Page 317 - Enhanced Oil Recovery in Shale and Tight Reservoirs

290 Enhanced Oil Recovery in Shale and Tight Reservoirs

their porosity and permeability, the lower their experimental velocity was
compared with the theoretical velocity.
Yang et al. (2016) studied water imbibition in pores of difference sizes by
nuclear magnetic resonance (NMR). According to the NMR theory, the
transverse relaxation (T 2 ) is inversely proportional to the surface-to-volume
ratio (S/V) of a porous medium (core), because the bulk relaxation and diffu-
sion relaxation can be negligible in porous media. As the pores become
smaller, the S/V becomes larger, and then T 2 will be smaller. Yang et al.’s
experimental data show that T 2 became smaller as more water imbibed, indi-
cating that water imbibed into the core initially into larger pores and later into
smaller pores. This is consistent with the Washburn (1921) equation, and it is
also consistent with Mirzaei and DiCarlo’s (2013) work. Imbibition in larger
pores is higher than that in smaller pores, because smaller pores have higher
friction, although they have higher capillary force. However, an earlier paper
(Meng et al., 2015) from the same research group showed that T 2 becomes
larger as the water imbibition time became longer. The data, which were
not consistent with the spontaneous imbibition theory by capillary pressure
(Washburn, 1921), might be affected by other unexplained factors. Further-
more, Wang et al. (2015b) observed in laboratory that as the core permeability
was higher, or equivalently, as the oil viscosity was lower, the oil recovery
from spontaneous imbibition was higher.

10.4 Effect of initial wettability and wettability
alteration

In the base shale model discussed earlier, the rock is initially oil-wet. If
no surfactant is added in the water solution, the rock remains oil-wet, and no
oil can be recovered by the countercurrent oil-water flow. The preceding
section shows that wettability alteration is a very slow process, especially
in a shale or tight reservoir. Some reservoirs are initially water-wet. Then
let us see how fast and how much oil can be produced from a shale reservoir.
Table 10.1 shows that the oil recovery factor by spontaneous water imbibi-
tion from the shale rock being initially water-wet is 38% for 138 days. For
comparison, the oil recovery factor from the conventional rock is also shown
in the table which is 42.6%. For both the conventional rock and the shale
rock, the recovery factors are zero if they are initially oil-wet and no surfac-
tant is added to alter the wettability. These results indicate that the initial
wettability is very important, which is consistent with Bourbiaux and
Kalaydjian’s (1990) experimental data; if the rock is initially water-wet,