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(a)
(b)
(c)
(d)
Figure 8.2.2 Mechanisms of CO 2 trapping
(a) Stratigraphic trapping: the injection of CO 2 displaces the brine from the aquifer,
leaving a large plume of supercritical CO 2 . This supercritical CO 2 is less dense than
brine, so because of buoyancy effects it will move toward the highest point of the
aquifer. The caprock prevents the CO 2 from moving out of the storage formation.
(b) Residual trapping: at the end of the CO 2 injection, as the CO 2 moves to the highest
point of the aquifer, the brine fl ows back into the trailing edge of the plume. As the
brine wets the rock, it preferentially fi lls the smallest pores and pore throats in the
rock, and CO 2 bubbles become trapped by capillary forces in the larger pores.
(c) Solubility trapping : the droplets of CO 2 that are residually trapped will slowly dis-
solve in the surrounding brine. The dissolution of CO 2 in brine generates carbonic
acid. The coexistence of supercritical CO 2 , brine, and carbonate minerals buffers
the pH of the brine to about 5.
(d) Mineral trapping : the low pH of CO 2 -equilibrated brine causes the weathering of
silicate mineral grains. The weathering reactions of certain silicate minerals release
divalent metals (Ca, Mg, Fe). These ions combine with CO 2 to form carbonate miner-
als (such as limestone, CaCO 3 ), the most stable state of carbon.
This trapped amount will depend on time but may constitute roughly
20% of the total amount of injected CO 2 ( Figure 8.2.3 ). In this capil-
lary trapping or residual trapping , part of the CO 2 becomes immo-
bilized as small bubbles at the trailing edge of the mobile CO 2 plume,
while the plume itself continues to migrate to the highest point in the
formation. Eventually, CO 2 will dissolve in the brine, a process known
as solubility trapping . Finally, the dissolved CO 2 will react with min-
erals such as feldspars to liberate cations (e.g., Mg 2+ , Fe 2+ , Ca 2+ ), and
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