ISOTOPIC EVIDENCE OF MICROBE-WATER-ROCK INTERACTIONS IN SHALE GAS PRODUCED WATERS

Isotopic characterization shale gas produced waters plays a key role in determining microbe-water-rock interactions in deep shale formations. Microbial activity within a natural gas reservoir can be beneficial (e.g. microbial methane contribution), or detrimental (e.g. well infrastructure corrosion due to H₂S production, precipitates that might plug fractures) depending on the nature of the microbes present. To test the efficacy of stable isotopes to predict microbial interactions, produced waters were collected from Marcellus Shale wells in Pennsylvania. The carbon isotope value of dissolved inorganic carbon (δ¹³C_DIC) in produced water samples collected from Marcellus wells range between +10 to 29 ‰ V-PDB respectively. Such high δ¹³C_DIC signatures are indicative of microbial methanogenesis suggesting that methanogens might exist in these deep shale formations. Quantitative polymerase chain reaction (qPCR) analysis of a produced water sample from the Marcellus Shale well detected the presence of Methanosarcinales sp. These methanogens are known to use a wide variety of organic substrates (inc. methanol, methylamines, and acetate) during the production of methane. Temporal monitoring of δ¹³C_DIC of flowback waters from two gas well shows a rapid increase in δ¹³C_DIC signatures during the first few days, followed by a slow increase over a period of several months. The rapid enrichments trends during early phase could possibly reflect dissolution of ¹³C enriched reservoir rocks during the initial phase of hydraulic fracturing. On the other hand, the slow increase in ¹³C values over time could possibly indicate microbial reactions in the reservoir that utilize ¹²C for their metabolic processes. Our preliminary data demonstrates how temporal isotope monitoring in conjunction with genomic tools could provide valuable insight into the microbe-water-rock interactions that occur after hydraulic fracturing fluids are introduced into these deep shale systems. Understanding these biogeochemical reactions is essential for developing new methodologies for preventing well souring/corrosion, bioremediation of organic contaminants, enhanced shale gas recovery and production.