WATER OF CONDENSATION: A NEWLY IDENTIFIED WATER END-MEMBER IN SHALE GAS WELLS

Time-series produced water (PW) chemical data from shale gas wells commonly exhibit a rapid increase in salinity and enrichment in δ¹⁸O values, during the first 1 – 3 weeks of production. These compositional changes are attributed to a transition from extraction of mostly injected water immediately after hydraulic fracturing to mostly naturally-occurring formation water over time, reaching a plateau of stable composition defined by maximum salinity and enriched δ¹⁸O values. We present a new pattern of low salinity and isotopically light water observed in some Marcellus shale wells during later stage (i.e., months or more) production as the result of water condensing out of the gas phase. The late-stage shift in the chemical and isotopic PW composition is most common in Marcellus shale gas wells producing <1bbl/mmcf (daily average) and is consistent with equilibrium conditions for Marcellus gas at reservoir temperature of ~60°C (0.3 bbl/mmcf). The water vapor, which is relatively fresh, is hypothesized to condense as fluid temperatures decrease once it leaves the reservoir. Geochemical evidence for water of condensation was identified ~45 years in deep gas systems (>3000 m, >100° - 150°C), but isotopic evidence is lacking and the process has not been shown to occur in much shallower shale gas plays.

End-member mixing between water vapor in equilibrium with liquid water at reservoir temperatures and formation water compositions is able to recreate most of the observed isotopic compositions from these PW data. That being said, the proportion of water of condensation estimated based on δ¹⁸O and δ²H compositions versus chloride concentrations differ, with the latter yielding lower proportions. Modifying the composition of water vapor due to Rayleigh fractionation in the gas reservoir produces more similar results between the two methods suggesting that isotopic fractionation may be an important process affecting the δ¹⁸O and δ²H values of PW as the formation “dries out”, enriching the isotopic and geochemical composition of downhole formation fluids over time. Results from this research suggest for the first time that water of condensation may be present in other shale gas reservoirs, especially from wells with low water/gas ratios, and should be investigated as a potential end-member in these systems.