Deep Underground Storage of C02

There have been suggestions that the C02 output from power plants could be pumped deep underground into cracks and pores in common alkaline rocks such as calcium aluminosilicates; there, the rocks could react with the gas, in microorganism-catalyzed processes, to produce calcium carbonate and thereby store the CO,. Such carbonate minerals are known to be present in deep caves in Hawaii and elsewhere, so the process could well be feasible if the reactions occur quickly enough. Recently, Norway has started to pump concentrated C02 gas into sandstone rocks located a kilometer below the North Sea; the pores in the rock were left empty by extraction of natural gas from them in the past. The C02 could react with the rock and thus be immobilized.

In the short run, the easiest route to begin sequestration of carbon dioxide is probably to inject it into reservoirs containing crude oil or natural gas. However, the total capacity for carbon dioxide storage by such enhanced oil recovery, labeled EOR in Figure 7-8, is less than 100 Gt. This technology is already used to enhance the recovery of oil in some fields, though currently most of the C02 is again recovered and reused.

In an interesting international project currently under way, 5000 tonnes a day of liquefied C02 are sent through a 325-km-long pipeline from a coal gasification plant in North Dakota to an oilfield in Weyburn, Saskatchewan. The gas is injected through pipes 1500 m underground, allowing more oil to be extracted from the field and sequestering most of the carbon dioxide in the brine of the oil reservoir. In 2004, after the project had been under way for four years, about 3.5 million tonnes of compressed carbon dioxide gas (of a projected 20 million tonnes eventually) had been sequestered at Weyburn. The injected C02 that accompanies the oil extracted from the field is captured and re-injected underground. Monitoring of the site found no significant amount of the gas to be escaping through the rocks and soil of the area. Any injected carbon dioxide escaping from the area could be identified by the characteristic low 13C/12C isotope ratio of the North Dakota fossil-fuel source; artificial tracer gases were also added to the injected gas to further monitor potential emissions from the site. Analysis of the fluid that accompanies oil produced in this way revealed that the carbon dioxide has been dissolving in the reservoir brine and has been trapped by some of the minerals in the reservoir. A similar sequestration project is planned for Norway, where carbon dioxide emissions from a plant that produces methanol from natural gas would be piped to offshore oil fields and injected into undersea reservoirs to help force oil to the seabed surface.

Depleted oil and gas reservoirs could be used to store carbon dioxide (Figure 7-9). These underground caverns are known to be stable, since they have held their original materials for millions of years. Carbon dioxide storage in coal seams that lie too far underground to be mined may also be feasible. Pumping C02 into the coal helps it release adsorbed methane, which can then be pumped to the surface and used. Several hundred gigatonnes of carbon dioxide could be sequestered in coal.

Much larger in volume and capacity than oil and gas reservoirs are saline aquifers, large formations of porous rocks that are saturated with salty water and that lie far underground, well below fresh-water supplies. Carbon dioxide injected into such an aquifer initially remains a compressed gas or a supercritical liquid, but some slowly dissolves in the sometimes very alkaline brine (Figure 7-9). The brine is contained mainly in small pore spaces occupying about 10% of the volume of the porous rock. If carbon dioxide is to be stored underground, a depth of more than a kilometer is required so that its density is comparable to that of water. Even so, it is likely to rise to the top of whatever geological formation encloses it and to spread laterally. Consequently, the caprock overlying the formation must be one that is secure if significant amounts of C02 are not to migrate upward through the soil and into the atmosphere over time. The stability of each aquifer to potential seismic activity and gas leakage must be individually assessed before it is used.

The Norwegian energy company Statoil has already demonstrated the feasibility of this approach by annually storing about a million tonnes of carbon dioxide (a 9% impurity that must be removed from their crude natural gas) in a saline aquifer that lies 1000 m under the floor of the North Sea. Interestingly, Statoil found it cheaper to sequester C02 this way than to pay the $50 per tonne carbon tax the Norwegian g overnment has instituted. The North Sea aquifers are sufficiently large to absorb all emissions of carbon dioxide produced by Europe for many hundreds of years. Experiments to store carbon dioxide in saline aquifers are under way in several locations in the world, including Texas and Japan. In the United States, large saline aquifers are found in the states lying just below the lower Great Lakes, in southern Florida, in northeastern Texas, and in the northern midwestem states.

Continue reading here: Removing C02 from the Atmosphere

Was this article helpful?

0 0