Page 322 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Spontaneous imbibition 295

Figure 10.8 Imbibition oil recovery versus interfacial tension for an Ordos Basin core
(Wang et al., 2015b).

permeability and other experimental details, for example, the surfactant
used, were not reported in their paper. Their conclusion might not univer-
sally hold as there was only one data point that deviated from the thread of
the rest of the data points.

10.5.2 Simulation analysis

Some surfactants like anionic surfactants can reduce water-oil IFT but may
not change rock wettability (Sheng, 2013a). Table 10.2 shows the effect of
IFT reduction by such type of surfactants. The simulation base shale model
was referred earlier. For the conventional rock, when the IFT is higher than
0.049 mN/m, oil cannot be recovered by water imbibition without chang-
ing oil-wetness to water-wetness by surfactant. The capillary number in the
9
simulation model is in the order of 10 . Such low capillary number cannot
reduce residual saturations or increase relative permeabilities. Also, such
intermediate IFT makes the capillary pressure negatively too large so that
oil cannot flow out of the oil-wet rock.
For the case of initially oil-wet shale rock, no matter what values of the
IFT, no oil can be recovered because of the very low permeability, in addi-
tion to the reasons mentioned above for the conventional rock. This is a very

Table 10.2 Oil recovery factors (RF, %) at different IFTs (no wettability alteration).
Conventional rock Shale rock
Permeability, mD 120 3.3E-04
Porosity 0.24 0.1
RF at 20 mN/m (initially oil-wet rock) 0 0
RF at IFT ¼ 0.049 mN/m 0 0
RF at IFT ¼ 0.0323 mN/m 20 0
RF at IFT ¼ 0.008 mN/m 42 0