How Rock Properties Shape Carbon Dioxide Flow, Determining Carbon Capture And Storage Performance

Carbon capture and storage or ‘CCS’ – capturing CO₂ emissions from industrial processes and injecting it deep underground – has been recognised by the IPCC for its potential to help nations meet net-zero targets by 2050.

For this reason, interest in CCS is growing, but key questions remain as to its feasibility and costs. One priority for geologists is to determine which geological characteristics ensure the safest CO₂ storage, without leaks to the environment.

Computer models allow geologists to look at how plumes of CO₂ might move when injected underground, but these forecasts carry uncertainties because of our limited knowledge about subsurface rock properties. That's because geologists can only directly see belowground at specific locations where vertical boreholes have been collected; everywhere else, they must infer what lies below based on surface observations.

The most detailed subsurface maps have a fairly grainy resolution, typically capturing features only at the 1-10 metre scale. “In general, there’s a lot of uncertainty about the subsurface geology, and the impact that belowground heterogeneities might have on CO₂ storage,” said PhD student Emily Flicos, who is based at the Department of Earth Sciences and Institute for Energy and Environmental Flows.

In a recent paper, Emily used computer models to investigate how variations in rock permeability controls the way injected CO₂ spreads underground (permeability is the ability of the CO₂ to penetrate the rock, which depends on the size and connectedness of gaps between the grains within a rock).

Rocks such as sandstone are some of the best reservoirs because they have plenty of interconnected pore spaces and therefore act like a sponge to soak up CO₂. Impermeable rocks, such as shale, have particles that are tiny and tightly packed – making them useful as caprocks to contain CO₂ once pumped into a permeable layer.

In her simulations, Emily set up a rock with horizontal variations in its permeability. She then introduced a plume of CO₂ into the rock and observed how the plume head propagated. She found that the plume head moved faster through higher permeability areas and slower in lower permeability zones. Over time, these horizontal variations in flow gave the plume head a steady, undulating profile that mirrored the underlying permeability pattern.

“How the plume head behaves is a deciding factor in the plume’s overall behaviour,” explained Emily. “Typically, you might want to know whether the nose could reach a fault or other geological feature that might encourage a leak. That’s important in trying to limit and contain environmental damage and make the technology as safe as possible.”

According to the IPCC, the risk of leaks is very low so long as geological sites are selected, designed and managed carefully. Understanding the circumstances under which CO₂ might leak is therefore key to prevention.

Another factor Emily has explored in an upcoming publication is how the slope of underground rocks determines the flow of injected CO₂. “Intuitively, you might think that steeply dipping rocks encourage faster flow,” she said, “but it turns out that the steeper the underground slope, the less distance the CO_₂ plume can travel before its wake becomes trapped.”