Page 422 - Petrophysics 2E

390 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

area on the particles. When the injection brine has the same salinity as
the connate water, the exposed oil-wet portions of the particles retain
droplets of oil in some pores and filaments of oil fill a large number of
pores. When the injection-brine salinity is decreased, the equilibrium of
the electrical double layer (in the water between the particles) is upset,
causing expansion of the double layer and thus release of the particles
from the pore walls [15]. When oil is resting on layers of particles,
the injection brine displaces the particles with attached oil droplets
(and the oil filaments filling the pores). The cumulative mobilization
of particles and oil can produce a significant increase in oil recovery.
A tenfold decrease of injection-brine salinity increased oil recovery
at water breakthrough from 56.0% to 61.9% and ultimate waterflood
recovery from 63.6% to 73.2% [MI.

ALTERATION OF WETTABILITY

Wettability is perhaps the most important factor that affects the rate
of oil recovery and the residual oil saturation, which is the target of
enhanced oil recovery technology. Wettability controls the rate and
amount of spontaneous imbibition of water and the efficiency of oil
displacement by injection water, with or without additives.
The study of the effects of wettability on oil recovery is facilitated by
using additives to treat the rock surface so as to produce direct changes
through all degrees of wettability from water-wet, neutral, mixed, or
fractional to strongly oil-wet. In addition, water- and oil-soluble additives
are used to change, or establish, a particular state of wettability.
TREATMENT THE ROCK
OF

Several methods have been used to alter wettability:

(1) treatment with organosilanes of general formula (CH&3iClx; the
silanes chemisorb on the silica surface, producing HCl and exposing
the CH; groups which produce the oil wetting characteristics;
(2) aging under pressure in crude oil;
(3) treatment with naphthenic acids;
(4) treatment with asphaltenes; and
(5) addition of surfactants to the fluids.

Treatment of the cores or sand is conducted by first cleaning with
solvents, acids, steam, or heating to 250°C to destroy organic materials;
however, heating to such a high temperature dehydrates the clays and
changes the surface chemistry of the rock. After cleaning, the core is