Page 424 - Petrophysics 2E

392 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

(2) the greatest amount of recovery occurs with neutral-wet systems
(8%- 10% higher); and
(3) the least amount of oil recovery was obtained from the oil-wet
samples, which exhibited early water breakthrough followed by a
low rate of oil recovery.

Kowalewski et al. found that they could control the changes from
water-wet to neutral using Berea cores, n-decane, and oil-soluble
hexadecylamine [93]. Waterfloods resulted in an almost linear correlation
between the concentration of the amine and an increase of oil recovery
(decrease of S,) as the system changed from water-wet (r, = 0.7) to
neutral (IA = 0.05). Langmuir isotherms were used to test the amounts
of amine adsorbed on crushed rock from various concentrations that
were used: the results ranged from 0.007 to 0.230 mg/g of rock.
Thus the change of wettability was directly related to the amount
of hexadecylamine that was adsorbed from the oil. Donaldson et al.
developed a method for determining the Iangmuir and Freunlich
isotherms and calculating the thermodynamic heats of adsorption of
organic compounds on sandstone cores [94]. The adsorption isotherms
showed maximum 'amounts of adsorbed compounds that varied from
0.200 to 10 mg/g of sandstone. The rates of adsorption at various
temperatures were also measured.
Many studies of the feasibility of using surfactants and caustics
dissolved in water to enhance the rate and total recovery of oil
from sandstone cores have been made [95, 961. In addition, the
U.S. Department of Energy conducted several field tests to evaluate
the potential of surfactant/polymer water floods for mobilization of
residual oil [97]. Surfactants and caustics lower the interfacial tension
and, intuitively, this should result in economically enhanced oil
recovery, but the results have generally been disappointing; enhanced
recovery (recovery of more oil than the So, of waterfloods) is usually
less than 5% regardless of the applied technology (surfactant/water,
surfactant/polymer, surfactant/CO2, foam floods, and surfactant/thermal
recovery). The poor results are attributed to adsorption and precipitation
caused by divalent cations in the oilfield brines. The early depletion of
the surfactant from the injected water solution rapidly diminishes the
effectiveness of the surfactants.
Standnes and Austad made a careful study of changes of wettability
from oil-wet to water-wet in chalk cores, using spontaneous imbibition
with anionic and cationic surfactants [981. The anionic surfactants were
ineffective; however, the cationic surfactants changed the wettability
from oil-wet to water-wet and produced as much as 70% of the original
oil in place compared to a maximum of 10% production using brine
alone. The enhanced production and change of wettability caused by