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MAKING SURE THAT captured carbon dioxide doesn’t leak from underground stores could be as simple as installing a pump and a mixer, a chemical engineer suggests.

Steven Bryant, associate professor of chemical engineering at the University of Texas in Austin, says that instead of injecting compressed CO₂ directly into underground formations, engineers should look for deep formations filled with saline. The salt water can then be pumped out, mixed with compressed CO₂, and returned underground. Mixing the CO₂ with salt water makes it much more dense than the surrounding salt water, which inherently minimises the risk of any leakages. By contrast, pure CO₂ is considerably more buoyant than salt water, which has raised fears, particularly in the popular press, that geological CO₂ storage could turn the world’s oceans into a huge soda fountain.

Bryant says he had previously assumed that dissolution would be too expensive to be practical, but on closer investigation the cost was considerably less than he’d expected.  "Our work shows that this alternative process does cost more than the standard approach, but not prohibitively more. In essence, the incremental cost can be regarded as the price of risk reduction. This is an important consideration because all stakeholders will want the greatest assurance of secure storage for the lowest cost." He tells tce: "Having confirmed plausibility, my group is now going back to fill in the technical details of how one would go about implementing this."