Introduction to Geological Sequestration - Introduction to Carbon Capture and Sequestration - page 356

The fi rst idea would be to eliminate the capture process and inject
the fl ue gasses directly. Let us assume that just below the power plant
we have an ideal geological formation in which we can inject our fl ue gas.
We will assume that this ideal geological formation has the shape of a
disk with a thickness of 10 m. The question now is: what will be the radius
of our disk fi lled with CO 2 after 50 years of injection?
Before answering this question we have to know a bit more about the
geological conditions. To compute the density of our gas, we need to
know the temperature and the pressure in the target formation at our
injection site. The deeper our injection zone, the higher the temperature.
The geothermal gradient can range from 15-30 C/km depending on the

geothermal activity of the region. Pressure also increases with depth
according to the hydrostatic pressure gradient (about 0.1 bar/m). If we
assume that we inject the fl ue gas at a 2 km depth in a region of small
thermal activity, and we know that at ambient conditions the density of
air is 1.3 kg/m 3 , then the average density of air in the storage formation
(200 bar, 60

°

C) is 200 kg/m 3 . We see that at storage conditions, the vol-
ume of our fl ue gas decreases to 4 x 10 9 m 3 .
We now have to know a little more about the geological formations.
The formations in which we would inject are very similar to those that store
natural gas. In the next chapter, we will discuss in more detail what these
formations look like, but for our calculation the most important factor is
the porosity: if we inject our fl ue gas, what percentage of the rock volume
will be occupied by the gas? We assume optimistically that the porosity is
about 40% and that our injection accesses 50% of the available pore
volume. This gives us a capacity of 20% of the total volume of our forma-
tion. Hence, we need for our total 50 years of fl ue gas production a geo-
logical formation with a total volume of 2 x 10 10 m 3 . If we assume that we
inject into a disk of 10 m thickness, this gives us a radius of 25 km.
Let us now compare this number with the situation in which we cap-
ture the CO 2 . Because we concentrate the CO 2 and thus decrease the
nitrogen fraction in our capture stream, we decrease the volume by a
factor of 7.7. In addition, the density of CO 2 at fl ue gas conditions is
1.7 kg/m 3 , and 2 km below the surface it is 725 kg/m 3 (200 bar, 60

°

C, see
Figure 8.1.1 ). Unlike air, CO 2 is supercritical at these conditions and
hence has a much larger density. The effective volume of the geological
formation that we need for sequestering the captured CO 2 decreases by
an additional factor of nearly 4, to 7 x 10 8 m 3 , which would give us a
radius of 5 km.

°