Page 323 - Enhanced Oil Recovery in Shale and Tight Reservoirs

296 Enhanced Oil Recovery in Shale and Tight Reservoirs

important observation. It means that conventional IFT reduction mecha-
nism by surfactants may not be effective in shale or tight reservoirs for spon-
taneous imbibition. It should be mentioned that in those simulation models,
the residual saturations (oil, water surfactant phases) are all assumed zero, and
the maximum relative permeabilities of oil, water, and surfactant phases are
all assumed one, and their exponents of relative permeabilities are also
assumed one as well at their high capillary numbers. These values of the pa-
rameters represent the best benefits a surfactant can provide in terms of IFT
reduction. In other words, although the models have made the best use of
IFT reduction mechanism, oil cannot be recovered from the shale rock.
The discussion here demonstrates that wettability alteration is fundamental
in shale reservoirs in terms of oil recovery by spontaneous imbibition.

10.6 Effect of diffusion

In the base shale model, the effective molecular diffusion coefficient is
11 2 5 2
7 10 m /s (6.5 10 ft /day). The value is in line with typical sur-
2
factant diffusion coefficients on the order of 10 11 e10 10 m /s (Lindman
et al., 1980; Cazabat et al., 1980; Chou and Shah, 1980; Weinheimeret
et al., 1981). When the diffusion coefficient is of 0.1, 1, 10, and 100 times
2
7 10 11 m /s, their oil recovery factors are represented in Table 10.3.Itis
assumed that surfactant changes the oil-wetness to intermediate wetness
(u ¼ 0.5). First, it is assumed that the IFT is reduced to 0.008 mN/m by sur-
factant. Oil can hardly be recovered by 138 days of imbibition, no matter

Table 10.3 Recovery factors at different diffusion coefficients.
By 138 days By w1.3 million days
2
IFT ¼ 0.008 mN/m, the base diffusion coefficient D ¼ 7E-11 m /s
0.1D 0.01 21.8
Base D 0.01 21.9
10D 0.01 22.0
100D 0.01 22.1
IFT ¼ 20 mN/m
0.1D 4.7
Base D 4.9
10D 5.03
100D 5.04