The world's nations want to agree on steps to counter climate changes due to greenhouse gases, so very soon the ways we handle fossil fuels will begin a drastic change. As we look forward to new technology that reduces greenhouse gases, a prime question is how to keep contained the most significant greenhouse gas, carbon dioxide. Here are two approaches to that sequestration problem.
Geological Sequestration
Right now, petroleum producers are using carbon dioxide to push oil and gas out of deposits that are running low on pressure. In fact, they are eager for supplies of the gas. This technology thus could produce money in two ways, by serving this market for CO2 and by drawing more energy from existing deposits.
A depleted oil or gas reservoir is a natural place to store CO2, in that the void space is being vacated already. Also, the natural barriers that preserved the oil or gas over geologic time should be strong enough to hold the CO2 well into the future.
Another attractive site for this kind of sequestration is coal beds that are unsuitable for mining. Here, in fact, the coal can hold on to the CO2 by the chemical process of adsorption while releasing methane, so we could get some natural gas at the same time.
Finally, deep deposits of salty groundwater look like good possibilities. These deep saline aquifers are quite common around the world and offer large amounts of underground space.
One disadvantage of all these is that we don't yet know their disadvantages. For instance, the rock formations may leak, or deep brines may change chemistry given so much CO2. But experiments have begun, and we will inevitably learn what we're doing.
There may be policy problems with geological sequestration. Petroleum producers have an incentive to pump CO2 underground, but less incentive to make sure it stays there for centuries. That's analogous to the problems we have with the waste products of nuclear energy. Resistance from industry may work against what is best for the long term.
There are other policy questions for which governments may need scientific guidance that serves a long-term vision. Disposing of such large amounts of waste CO2 is a one-way, one-time process that should not be considered a permanent path of technology. And while geological sequestration would allow fossil-fuel use to have smaller climate effects, it would not reduce the other environmental effects of large-scale mining such as land destruction and acid drainage. Finally, geological sequestration could let us keep relying on fossil fuels longer than we should instead of moving to renewable, sustainable energy sources.
Mineral Sequestration
Most of the world's carbon is locked up in minerals. We could do the same with waste carbon dioxide.
Earth's long-term carbon cycle takes metal ions from rocks, mainly calcium and magnesium, and combines them with CO2 and water to yield various carbonate minerals, mostly calcite and dolomite, and a residue of clay. Plate tectonics carries these down into the hot mantle, where CO2 and water are liberated again and the metal ions return to fresh magma. (Living things help the cycle by making seashells from calcite.)
Industrial processes are being designed that mimic or interact with this mineral cycle. One of these being developed under the name GreenCarbon begins with the metal-rich minerals olivine and serpentine, found in ultramafic rocks. Finely crushed stone is combined with CO2 to yield carbonate minerals and other byproducts. Another proposal called the House process, based on acid-washing basalt, acts indirectly by affecting the oceanic carbon cycle. It would yield industrial-quality acids and alkalis.
Both of these technologies, and others being proposed, would not only neutralize CO2, but also create new sources of industrial materials and reuse waste material from current mining. Their promise, then, goes beyond greenhouse gases to offer ways of avoiding new mines. They would improve upon geological sequestration in that respect. But they still would serve to stretch fossil-fuel technology longer when the objective is to stop burning fossil fuels entirely.
But we won't stop burning fossil fuels anytime soon. As long as sequestration does not impede more sustainable energy sources, it's a technique that can buy us more time.
