Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 393

Table 9.3.1 Density and viscosity of water, brine and different gasses

(Pressure/
Temperature) Brine Water Air CO 2 CH 4

Density
(kg/m 3 )

Ground
surface

(0.1 MPa,10 ° C) 1,190 990 1.2 1.9 0.68

Deep system
(2 km)

(20.1 MPa, 60 ° C) 1,200 1,000 200 725 130

Viscosity
(10 − 6 Pa s)

Ground
surface

(0.1 MPa,10 ° C) 1,800 1,300 18 14 11

Deep system
(2 km)

(20.1 MPa, 60 ° C) 940 470 24 60 18

Density of pure water and sodium chloride brine at two different representative conditions [9.11].

The prevailing temperature in deep sedimentary basins is controlled
by the geothermal gradient, generally around 25°C/km. In addition to pres-
sure and temperature, composition controls the properties of the fl uids.
Groundwater at depths greater than several hundred meters is typically
salty. This is the result of long-term dissolution of minerals and the ten-
dency for downward migration of saline water due to its greater density
relative to fresh water. At depths of 1 km or more, groundwater is often very
salty. If its salinity is greater than that of seawater ( 35,000 mg/L (ppm)), we

refer to this water as brine. The densities and viscosities of pure water and
of brine at representative 2 km deep subsurface and shallow ground sur-
face conditions are compared in Table 9.3.1 . Due to its relative incom-
pressibility, the density of groundwater does not vary much with depth. The
viscosity of a fl uid increases at higher density but decreases at higher
temperature. Table 9.3.1 shows that the temperature effect dominates.

∼

Properties of CO 2
As we saw in Chapter 1, the critical temperature and pressure of CO 2 are
31°C and 7.4 MPa, respectively. In what seems like a quirk of nature, the
hydrostatic pressure and geothermal gradients in the subsurface at
around 800 m depth coincidentally bring CO 2 to its critical point. Under
typical static conditions free phase CO 2 will be either gaseous, e.g., at
shallow depths, or supercritical, e.g., in the subsurface at depths greater
than approximately 800 m [9.11].