Tracing produced water origins from
wells hydraulically fractured
with freshwater-based fluids is sometimes predicated on assumptions
that (1) each geological formation contains compositionally unique
brine and (2) produced water from recently hydraulically fractured
wells resembles fresher meteoric water more so than produced water
from older wells. These assumptions are not valid in Williston Basin
oil wells sampled in this study. Although distinct average 228Ra/226Ra ratios were found in water produced from the
Bakken and Three Forks Formations, average δ2H, δ18O, specific gravity, and conductivity were similar but exhibited
significant variability across five oil fields within each formation.
Furthermore, initial produced water (“flowback”) was
operationally defined based on the presence of glycol ether compounds
and water from wells that had produced <56% of the amount of fluids
injected and sampled within 160 days of fracturing. Flowback unexpectedly
exhibited higher temperature, specific gravity, conductivity, δ2H, and δ18O, but lower oxidation–reduction
potential and δ11B, relative to the wells thought
to be producing formation brines (from wells with a produced-to-injected
water ratio [PIWR] > 0.84 and sampled more than 316 days after
fracturing).
As such, establishing an overall geochemical and isotopic signature
of produced water compositions based solely on chemical similarity
to meteoric water and formation without the consideration of well
treatments, well completion depth, or lateral location across the
basin could be misleading if these signatures are assumed to be applicable
across the entire basin. These findings have implications for using
produced water compositions to understand the interbasin fluid flow
and trace sources of hydraulic fracturing fluids.