Page 420 - Petrophysics 2E

388 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

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Figure 6.11. Ultimate recovey as a function of wettability Maximum otl recovey
apparently occurs in neutral or sligbtly oil-wet rocks

a function of its mobility, which in turn is a function of capillary size and
wettability. The wetting phase has a lower mobility if it is located in the
smaller pores and is adhering to the rock surface. In an oil-wet system,
water breakthrough occurs very early in the flood; in fact, it may occur
before oil is produced if the water/oil viscosity ratio is very low. After
water breakthrough, production of oil continues with an ever increasing
water-to-oil producing ratio until a decision is made with respect to the
practical Sor for the waterflood.
The importance of the measurement of wettability is best illustrated
by the performance of waterfloods for systems at various states
of wettability. Donaldson et al. treated 30-cm-long sandstone cores
with increasing concentrations of an organosilane compound to
make progressively more oil-wet cores [48]. The cores were then
saturated with brine and reduced to the irreducible water saturation
by displacement of the brine with oil. A small piece of each core was
removed and tested for wettability, and waterfloods were conducted
using the remaining 25cm cores (Figure 6.9). A wettability range from
0.649 (strongly water-wet) to - 1.333 (strongly oil-wet) was achieved,
and the waterfloods show that a strongly water-wet system will have
breakthrough of water after most of the production of oil has taken
place and that very little production of oil will take place after water
breakthrough. As the system becomes more oil-wet, water breakthrough
occurs earlier in the flood and production continues for a long period after
water breakthrough at a fairly constant water/oil production ratio. Similar
work also was presented by Emery et al., using packs of unconsolidated
sand [8]. They obtained progressively more oil-wet sandpacks by varying
the aging time of the cores, at 71°C and 7 MPa pressure, from 5 to
1000 hours (Figure 6.12). Their results also show that, for a specific