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

Abstract

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 with an isotopic signature consistent with migrated production 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, likely reflecting the use of oil-based drilling muds. 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.

Environmental risks and opportunities of orphaned oil and gas wells in the United States

“…Methane is not the only component of natural gas, and we can expect other non-methane air pollutants, including benzene, a known carcinogen, to be co-emitted [31,32]. Other non-methane air pollutants that can be co-emitted include hydrogen sulfide, toluene, ethylbenzene, and xylenes [33,34].…”

Highly Uncertain Methane Leakage from Oil and Gas Wells in Canada Despite Measurement and Reporting

“…In Canada, fluids entering this space are intentionally vented at the surface to protect groundwater, resulting in surface casing vent flow (SCVF). − SCVF is not particular to Canada, but many other jurisdictions (e.g., United States, China) − opt to keep the fluids sealed in the surface casing. While this may reduce methane emissions to the atmosphere, it can increase the risk of well blowouts and/or groundwater contamination. ,, Fluids may also escape the well entirely, traveling through the surrounding subsurface, referred to as “gas migration” (GM). , While GM generally emits much less methane than SCVF, this intrusion of petroleum fluids into the subsurface can pose a risk to groundwater.…”

“…14 , 23 , 26 Fluids may also escape the well entirely, traveling through the surrounding subsurface, referred to as “gas migration” (GM). 27 , 28 While GM generally emits much less methane than SCVF, this intrusion of petroleum fluids into the subsurface can pose a risk to groundwater.…”