks associated with Shale gas drilling there is a need to 1) understand baseline variations in dissolved methane concentrations, and 2) develop a suite of natural geochemical tracers that can track sources of dissolved methane in the groundwaters and surface waters of the area.

In this study, groundwater samples were collected from aquifers overlying Marcellus shale in north-central West Virginia, with no active shale gas drilling in the study area during collection. Samples were analyzed for C and H isotopic signatures and concentrations of dissolved methane, dissolved CO2 concentrations, and C isotopic signatures of dissolved inorganic carbon. To understand the isotopic compositions of possible sources of methane in study area, samples were collected from producing wells in shallow Devonian sands and Marcellus Shale, deeper gas producing formations, and a coalbed methane well. The isotopic and geochemical data suggests that dissolved methane in the sampled groundwaters is primarily generated by deeper thermogenic sources and/or microbial sources via CO2 reduction in deeper marine formations. Furthermore, samples with higher methane concentrations do not show a clear correlation with abandoned oil and gas wells, coalbed methane wells and/or abandoned mine lands. We propose that methane from deeper sources is migrating into the sampled groundwater aquifers through natural fractures and faults over geological time scales.