Using noble gases to trace the migration of hydrocarbons into shallow aquifers: separating signal from noise with inverse modelling techniques
Abstract
Hydrocarbon fluids can potentially leak into shallow aquifers during the production of conventional and unconventional systems, as well as during fluid injection for enhanced oil recovery and wastewater disposal, or from surface disposal ponds. Alternatively, hydrocarbon compounds may naturally occur in groundwater due to microbial activity or tectonically driven migration. The protection of groundwater resources requires robust analytical techniques to distinguish between natural and anthropogenic modes of hydrocabron migration into freshwater aquifers.
Noble gases are conservative tracers and are particularly powerful tools for tracing fluid migration in the subsurface. The noble gas composition of water is controlled by a number of factors, such as recharge temperature, water salinity and amount of excess air. These mechanisms are well understood and therefore allow us to constrain a baseline even in localities where no water data exists prior to the suspected interaction with hydrocarbons. Deviation from the equilibrium conditions may indicate interaction with natural gas or oil. We introduce an inverse modelling technique [e.g., 1] which allows us to statistically interrogate the incursion of deep fluids into the shallow groundwater system. We consider the expected atmospheric noble gas fractionation patterns in groundwaters interacting with different volumes of oil and gas in reservoir and atmospheric conditions. The technique allows us to identify natural gas migration through groundwater with high confidence, particularly where the volumes of fugitive natural gases are above the water solubility levels. Interaction with the oil phase produces a predictable fractionation pattern, however, the signal overlaps with that produced by groundwater equilibration at increasing recharge temperatures and higher salinity conditions. To further distiguish between these different processes, we incorporate radiogenic noble gas fluxes to groundwater for comparison to radiogenic noble gases introduced by fugitive hydrocarbons. This approach allows us to define the extent to which deep fluid contributions can be identified from background noble gas signatures in shallow groundwater in a statistically rigorous way. [1] Ballentine and Hall 1999, GCA 63, 2315-2336- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H51J1626K
- Keywords:
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- 3305 Climate change and variability;
- ATMOSPHERIC PROCESSES;
- 1803 Anthropogenic effects;
- HYDROLOGY;
- 1829 Groundwater hydrology;
- HYDROLOGY;
- 1875 Vadose zone;
- HYDROLOGY