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12.8 CO 2 Storage 385

According to Darcy’s law, the volume flow rate of a fluid through the porous
medium is

kA dp
J ¼ ð12:64Þ
l dx
3
where J is the volume flow rate in m /s, k is he permeability of the porous medium
2
2
(m ), A is the area (m ), µ is the fluid viscosity (kg/m · sorPa · s), dp/dx stands for
the pressure gradient in Pa/m over flow direction. One can estimate the bottom hole
pressure corresponding to a certain CO 2 injection rate or leaking rate.
Solubility trapping Solubility trapping is one major trapping mechanism in
saline aquifer storage of CO 2 . Dissolution of CO 2 in formation water can last as
long as 1,000 years, depending on the brine composition and pH as well as the
mineralogy of the reservoir.
Mineral trapping Mineral trapping is resulted from the chemical reactions
2+
2+
between CO 2 and metal ions such as Ca ,Fe 2+ and Mg , which are rich in the
surrounding rocks. As the carbonates produced precipitate in the rock pores
the reactions slow down over time. As a result, it takes 10–10,000 years to saturate
the pores. Meanwhile, the chemical reactions take place in the pores of the cap rock,
improving the integrity of cap rock over time.

12.8.4 Deep Ocean Storage

The ocean naturally traps over 143,000 Gt of CO 2 , which is 50 times more than that
in the atmosphere. The uptake of CO 2 in ocean has been increasing over the past
centuries as a result of the increasing atmospheric CO 2 concentration. However, it
still can hold much more! A comprehensive documentation of deep ocean CO 2
storage is given by Caldeira et al. in the form of a SRCSS Special Report to IPCC
[32] (Chap. 6, Ocean Storage).
Similar to what was introduced in mass transfer in absorption (Sect. 2.3.4),
natural CO 2 storage in ocean is a chemical absorption process. CO 2 enters ocean
through the surface water, where the equilibrium concentration is governed by
=H, where H is the Henry’s law constant for CO 2 -sea
Henry’s law, x CO 2 ¼ P CO 2
water system.
The solubility of CO 2 in seawater is not constant everywhere; it depends on the
pressure, salinity, pH, and temperature of seawater.
Deeper into the ocean, CO 2 goes through chemical reactions. Several simplified
chemical equilibriums define the process as
þ
CO 2 þ H 2 O $ H 2 CO 3 $ HCO þ H $ CO 2 þ 2H þ ð12:65Þ

3 3