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The Columbia basalt is volcanic, rather than sedimentary, rock.
But based on the Energy Department's core samples of the area and other
research, McGrail believes the Columbia group has real sequestration potential.
Lava oozing out of a fissure can contain high volumes of
trapped gas, such as sulfur dioxide and CO2. These gases will push toward the
top of the flow to escape. As the lava begins to set, some of the gas is trapped
in bubbles, which form the pores or vesicles that are the targets of CO2
injection. The more bubbles, the more surface area is available for the CO2 to
make contact with basalt's minerals. The cylindrical cores McGrail has studied
are about three inches in diameter and clearly show the boundaries between lava
flows, interrupted periodically by thinner, small-grained layers from
non-eruptive periods, when windblown soil, volcanic ash, and other materials
drifted across the landscape.
Because the Columbia basalt is made up of many separate flows,
it has numerous alternating porous and dense layers. McGrail thinks the former
can absorb and transform large amounts of CO2 and the latter can serve as an
effective caprock, aided by the occasional sedimentary layer that lies in
CO2 and basalt have attributes whose combination could be a
marriage made in heaven. At depths below around 3,000 feet, CO2 becomes
supercritical — that is, it turns into a liquid slightly less dense and much
runnier than water. Injection pressure and the weight of the earth above it
will force the CO2 to dissolve in groundwater residing in aquifers and
distributed throughout the small cracks and holes in porous sections of basalt.
As anyone familiar with Perrier can attest, dissolved CO2 makes water fizzy;
this sparkling, mildly acidic "pore water" reacts with minerals in
the basalt, principally calcium, and eventually breaks up CO2 molecules,
sequestering their carbon in solid deposits of calcium carbonate, also known as
limestone. Over long time periods, further reactions convert the available
elements into even more stable types of rock, such as olivine.
When McGrail first started working on basalt sequestration, he
thought it a wacky idea. Experience has since changed his mind, but other
experts still question the details.
David Keith, a professor of chemical and petroleum engineering
and economics at the University of Calgary, says bluntly, "I don't think
(basalt) is that important. Saline (aquifer) capacity is gigantic. I think
(basalt) doesn't matter for a long time. We're not going to run short for half
a century even if we do this at a huge scale."