Confused by carbonates

Somebody please help!

I’m having great difficulty reconciling two things that I’ve read:

1. There is a carbonate “saturation horizon” at a specific depth in the oceans. Below this depth carbonates dissolve because of the high pressure. (The “saturation horizon” depth is also less where it is colder).

What’s going on is that there is a chemical equilibrium:

Ca2+ + HCO3 <–> CaCO3 + H+

Adding CO2 to the oceans – a result of adding it to the atmosphere – makes the problem worse. It acidifies the water, driving this equilibrium to the left, in effect dissolving carbonates, such as the shells of marine organisms. (The big danger is that this process will raise the carbonate saturate horizon to the surface in the polar oceans, leading to a sudden increase in acidity in the absence of the carbonate buffer, which will reduce the ability of the ocean to absorb carbon dioxide, as well as prevent organisms from making carbonate exoskeletons).

2. There is a plan afoot to dump carbon dioxide underground, in gas and oil fields and in saline aquifers (”carbon capture and sequestration” or CCS). There was an interesting article on this by Fred Pearce in last week’s New Scientist (subscriber’s only, I’m afraid). Now, said Fred, “… the chemical reactions might gradually convert the CO2 into carbonate rock…”. But Fred also mentions the Frio project when the CO2 “…acidified the brine allowing it to dissolve metal-oxide minerals in the rock…” which “…might eventually create tunnels in the cap rock through which CO2 might escape”.

My question is, why wouldn’t the CO2 in general form an acid (I assume there’s plenty of water about) and dissolve the rock? In particular, how could it form carbonate rock, when, as we see in the oceans, CO2 in solution forms an acid which dissolves carbonate rock – more effectively at pressure? Surely this could only happen once all the CO2 had been converted to some intermediate form? – since otherwise any remaining CO2 would form acid and dissolve the carbonate. Can we therefore always rely on the sequestered CO2 staying where it’s put?

Of course, I’ve consulted “Sustainable Fossil Fuels” by Mark Jaccard who notes that: “… the CO2 may eventually either dissolve into the aquifer water (hydrodynamic trapping) or precipitate as a solid carbonate mineral by reacting with the surrounding rock (mineral trapping).” OK…

When I look at the IPCC Special Report on CCS, I see they go into all this in more detail, of course. I guess I’m happy with the chemistry – on its own – and I’m happy with the mechanics – permeability, cap-rocks etc. – on its own. It’s the interaction between the chemistry and the physics of the rock formations that bothers me. The IPCC notes that:

“Reaction of the dissolved CO2 with minerals can be rapid (days) in the case of some carbonate minerals…” (section 5.2.2.3, p.209).

and that:

“Reaction of the CO2 with formation water and rocks may result in reaction products that affect the porosity of the rock and the flow of solution through the pores. This possibility has not, however, been observed experimentally and its possible effects cannot be quantified.” (section 5.2.2, p.210).

Perhaps we’d better quantify it before we get our hopes for CCS up too high. What was the Frio project if it wasn’t an experiment? Puzzlingly, the IPCC report only mentions the Frio project as 1600tCO2 “pilot” (Table 5.1, p.201) and one of several that:

“…demonstrate that subsurface injection of CO2 is not for the distant future, but is being implemented now for environmental and/or commercial reasons.” (section 5.1.2, p.204).

What is this? A scientific evaluation or a sales brochure?

In general, is the approach being adopted to evaluating CCS one of identifying all the problems so that we can avoid them when we roll-out the technology, or one of trying to show that there are no problems, so that we can carry on planning to burn fossil-fuels (and building coal-fired power-stations) with as few qualms as possible?

One other annoying fact: liquid CO2 is less dense than water, so if there is enough pressure and the reservoir is not sealed, it’s the CO2 that will leak out, not the H2O.

Any comments that might help allay my fears are more than welcome.