Page 145 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Optimization of huff-n-puff gas injection in shale and tight oil reservoirs 129

in the puff mode, and vice versa. They found that the asynchronous injection
outperforms the synchronous injection. This probably results from that the
energy is more effectively used during the asynchronous injection.

5.4.2 Optimum soaking time

Experimental data show that the oil recovery will be higher within a single
cycle if soaking time is added or a longer soaking time is used (Gamadi et al.,
2013). However, if the total experimental time of huff, soak, and puff is the
same, more oil can be recovered without soaking time or shorter soaking
time (Yu and Sheng, 2015). The field tests reported by Monger and Coma
(1988) used the soaking time of 18e52 days; the results did not show the
sensitivity of soaking time. Their laboratory tests showed that soaking time
improved recovery of waterflood residual oil in the cores. But the improved
oil recovery was mainly from the subsequent waterflooding period. Sanchez-
Rivera et al.‘s (2015) simulation result shows a lower oil recovery at a longer
soaking time; adding molecular diffusion does not change the result.
To find out the effect of soaking time, a new case H300S100P300 is
created. In this case, 100 days of soaking time is added, and the total number
of huff-n-puff cycles remains the same as H300P300. The total number of
cycles for 10,950 days is about 17 for the two cases. But the total elapse
time for H300S100P300 is increased to 12,650 days (¼10,950 þ 1700).
Its oil recovery factor is 21.39%, higher than 21.2% from H300P300, as
presented in Table 5.5. This result is consistent with the experimental
observations mentioned above.
However, if the total operation time is fixed at 10,950 days, and the huff
time of 300 days is split into 200 days of huff time and 100 days of soaking
time in H200S100P300, the oil recovery factor becomes 17.7% that is lower
than 21.2% in H300P300 (Table 5.5). This result is consistent with that in a
condensate reservoir case (Sheng, 2015b).

Table 5.5 Effect of soaking time.
Case Huff, days Soak, days Puff, days Oil RF, %

Primary 0 0 10,950 11.42
H300P300 300 0 300 21.20
H300S100P300ext 300 100 300 21.39
H200S100P300 200 100 300 17.70
H300S5P300 300 5 300 21.01
H300S50P300 300 50 300 20.71
H300S100P300 300 100 300 20.33
H300P300Diff 300 0 300 23.40
H300S100P300Diff 300 100 300 22.71