Global Warming - Energy and the Environment - page 289

reservoirs have a limited capacity for sequestering CO 2 . However, the potential storage capacity
of the world's proven oil and gas fields is about 140 Gt carbon, assuming they all will be depleted
eventually (which will occur in the not-too-distant future, say within 50-100 years). The problem
with using oil and gas reservoirs for sequestering CO 2 is not only the limited capacity, but also
the fact that very few existing large coal-fired power plants are within transport distance to the
reservoirs. Furthermore, urban-industrial areas where new power plants are going to be built are
usually far away from oil and gas fields. The transport cost of piping supercritical CO 2 is significant.
Currently it is estimated at $2 to $7 per metric ton of CO 2 per 250-km distance, depending on pipe
diameter (the larger the diameter, the lower the cost). Thus, a 1000-MW coal-fueled power plant
may have to spend in the range of $12 to $56 million per year for transporting the CO 2 to a depleted
oil or gas field that is 250 km away. This cost is in addition to the cost of capturing and compressing
the CO 2 at the power plant.
Sequestering CO 2 in depleted and semi-depleted oil and gas reservoirs can play a role in
mitigating global warming, albeit on a limited scale and at an economic cost that could not be
recovered from the price of electricity presently charged to customers.
10.4.4.2 Deep Ocean
The ocean is a natural repository for CO 2 . The ocean is vast: It covers about 70% of the earth's
surface, and the average depth is 3800 m. There is a continuous exchange of CO 2 between the
atmosphere and the ocean. We have seen in Section 10.4.1 that the ocean absorbs about 92 Gt y − 1
of carbon from the atmosphere, while it outgasses into the atmosphere about 90 Gt y − 1 . Thus, the
ocean is a net absorber of carbon, which probably is part of the reason that CO 2 concentrations
in the atmosphere do not increase as fast as expected from anthropogenic emissions. Most of the
ocean-atmosphere carbon exchange occurs within the surface layer of the ocean, which is on the
average about 100 m deep. This layer is more or less saturated with CO 2 . Between 100- and 1000-m
depth the temperature of the ocean is steadily declining (the so-called thermocline ). Beneath about
1000 m depth, the temperature is nearly constant, between 2 ◦ C and 4 ◦ C, and the density increases
slightly because of hydrostatic pressure. This makes the deeper layers of the ocean very stable;
and the “turnover” time—that is, the time it takes for the deep layers to exchange waters with the
surface layer—is long, on the order of hundreds to thousands of years.
The deep layers of the ocean, 1000 m or deeper, are highly unsaturated in regard to CO 2 .
The absorptive capacity of the deep ocean is estimated on the order of E(19) tons of carbon, so
conceivably all the carbon residing in fossil fuels on earth could be accommodated there, without
reaching even near the saturation limit. Because CO 2 emitted into the atmosphere eventually will
wind up in the deep ocean, an artificial injection of CO 2 at depth would merely short-circuit the
natural cycle that takes hundreds to thousands of years.
Carbon balance models predict that the atmospheric concentrations of CO 2 will increase more
or less in proportion to the amount of fossil fuel combustion. Peak concentrations will occur
somewhere in the twenty-third century, after which they will slowly decline because (a) mankind
will run out of fossil fuels and (b) the ocean will slowly absorb the excess CO 2 that built up in
the atmosphere. In about a 1000 years, a new equilibrium will be reached with atmospheric CO 2
concentrations slightly higher than today. The purpose of deep ocean injection of CO 2 is to shave
off that peak that would be building up in the next 200-300 years. Figure 10.11 shows how the
peak could be minimized by injecting incremental amounts of CO 2 into the deep ocean.
An injection of CO 2 at 1000 m or deeper is deemed necessary not only because the deep
layers are unsaturated with regard to CO 2 , but also on account of the physical properties of CO 2 .