Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 427

Figure 9.7.3 Residual trapping
Schematic fi gure showing a CO 2 plume immobilized by residual gas trapping hundreds
of years after the end of CO 2 injection. The fi gure neglects any stratigraphic, solubility,
or mineral trapping.

trapped by residual gas saturation, even in the absence of a closed struc-
ture (anticline or dome) and associated caprock. Residual CO 2 trapping,
then, is one of the key mechanisms of carbon sequestration ( Question
9.7.1 ). Higher residual trapping yields smaller plume migration distances,
greater storage security, and higher storage capacities.
Residual gas trapping is measured using core fl ooding experiments,
where core samples of reservoir rocks, initially brine-saturated, are suc-
cessively invaded with CO 2 , then subject to brine imbibition at controlled
pressure and temperature. The value of S g at the beginning of brine imbi-
bition is known as the initial CO 2 saturation S g,i . The steady-state value
of S g reached during brine imbibition is the residual CO 2 saturation S g,r
( Figure 9.7.4 ). In the fi nal state, any CO 2 that remains in the column con-
sists of immobile, disconnected saturation (bubbles). At present, rela-
tively few high-quality data are available on the S g,r values of CO 2 in
reservoir rocks. Experimental results have been obtained only for small
core samples (with lengths 5 cm) and their applicability at much larger

scales (meter to kilometer scales of reservoir models) is unclear. Values
used in fi eld scale models and storage capacity estimates range from
S g,r

∼

0.05 to 0.4 (see Table 9.7.1 ).
Given the importance of residual trapping, current research is
focused on obtaining a better understanding of how residual trapping is
related to the properties of the rocks. Such understanding is important in

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