New research showing how naturally occurring noble gases can be used to track the movement of carbon dioxide (CO2) injected underground could provide a reliable monitoring technique for carbon storage operators worldwide.
With nations having committed to rapidly reducing carbon emissions to slow climate change, one approach is to deploy geological CO2 storage until cleaner energy sources are fully in place.
Storage operators must ensure that injected CO2, captured from industrial processes or power generation, is stored securely. That means being able to track its movement below ground and identify its source.
The new method developed by scientists at SCCS partner institute, the University of Edinburgh, stems from previous research suggesting that chemical tracers provide a fingerprint for injected CO2, which can be clearly distinguished from natural sources. However, the movement of these tracers below ground was poorly understood and it was uncertain whether they could act as an early warning of migrating CO2.
Using mass spectrometry, the researchers conducted time trials of the gas with different chemical tracers – including noble gases and sulfur hexafluoride – through samples of sandstone.
Their results, published in the journal Chemical Geology, show that the tracers take different times to travel through the same length of rock, with all of them arriving ahead of the CO2. The findings suggest that, if the tracers were added to CO2 before storage, any movement of the gas once injected into the store could be quickly detected.
Dr Rachel Kilgallon, formerly a PhD candidate at the School of GeoSciences, who undertook the study said:
Our work highlights that added tracers would provide an early warning of any unplanned migration of the injected CO2. Our findings confirm results from large-scale experiments in the USA, where tracers were seen to travel faster than CO2 through the rocks.
Dr Stuart Gilfillan, also of the School of GeoSciences, who co-ordinated the study said:
This new knowledge about the behaviour of CO2 and tracers in the subsurface will help in the development of monitoring techniques to ensure secure carbon storage. We found that the time taken for krypton and xenon to flow through the rock sample was almost identical to that of the standard industry tracer, sulfur hexafluoride. As sulfur hexafluoride is a potent greenhouse gas, our results show that climate-friendly krypton and xenon can be used instead in future tracing applications.
The paper, Experimental determination of noble gases and SF6, as tracers of CO2 flow through porous sandstone, is available online.