A combined 228Ra/226Ra and 87Sr/86Sr approach to identify the production of out-of-zone formation fluid from Marcellus shale gas wells

Abstract

The Marcellus shale is generally characterized by low to irreducible water saturation, and gas wells in this formation typically produce small volumes of produced water compared to the volume of fracturing fluids injected. However, some Marcellus wells produce larger volumes of high-salinity fluids months or years into production, pointing to the possibility of out-of-zone water production. To identify the origin of these high-salinity fluids, we utilize a combination of ²²⁸Ra/²²⁶Ra activity and ⁸⁷Sr/⁸⁶Sr ratios measured in Marcellus produced water samples. Key findings show that: i) bulk rock Th/U and Rb/Sr mass ratios, in concert with fresh water ⁸⁷Sr/⁸⁶Sr leachates, can be used to develop predicted ²²⁸Ra/²²⁶Ra and ⁸⁷Sr/⁸⁶Sr ratios of pore waters within different formations, ii) the ²²⁸Ra/²²⁶Ra and ⁸⁷Sr/⁸⁶Sr ratios of Marcellus produced water are dominated by the Ra²⁺ and Sr²⁺ content of downhole formation fluids and water-accessible fractions, and iii) Marcellus wells with higher water-to-gas production ratios (e.g. >1 bbl water/10⁶ ft³ gas, or >5.6 m³ water/10⁶ m³ gas) and elevated TDS (>100,000 mg/L) exhibit lower ⁸⁷Sr/⁸⁶Sr ratios, consistent with a significant contribution from exogenous Onondaga pore fluids. The approach developed here has potential utility for identifying out-of-zone water sources in other oil and gas basins.