Page 126 - Enhanced Oil Recovery in Shale and Tight Reservoirs

112 Enhanced Oil Recovery in Shale and Tight Reservoirs

Figure 4.27 Changes in oil viscosity, pressure, and oil saturation at Block (10,1,1) for
methane injection into a core of Fluid A.

pressure, and oil saturation at Block (10,1,1) for methane injection into a
core of Fluid A. Keep in mind that total 24 blocks are used in the model
with the huff-n-puff block in (1,1,1). On the pressure curve, every peak
represents the end of huff or injection period, and every trough represents
the end of puff or production period. Within a cycle of huff and puff, the
trend of oil saturation change is opposite the trend of pressure change.
In other words, when the pressure increases during the huff, the oil saturation
decreases because higher pressure vaporizes the oil (the dew point pressure is
2795 psi). Note that the oil saturation is below 0.2 in almost the entire huff-n-
puff process. The residual oil saturation in the model is 0.25. Therefore, the
oil saturation decreases because of vaporization. Overall, the oil saturation de-
creases with cycle, and the peaks of oil viscosity slightly increase with cycle.
The increase in oil viscosity with cycle is because light components are vapor-
ized preferentially in early cycles, resulting in heavy components remaining in
later cycles. Fig. 4.28 shows the mole fraction of butane at Block (10,1,1) de-
creases, while the mole fraction of decane increases with cycle. The discussion
here also reveals viscosity reduction may not be a dominant mechanism for a
low-viscosity condensate recovery.