Page 137 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Optimization of huff-n-puff gas injection in shale and tight oil reservoirs 123

Table 5.4 Effect of huff-n-puff times.
Case Huff, days Puff, days Oil RF, %

Primary 0 10,950 11.42
H100P100 100 100 15.12
H100P300 100 300 15.05
H300P300 300 300 21.20
H300P100 300 100 15.38
H300P200 300 200 19.49
H300P350 300 350 20.95
H300P450 300 450 20.57
H300P600 300 600 20.12
H100P100qx3 100 100 23.33
Primarytrans0.33 0 10,950 9.46
H100P100trans0.33 100 100 15.53

huff and the following number represents the huff time in days; P means puff
and the following number presents the puff time in days. This naming
applies to the subsequent case numbers. The block pressures at block
(2,16,1) near the injection well at block (1,16,1) for 100 and 300 days of
huff time are shown in Fig. 5.5. It shows that the pressure for 100-day
huff time is raised to be less than 4000 psi, while the pressure for 300-day
huff time reaches around the maximum allowable 7800 psi. Then the draw-
down pressure to produce oil from the 100-day case is almost half of that
from the 300-day case. 100-day huff time is not long enough to boost the
pressure (energy).
For the same huff time of 100 days, when the puff time is increased from
100 days (Case H100P100) to 300 days (Case H100P300), the oil recovery
factor decreases by 0.07% from 15.12% to 15.05%, indicating the puff time is
not important. The near-wellbore block pressures are shown in Fig. 5.6.
When the huff time is 100 days, the pressure near the well is relatively
low (4000e5000 psi). Then the production drawdown and the oil rate
will be low. Therefore, a longer puff (production) time may not be helpful
to oil production. If it is too long, effective operation time is lost.
Comparing the two cases, H300P300 and H300P100, the oil recovery
factor for H300P100 decreases by 5.82%, from 21.2% to 15.38%, for
H300P300. The puff time is important in these two cases. The block
pressure at block (2,16,1) near the well (fracture) block (1,16,1) in
H300P100 is shown in Fig. 5.7. It shows that the pressure reaches the
injection pressure during the huff period, but it cannot deplete to the set
puff pressure of 500 psi at the end of 100 days of puff, before the well is