Thinking through carbon sequestration

I've been trying to figure out what to say to my classes about carbon sequestration. As Brian mentioned in a comment, if we're going to lock carbon dioxide in rocks (well, artifically*), geologists are going to need to be involved.

I ought to be able to think through the problem from first principles. Carbon dioxide is part of all sorts of natural systems, after all. When it dissolves in water, it makes the weak acid that is responsible for much of the natural weathering of silicate minerals. It's released by metamorphic reactions, and the relative amounts of carbon dioxide and water in metamorphic fluids is important in determining which minerals are stable. It's one of several gasses that dissolve in magmas, and is important in its own ultra-weird magmas: carbonatites, which erupt molten baking soda in the East African Rift (and which concentrate rare earth elements in old deposits). At low temperatures, carbon dioxide, carbonic acid, bicarbonate, and carbonate ions make a fascinating buffer system. (I don't think I explained that one very well, the one time I taught environmental geochemistry. But it's still a fascinating system.)

And that's just the chemistry. There are also questions about fluid flow through rocks and fractures, and about possible rock fracture associated with high fluid pressures. It's hard to know exactly where to start, especially because the info sources that I've read don't try to explain things from first principles.

So how, exactly, is this carbon sequestration supposed to work, and how does it relate to all the various CO2 factoids that I've accumulated over time?

The answer depends on the rock and fluid involved, it seems. A number of possible environments have been proposed, and the issues are somewhat different for each one.

1) Use waste CO2 to enhance recovery of oil and gas. I was vaguely aware that carbon dioxide was used to help recover more oil from old oil fields (mostly because CO2 is produced near Durango - it's actually a commercially produced commodity). I didn't know much about how it works, though - was it used to increase the fluid pressure in the rock, and force the oil out, or did the CO2 dissolve in the oil and change its properties? It turns out that both happen. Yes, the carbon dioxide changes the fluid pressure. But it also dissolves in the oil and makes it flow more easily, which makes it possible to recover more of the oil.

This is already being done - in fact, carbon dioxide is produced commercially to be used in oil fields. So it makes sense to capture waste CO2 and use it instead. (Perhaps this should be considered CO2 recycling rather than sequestration, however - I assume that CO2 dissolved in the oil comes back to the surface with the oil. Not that there's anything wrong with recycling - make less waste, use if for useful purposes instead. But it doesn't take the CO2 away forever.)

2) Pump the CO2 into coal. The methane that adsorbs onto the surfaces of coal has become a commercially important source of natural gas (especially in the San Juan Basin, just south of Durango). Traditionally, the methane is released by pumping water out of the coal. But carbon dioxide also adsorbs onto the surfaces of coal. Maybe CO2 could be used to enhance coal-bed methane production, too.

This technique is being tried in my backyard. If it works, it's got a lot of potential, because coal-bed methane and coal-burning power plants (as a source of carbon dioxide) can be very near one another. (In fact, there are currently two coal-fired power plants near Farmington, New Mexico.) I don't know much about the surface chemistry of coal, so I don't have a good sense of what factors could make this work or not.

3) Pump the CO2 into deep, salty formation water. This is the target of the experiment that Lee Allison described today. I'm not entirely certain of the characteristics of the ideal "saline formation" sequestration project. I think the idea is that a) the salty water is isolated from useable groundwater, probably by some kind of low-permeability cap (like a classic oil reservoir would have); and b) the salty water would make a good chemical buffer for the carbonic acid. I'm not sure of the chemistry of the buffering - this is where I wish that I remembered the complications that happen in the systems of carbonic acid, bicarbonate, and carbonate when there's something in the system other than calcite and carbon dioxide. (I'm guessing that the high concentration of dissolved solids helps buffer the system, since the focus isn't entirely on limestones.) I'm also not sure how the dissolution of carbon dioxide in water affects the potential for problems with increased pore fluid pressure. (Increasing the amount of fluid in rock can make rock break. If you want to get oil out of the rock, this is a good thing; if you want to keep carbon dioxide from escaping, I'm guessing that it would be bad. Unless you can make the reservoir rock more permeable without breaking the less permeable cap?)

And this doesn't include ideas about using carbon dioxide to speed the weathering of silicates. (I think that's partly what's going on with the suggestions to sequester carbon dioxide in basalt, for instance.)

So I'm not sure how to discuss the issue in intro classes. I think I need more information. I would like to tell them about the ideas that have been proposed, however - particularly because some are happening in our own backyard.

*Carbon is naturally locked in rocks like coal, oil shale, and limestones. We let it out when we burn oil, coal, or natural gas, or when we make cement. But natural processes don't remove carbon as fast as we burn it for energy. So if we want to use fossil carbon for energy, and we don't want to deal with the consequences of putting all that carbon dioxide in the atmosphere, we've got to do something to speed up the process.