One magic bullet that engineers like to talk about for global warming is carbon capture and sequestration (CCS) — pumping carbon dioxide into the right water-saturated geological formations just might form carbonate minerals, converting carbon dioxide gas (greenhouse gas!) to a greenhouse-neutral rock. There are some hurdles, though, that haven’t quite been resolved. Most of the current CCS projects inject the captured gas into oil formations to enhance oil recovery — this is means that some of the gas often comes back out with the oil. It’s a different task to inject carbon dioxide to form minerals.
First, you have to have the right geochemistry in the water and in the rock. Carbonates are actually really slow to precipitate, and in some cases in the lab, oversaturated solutions took months to precipitate out solids (chemistry note: thermodynamics is separate from kinetics). Second, you have to get the pressurization right in the geological formation. On the one hand, this means putting in the proper well casing and piping (the “walls” of the well hole) — see the BP oil spill for an example of improper well construction. On the other hand, this means ensuring that the geological formation doesn’t open up cracks under pressure and leak that carbon dioxide back to the surface. (This is the suspicion of what might go wrong in some fracking, for example: that certain formations might open preferential pathways for migration of gases and solutions upward into overlying aquifers of drinking water. This is also an unknown, but not one that I’ll address here.) Finally, there’s the risk that injecting pressurized fluids will lead to increased seismic activity. This is currently being documented as it occurs, but we’re far from predicting when and where it might occur (other than “in locations where we injected stuff…after we injected the stuff” which isn’t that helpful for planning or engineering purposes).
Well, as usual, scientists are working on stuff that will eventually be useful. A recent paper evaluated a new way to detect carbon dioxide leaking into shallow soils from deep formations. Traditional technology has required an extensive collection of background data to understand how carbon dioxide might naturally move through a geological formation. But the new method uses measurements of nitrogen, oxygen, carbon dioxide, and methane in soil gas to distinguish between naturally occurring processes and an extra influx of carbon dioxide from, say, a CCS injection site.
This is pretty sweet, because knowledge is going to save money on these projects, as well as give clearer answers that the traditional method could. Plus, this will resolve a big question, at least in my mind, as to the viability of this technology. We can deal with it if the kinetics of carbonate mineralization are slow underground, but if the carbon dioxide gas leaks back out, it isn’t really sequestered after all…