We can bury our excess CO2, but will it escape?

Critics of carbon storage have well-founded concerns about the practice’s potential risks. Past studies have suggested that carbon dioxide stored underground could corrode the rock layers above it and escape.

A new study suggests that carbon storage is much safer than previously believed. Researchers at the University of Cambridge studied carbon dioxide that has naturally accumulated in deep underground reservoirs for around 100,000 years, and found that it has not significantly corroded the rock above. The results, published in the journal Nature Communications, imply that the greenhouse gas could be safely stashed underground without leaking back out.

Experts say that if carbon dioxide from power plants and industrial smokestacks is to be collected, compressed, and pumped underground, it needs to remain there for at least 10,000 years to avoid added global warming effects.

Carbon storage proposals call for CO2 to be pumped into porous rock layers. To keep it trapped underground, the key is to have a layer of impermeable “cap rock” on top. But there have been concerns that as the carbon dioxide mixes with water, the acidic brine that’s formed can slowly corrode the cap rock, making it more permeable. Some laboratory and computer modeling studies have supported this theory.

For the new study, the Cambridge researchers drilled down hundreds of meters into a natural carbon dioxide reservoir near Green River, Utah, and collected samples of cap rock and the fluid in the reservoir. They analyzed the chemical composition of the fluid and the rock, and measured the corrosion of the minerals in the rock.

Based on their analysis and computer simulations of the chemical reactions between the rock and brine, they estimated that the reservoir was 100,000 years old. Yet, the corrosion of the rock was only limited to a 7 cm-thick layer.

Previous studies were done for a short time period in the lab, or were based on theoretical models. This first field study of natural carbon dioxide reservoirs more accurately predicts how the gas behaves underground, said Mike Bickle, director of the Cambridge Centre for Carbon Capture and Storage, who led the work.

Of course, because different geological sites have different mineral compositions and chemical characteristics, the researchers caution that the safety of storage sites will have to be considered on a case-by-case basis. —Prachi Patel | 4 August 2016

Source: N. Kampman, et al. Observational evidence confirms modelling of the long-term integrity of CO2-reservoir caprocks. Nature Communications 2016. DOI: 10.1038/ncomms12268.