Environmental Factors Associated With Natural Methane Occurrence in the Appalachian Basin

Abstract

The recent boom in shale gas development in the Marcellus Shale has increased interest in the methods to distinguish between naturally occurring methane in groundwater and stray methane associated with drilling and production operations. This study evaluates the relationship between natural methane occurrence and three principal environmental factors (groundwater redox state, water type, and topography) using two pre-drill datasets of 132 samples from western Pennsylvania, Ohio, and West Virginia and 1417 samples from northeastern Pennsylvania. Higher natural methane concentrations in residential wells are strongly associated with reducing conditions characterized by low nitrate and low sulfate (However, no significant relationship exists between methane and iron [Fe(II)], which is traditionally considered an indicator of conditions that have progressed through iron reduction. As shown in previous studies, water type is significantly correlated with natural methane concentrations, where sodium (Na) -rich waters exhibit significantly higher (p<0.001) natural methane concentrations than calcium (Ca)-rich waters. For water wells exhibiting Na-rich waters and/or low nitrate and low sulfate conditions, valley locations are associated with higher methane concentrations than upland topography. Consequently, we identify three factors (''Low NO 3 − & SO 4 2− '' redox condition, Na-rich water type, and valley location), which, in combination, offer strong predictive power regarding the natural occurrence of high methane concentrations. Samples exhibiting these three factors have a median methane concentration of 10,000 μg/L. These heuristic relationships may facilitate the design of pre-drill monitoring programs and the subsequent evaluation of post-drill monitoring results to help distinguish between naturally occurring methane and methane originating from anthropogenic sources or migration pathways.

Controls on Methane Occurrences in Shallow Aquifers Overlying the Haynesville Shale Gas Field, East Texas

“…On the other hand, many samples with low‐methane show some sulfate (Figure S17). Positive correlation of dissolved methane concentration with that of chloride (Figure S21 crossplot) was already suggested by the Piper diagram; correlation with sodium is also positive (Figure S22) and stronger than that with chloride, an observation previously made by Molofsky et al () in the Marcellus Shale area. A few water samples, including one with some methane (HS46, 0.15 mg/L C1, −65.6‰ C1), show higher Ca concentrations.…”

“…Once it appeared that high‐dissolved methane water wells were clustered, additional sampling was conducted in and around the cluster in order to better define it. Wells were sampled only once, but recent studies on methane variability have suggested that, although indeed variable, methane concentrations do not change by orders of magnitude in a given well (Gorody ; Loomer et al ; Molofsky et al ; Smith et al ). The sampling occurred between February and June 2014.…”

Microbial and Isotopic Evidence for Methane Cycling in Hydrocarbon-Containing Groundwater from the Pennsylvania Region

“…Although, it has been detected in water wells of the Pennsylvania region for centuries (Molofsky et al, 2011, 2016), the origin of methane in aquifers and groundwater has remained misunderstood and it is likely that methane in the aquifers originates from a range of sources (Breen et al, 2007; Osborn et al, 2011; Wilson, 2014). Methane carbon and hydrogen isotopic ratios are frequently used to determine the possible source of methane in environmental samples (Whiticar, 1999).…”

“…Natural hydrocarbon and gas seepages are common in the Pennsylvania region and occurrence of methane in water wells has been observed for centuries (Molofsky et al, 2011, 2016). Therefore, this study aimed to investigate methane cycling and related microbial communities in methane-containing aquifers of the Pennsylvania region (Tioga County), from methane origin to methane oxidation.…”

Towards Quantifying the Likelihood of Water Resource Impacts from Unconventional Gas Development

“…In particular, there has been much discussion on the potential role of hydraulic fracturing, and opinion is divided on whether or not this poses a significant risk (Howarth et al 2011;O'Malley et al 2016;Darrah 2018). There are many recent case studies investigating the sources of methane in groundwater aquifers near shale plays (Molofsky et al 2016;Zhang and Soeder 2016;Nicot et al 2017aNicot et al , 2017bZhu et al 2018). 57, No.…”