Page 247 - Enhanced Oil Recovery in Shale and Tight Reservoirs

230 Enhanced Oil Recovery in Shale and Tight Reservoirs

oil-wet k r to the intermediately wet k r , the oil recovery is almost unchanged
because no p c is changed; but the ultimate recovery is higher than that in the
oil-wet case with IFT reduction. These results indicate that the relative
permeability dominates the oil recovery in the later stage.
One may ask why the oil recovery from the case “Only Pc altered” with p c
is even lower than that from the case “OW þ IFT” without p c . Refer to
Fig. 9.3, water saturation becomes higher at the later time, then the capillary
pressure becomes negative. Therefore, the capillary pressure becomes resistant
to flow at the later time, thus reducing oil flow. In the case of
“WW þ IFT”, a positive capillary pressure and beneficial (increased) k r ef-
fect must be effective only when some block wettability has been changed
from oil-wet to water-wet in the early time, as indicated in the figure that
the oil recovery in the early time is higher than the case of “Only kr altered.”
At a later time, relatively higher water saturation may lead to lower k r in
“WW þ IFT” than that in the “Only kr altered.” Also, because of ultralow
IFT, the capillary effect is minimum, and the oil recovery is dominated by k r .

9.5.2 Relative importance of wettability alteration and IFT
reduction
Wettability alteration may be caused by surfactant adsorption; with more
adsorption, wettability is altered more significantly. For a fixed amount of
surfactant injected, more surfactant adsorption leaves less surfactant available
for IFT reduction. Fig. 9.8 shows the effect of the adsorption on oil recovery
using the model in the preceding section. There are three cases: a base
adsorption, twice the base adsorption, and half the base adsorption. It shows
that when the adsorption is higher, the oil recovery factor becomes lower. It
is implied that the decreased IFT effect is more significant than the increased
effect of wettability alteration, owing to higher adsorption. Here the surfac-
tant assumption is assumed as the mechanism of wettability alteration. It im-
plies that the effect of IFT is more significant than that of wettability
alteration. This observation may be specific from this model. It also holds
from the discussions presented in the following section. However, for this
observation or conclusion to hold, the core permeability must be high
enough so that oil can move by gravity (for example). Capillary pressure
should not be the dominant driving force. Therefore, this conclusion may
not hold in formations with ultralow permeability.
Actually, Zhang et al. (2018) collected the 35 experimental data on sur-
factant solution imbibition in Wolfcamp and Eagle Ford cores. They corre-
lated the oil recovery factors with the contact angles, IFTs, and capillary