Page 416 - Petrophysics 2E

384 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

the world [64]: water-wet = 0-80°, intermediate-wet = 80-loo", and
oil-wet = 100-180". Using these criteria, they measured the wettability
of 161 cores composed of limestone, dolomitic limestone, and calcitic
dolomite, and found that 80% of the cores were oil-wet (Table 6.3).
Overall wettability and point-contact wettability are conditions
imposed on the boundaries of the water-oil and fluid-rock interfaces by
polar (NSO) compounds in the crude oil, depending on the chemical
properties of the water and rock surface minerals. An equilibrium
accumulation of surfactants at the interfaces can be destabilized by
changes of pH, water soluble surfactant, cationic concentration and
temperature. Once NSO compounds accumulate on mineral surfaces,
strong adhesive properties immobilize them and the contact area now
becomes oil-wet [14, 671. If the condition is distributed in a fragmented
(spotted) manner in the rock, a change in wettability from water-wet
to fractional wetting occurs. If the condition (precipitation of NSO
compounds) spreads through the rock, it establishes continuous oil-wet
zones in the pores of the rock; the wettability change from water-wet
will then tend toward an overall mixed wettability or, in an extreme case,
the fluid-rock system will change from water-wet to oil-wet.

EFFECT OF WETTABILITY ON OIL RECOVERY

Primary oil recovery is affected by the wettability of the system
because a water-wet system will exhibit greater primary oil recovery,
but the relationship between primary recovery and wettability has not
been developed. Studies of the effects of wettability on oil recovery
are confined to waterflooding and analyses of the behavior of relative
permeability curves. The changes in waterflood behavior as the system
wettability is altered are clearly shown in Figure 6.9. Donaldson et al.
treated long cores with various amounts of organochlorosilane to
progressively change the wettability of outcrop cores from water-
wet (USBM Iu = 0.649) to strongly oil-wet (I,, = -1.333) [48]. After
determining the wettability, using a smd piece of the core, they
conducted waterfloods, using a crude oil. The results show that as the
system becomes more oil-wet, less oil is recovered at any given amount
of injected water. Similar results have also been reported by Emery et al.
and Kyte et al. 18, 91.
Relative permeability curves are used for quantitative evaluation of
waterflood performance, and the effects of wettability can be observed
in changes that occur in the relative permeability curves (Figure 6.10).
In mixed wettability cases, however, the relative permeability of each
phase is a function of the saturation distribution of the two phases in the
rocks.