Noble Gases Document Where Air has been Injected into the Subsurface

The COGG program has been exploring the use of noble gases as a tool for detecting oil-field fluids in groundwater. The findings from these two publications document what noble gases trace best and when they are most effectively used.

Noble gases and hydrocarbons in groundwater (Karolyte and others, 2021)

Noble gases are used in geochemistry to infer the source of fluids and transport mechanisms between sources and sample.

In this study, this method effectively distinguished between underground migration of fluid and leakage of fluid from sump ponds in groundwater samples. The major difference was the presence of an air signature in the noble gas mixtures linked to sump ponds, while fluids migrating in the subsurface lacked this air signature.

This study was conducted in the same locations as other COGG geochemical analyses and successfully documented the same conclusions arrived at using the geochemistry data; this indicates comparability between the two methods.

Using noble gas methods in future COGG work is most beneficial when sources and pathways are hard to identify such as when independent chemical signatures are not detectable due to hydrocarbon degradation or are at very low concentrations.

Noble gas analyses and gas from well casings and produced waters (Tyne and others, 2021)

Noble gases can help characterize hydrocarbon reservoir composition and how it changes over time.

Samples taken during this study document the influence of enhanced oil recovery practices on variability in the producing zone of CA oil fields; most notably, air signatures are evident in reservoirs and strongly correlate with amount of injection that has taken place within 500 meters (0.3 miles) of a well.

Understanding the difference between natural hydrocarbon reservoir conditions and injection-modified compositions is critical for determining the source of hydrocarbons if they are observed in nearby groundwater. The presence of air documented by noble gas signatures could indicate migration of fluids affected by injection; the absence of air could mean either natural migration, or the migration of fluids from hydrocarbon reservoirs unaffected by injection. Distinguishing between oil-field water originating from a) a surface sump pond, b) subsurface injection, and c) unaffected by injection would require the use of multiple geochemical tracers such as dissolved solutes, groundwater age tracers, and noble gases.

Noble gases sampled in well casing gas had similar composition to gases in produced waters, indicating that future studies may use casing gas instead of produced water samples.