Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 426

continue until brine is in the continuous phase. The remaining bubbles of
CO 2 are trapped and the mobility of the CO 2 has now dropped to zero.
Once the permeability of the CO 2 phase has dropped to zero, we cannot
remove any additional CO 2 . After the experiment, we have again reached
the hydrostatic pressure, but we do not have the fully saturated brine
phase with which we started our experiment! This phenomenon by which
the relative permeability of a phase (brine during drainage, CO 2 during
imbibition) decreases to zero while a signifi cant quantity of that phase
(the residual phase saturation) remains present in the porous medium is
known as residual phase trapping .
This phenomenon of following different paths during drainage and
imbibition is called hysteresis and the resulting residual CO 2 saturation is
the basis of the capillary or residual trapping process that was introduced
in Chapter 8 ( Figure 8.2.3 and Movie 8.2.1 ).
Signifi cance of residual-phase trapping
Residual phase trapping has both a positive and negative effect on geo-
logical CO 2 sequestration.
The negative effect is that if we inject CO 2 in an aquifer, the residual
saturation of the water will reduce the effective capacity of a particular
geological formation.
The positive effect is illustrated in Figure 9.7.3 , which shows a post-
injection situation in which long-term CO 2 injection has produced a large
CO 2 plume that has migrated upward and is trapped against a gently
dipping caprock. At the leading edge of the CO 2 plume, water will be
displaced by CO 2 and the system will follow a drainage curve with
respect to relative permeability and capillary pressure. Along the drainage
curve, relative permeability of the rock to CO 2 is high and migration is
facilitated. At the trailing edge of the plume, the system will follow a wet-
ting curve as CO 2 migrates up-dip and groundwater imbibes the pore
space behind the plume. Along the wetting curve, relative permeability of
CO 2 in the rock is lower than along the drainage curve and, most signifi -
cantly, at the residual saturation of CO 2 , the relative permeability of CO 2
becomes zero. This residual CO 2 is effectively trapped. By this process,
an upward-migrating CO 2 plume eventually will lose all of its mobile frac-
tion to residual gas saturation. What this thought experiment illustrates is
that upward-migrating CO 2 plumes eventually become completely