A geochemical and multi-isotope modeling approach to determine sources and fate of methane in shallow groundwater above unconventional hydrocarbon reservoirs

Abstract

Due to increasing concerns over the potential impact of shale gas and coalbed methane (CBM) development on groundwater resources, it has become necessary to develop reliable tools to detect any potential pollution associated with hydrocarbon exploitation from unconventional reservoirs. One of the key concepts for such monitoring approaches is the establishment of a geochemical baseline of the considered groundwater systems. However, the detection of methane is not enough to assess potential impact from CBM and shale gas exploitation since methane in low concentrations has been found to be naturally ubiquitous in many groundwater systems. The objective of this study was to determine the methane sources, the extent of potential methane oxidation, and gas-water-rock-interactions in shallow aquifers by integrating chemical and isotopic monitoring data of dissolved gases and aqueous species into a geochemical PHREEQC model. Using data from a regional groundwater observation network in Alberta (Canada), the model was designed to describe the evolution of the concentrations of methane, sulfate and dissolved inorganic carbon (DIC) as well as their isotopic compositions (δ 34 S SO4 , δ 13 C CH4 and δ 13 C DIC) in groundwater subjected to different scenarios of migration, oxidation and in situ generation of methane. Model results show that methane migration and subsequent methane oxidation in anaerobic environments can strongly affect its concentration and isotopic fingerprint and potentially compromise the accurate identification of the methane source. For example elevated δ 13 C CH4 values can be the result of oxidation of microbial methane and may be

Comparison of isotopic compositions of hydrocarbon gas in shallow groundwater and a deep oil and natural gas reservoir in southeastern New Brunswick, Canada

“…Therefore, additional lines of evidence to distinguish between possible sources are required. Researchers have utilized other geochemical data such as the noble gases (Darrah et al 2014), groundwater inorganic geochemistry (Lautz et al 2014), and sulphur isotopes and geochemical modeling (Humez et al 2019). Here we use additional hydrocarbon compositional data coupled with geological information to support gas-origin interpretations.…”

Origin of sulphate in the unsaturated zone and groundwater of a loess aquifer

“…However, SO 4 in ecosystems does not behave conservatively due to the redox reactions of SO 4 (Devai & Delaune, 1995; Fry et al, 1988) and comprehensive distribution of sulphur‐contained minerals (Mayer, 2005). The geochemical characteristics and origins of SO 4 not only affect the carbon cycle through sulphuric acid weathering of carbonate rocks (Huang et al, 2019; Spence & Telmer, 2005) but also affect the nitrate cycle through sulphur oxidation and reduction (Aravena & Robertson, 1998; Humez et al, 2019; Kendall, 1998). Tracing the origin of sulphate is helpful to understand the hydrogeochemical processes of SO 4 when precipitation percolates through the UZ.…”

Reactive Transport Modeling of Natural Gas Molecular and Isotopic Evolution During Diffusive Transport in the Subsurface

“…Although it is recognized that identification of the source for hydrocarbon gas can be complicated by microbially‐mediated molecular and isotopic fractionations (Humez, Mayer, Nightingale, et al., 2016; Loomer et al., 2020; McMahon et al., 2017; Sherwood et al., 2016), mixing between sources (Moritz et al., 2015; Sherwood et al., 2016) and, less frequently, fractionations due to variable gas solubility (Eymold et al., 2018), possible fractionations resulting from diffusive transport are commonly neglected (Alperin et al., 1988; Darrah et al., 2015; Humez et al., 2019; Schloemer et al., 2018; Schoell, 1984; Whiticar, 1999). Recently however, Wanner and Hunkeler (2018) noted the importance of diffusive isotopic fractionation for a wide range of chemical species.…”