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c
(a)
(b)
injecting CO 2
d
b
imbibing
water
e
a
(c)
(d)
0
1
Water fraction
(e)
Figure 9.7.2 Capillary hysteresis
The graph gives the brine saturation as a function of the capillary pressure. We start with
a formation with only brine (a). If we increase the pressure of CO 2 above the hydrostatic
pressure, CO 2 will fl ow into the largest pores (b). The amount of brine will decrease
because of the increase in capillary pressure, we displace brine from increasingly
smaller pores. The brine saturation will decrease until we reach the minimum brine satu-
ration for which the brine mobility is zero because the brine is trapped in disconnected
capillaries (c). If we now decrease the CO 2 pressure, we see that brine is wetting the
grains (d) and fi lling the small pores fi rst, thus isolating the CO 2 phase. As a result, the
wetting curve has for the same capillary pressure a lower brine saturation. If we again
reach the hydrostatic pressure, droplets of CO 2 remain trapped in the formation (e).
in order to force CO 2 to invade the porous medium, and k r, i increases with
S i , as each fl uid fl ows more readily if it occupies a larger fraction of the
pore space. Characteristic curves have been extensively investigated in
the case where the wetting fl uid is water and the non-wetting fl uid is air
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