University of Bath: Putting the fizz into salty water

Pools of salty water (brine) trapped beneath the seabed offer an unparalleled opportunity to sequester carbon and keep it trapped for millennia. Yet research in this area remains rudimentary, as little is known about the way sodium chloride (salt) behaves when it’s combined with carbon dioxide several kilometres beneath the surface of the earth, where conditions of heat and pressure are extreme.

Now a study from the University of Bath is shedding new light on the way saline solutions act in deep geological formations (known as aquifers), paving the way for further research into CO₂ sequestration beneath the seabed. The final aim of this work is for pipes to carry CO₂ from the earth’s atmosphere into these aquifers, where it will be stored harmlessly, potentially forever.

Working out how CO₂-laden salty water behaves under extreme conditions in the presence of rock is important. Will it dissolve in the water and react with the rock, or will it simply bubble back to the surface at the first opportunity, like the bubbles from a bottle of cola after it has been shaken and opened?

In an ideal world, the combination of seawater and CO₂ under pressure will result in the formation of rock, though it is more likely that the blend will retain its liquid form. “Providing the rock above the solution is fault-free and impermeable, the CO₂ will stay there,” said co-author Professor Philip Salmon, from the Department of Physics.

For the study, published in the Journal of Chemical Physics, the researchers observed saline solutions under conditions of pressure and temperatures that mimic the conditions found in deep aquifers. Their ‘neutron diffraction’ technique allowed them to examine saline solutions in more extreme conditions than ever before. Using this technique, they studied different isotopes (or versions) of sodium chloride, allowing new insight into the way salty water behaves under different sets of pressure and temperature conditions.

The chemistry of salty solutions mixed with CO₂
Little is known about the chemistry of mixing saline solution and CO₂ at high pressures and temperatures. Previous experimental efforts to find answers have failed because the solution, under extreme conditions, is highly corrosive and destroys the lab equipment it’s contained within before results are yielded.

“Being able to hold these solutions without the apparatus falling to pieces was a big challenge,” said co-author Dr Anita Zeidler, also from the Department of Physics. “We overcame them through the design of high-pressure apparatus and a judicious choice of containment materials.”

Describing the research, Professor Salmon said: “Our experiments show that by using neutron diffraction, you can see how the salt ions and water molecules interact under quite extreme conditions of heat and pressure. Next, we’ll be attempting to dissolve carbon dioxide into the saline solutions. The results from these experiments will inform models on carbon sequestration mechanisms, with the end-goal being to find a way to safely sequester carbon dioxide in deep-sea aquifers.”

The sequestration of CO₂ in deep aquifers is one of the global strategies being explored for carbon capture and storage. Pilot plants have demonstrated the success of this strategy, but ramping-up the scale depends on solving some key issues, such as storage capacity. It‘s therefore important for scientists to improve their knowledge of the physics and chemistry of CO₂ in the environment into which it is injected.

The researchers hope to find collaborators with an expertise in corrosion and corrosion resistance before they start the next phase of their project. “We want to study saline solutions under even more extreme conditions that fully fit in with real-life conditions, and these conditions will be even more corrosive. So, we could really benefit from the input of a corrosion specialist,” said Dr Zeidler.