Composition and Origin of Surface Casing Fluids in a Major US Oil- and Gas-Producing Region

Fluids leaked from oil and gas wells often originate
from their
surface casinga steel pipe installed beneath the deepest underlying
source of potable groundwater that serves as the final barrier around
the well system. In this study, we analyze a regulatory dataset of
surface casing geochemical samples collected from 2573 wells in northeastern
Coloradothe only known publicly available dataset of its kind.
Thermogenic gas was present in the surface casings of 96.2% of wells
with gas samples. Regulatory records indicate that 73.3% of these
wells were constructed to isolate the formation from which the gas
originated with cement. This suggests that gas migration into the
surface casing annulus predominantly occurs through compromised barriers
(e.g., steel casings or cement seals), indicative of extensive integrity
issues in the region. Water was collected from 22.6% of sampled surface
casings. Benzene, toluene, ethylbenzene, and xylenes were detected
in 99.7% of surface casing water samples tested for these compounds,
which may be due to the presence of leaked oil, natural gas condensate,
or oil-based drilling mud. Our findings demonstrate the value of incorporating
surface casing geochemical analysis in well integrity monitoring programs
to identify integrity issues and focus leak mitigation efforts.