Methane Occurrences in Aquifers Overlying the Barnett Shale Play with a Focus on Parker County, Texas

Abstract

Clusters of elevated methane concentrations in aquifers overlying the Barnett Shale play have been the focus of recent national attention as they relate to impacts of hydraulic fracturing. The objective of this study was to assess the spatial extent of high dissolved methane previously observed on the western edge of the play (Parker County) and to evaluate its most likely source. A total of 509 well water samples from 12 counties (14,500 km ) were analyzed for methane, major ions, and carbon isotopes. Most samples were collected from the regional Trinity Aquifer and show only low levels of dissolved methane (85% of 457 unique locations <0.1 mg/L). Methane, when present is primarily thermogenic (δ C 10th and 90th percentiles of -57.54 and -39.00‰ and C1/C2+C3 ratio 10th, 50th, and 90th percentiles of 5, 15, and 42). High methane concentrations (>20 mg/L) are limited to a few spatial clusters. The Parker County cluster area includes historical vertical oil and gas wells producing from relatively shallow formations and recent horizontal wells producing from the Barnett Shale (depth of ∼1500 m). Lack of correlation with distance to Barnett Shale horizontal wells, with distance to conventional wells, and with well density suggests a natural origin of the dissolved methane. Known commercial very shallow gas accumulations (<200 m in places) and historical instances of water wells reaching gas pockets point to the underlying Strawn Group of Paleozoic age as the main natural source of the dissolved gas.

Numerical Modeling of Methane Leakage from a Faulty Natural Gas Well into Fractured Tight Formations

“…Significant progress has been made in collecting data in advance of drilling for shale gas and in developing more robust geochemical and hydrogeological approaches to evaluating the source and the occurrences of methane in shallow aquifers (Darrah et al 2015a(Darrah et al , 2015bHarkness et al 2017;Heilweil et al 2015Heilweil et al , 2016Humez et al 2016;Jackson et al 2013a;Moritz et al, 2015;Nicot et al, 2017aNicot et al, , 2017bSiegel et al 2015;Vengosh et al 2014;Warner et al 2012aWarner et al , 2012bWarner et al , 2014). Yet, there is a need for modeling approaches to constrain the pathways, timing, and mechanisms of hydrocarbon migration to and within shallow aquifers (Schwartz 2013).…”

“…Yet, there is a need for modeling approaches to constrain the pathways, timing, and mechanisms of hydrocarbon migration to and within shallow aquifers (Schwartz 2013). For example, aquifers rarely can be characterized as homogenous porous media (Gorody 2012;Darrah et al 2015a;Nicot et al 2017aNicot et al , 2017b. Modeling gas transport in shallow aquifers overlying unconventional hydrocarbon reservoirs is complicated by complexities in the physical setting.…”

“…Such advances are essential in order to distinguish between natural and anthropogenic sources of hydrocarbons in shallow aquifers and for source attribution in instances where contamination of groundwater by anthropogenic gas has occurred. The shallow bedrock aquifers overlying the Marcellus and Barnett formations are good examples of these fine-grained clastic rocks, characterized by an inherently low matrix permeability (Warner et al 2012a(Warner et al , 2012bDarrah et al 2015a;Nicot et al 2017b). For example, aquifers rarely can be characterized as homogenous porous media (Gorody 2012;Darrah et al 2015a;Nicot et al 2017aNicot et al , 2017b.…”

“…For example, aquifers rarely can be characterized as homogenous porous media (Gorody 2012;Darrah et al 2015a;Nicot et al 2017aNicot et al , 2017b. Typically, groundwater wells completed in these kinds of units produce water at relatively low rates (Nicot et al , 2017bDarrah et al 2015a). The shallow bedrock aquifers overlying the Marcellus and Barnett formations are good examples of these fine-grained clastic rocks, characterized by an inherently low matrix permeability (Warner et al 2012a(Warner et al , 2012bDarrah et al 2015a;Nicot et al 2017b).…”

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.…”