Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 394

Table 9.3.1 compares the density and viscosity of CO 2 at surface and
deep underground conditions with those of other gasses. Unlike CO 2 , meth-
ane and air are not supercritical at the deep conditions and therefore have
much lower (gas-like) densities. Because of the density increase for super-
critical CO 2 the viscosity also increases, yet the viscosity of supercritical
CO 2 is still about an order of magnitude lower compared to that of a liquid.
Figure 9.3.1 shows a modifi ed phase diagram convenient for under-
standing CO 2 properties in the subsurface as a function of pressure,
depth, and temperature. Superimposed on the diagram are the lines that
connect the temperatures and pressures for which CO 2 has a constant
density (isochores). In addition, the diagram shows two hypothetical
paths; one in which the temperature increases 15

° C per km and one for

30 C per km, which are the typical temperature gradients found in differ-

ent geological formations. If CO 2 rises from the depths to the atmos-
phere, it will follow a path between these 15

°

C boundaries, and
we see that there is a narrow (green) triangular region where it may transi-
tion to a liquid. Furthermore, the upward migrating CO 2 will undergo a
large expansion, during which the Joule-Thomson effect will cause the
fl uid to cool. Both effects may result in the formation of a liquid with very
different properties from either gaseous or supercritical CO 2 [9.12, 9.13].

° C and 30 °

Temperature (ºC)

− 20 0 20 40 60 80 100 120

0 4

20 kg m -3
50 kg m -3
100 kg m -3

Gas

1

100

Liquid

2

200

3

300

Supercritical

4

400

Figure 9.3.1 CO 2 phase diagram
Inverted portion of the CO 2 phase diagram relevant to sequestration with superimposed
constant density lines (isochores) convenient for understanding phase conditions of CO 2
in the subsurface. Figure reproduced with permission from Oldenburg [9.11] .