Page 146 - Enhanced Oil Recovery in Shale and Tight Reservoirs

130 Enhanced Oil Recovery in Shale and Tight Reservoirs

Now keeping the total elapse time (10,950 days) and the huff time
(300 days) unchanged, soaking times of 5, 50, and 100 days are added to
H300P300 in the cases of H300S5P300, H300S50P300, and
H300S100P300. The oil recovery factors for these cases are 20.01%,
20.71%, and 20.33%, respectively, as shown in Table 5.5. All these recovery
factors are lower than 21.2% in H300P300 without soaking time.
Therefore, soaking time was not added (Wan et al., 2013a, b; Wan et al.,
2014a, b; Sheng and Chen, 2014; and Wan and Sheng, 2015b; Meng
et al., 2015b).
In the above cases with soaking time, diffusion is not included. Soaking
may help gas diffuse into oil phase. Therefore, to study the effect of soaking
time, diffusion is added to create a case H300S100P300Diff. The oil
recovery factor is 22.71% that is lower than that in H300P300Diff without
soaking time (see Table 5.5). The total project time is the same in these two
cases. This comparison shows that including diffusion does not make soaking
time beneficial. Fragoso et al. (2018a) did not use soaking time in their
simulation study as the highest oil recovery could be obtained without
soaking time.

5.4.3 Number of cycles
More oil can be produced in each of early cycles than each of late cycle.
There should be an optimum number of cycles. Artun et al.‘s (2011)
simulation work indicated that an optimum number of cycles were two
to three based on net present value for a fractured conventional reservoir.
Yu and Sheng (2015) did 10 cycles of huff-n-puff experiments and the
cumulative oil recovered continued to increase with the cycle. Wan
et al.’s (2015) history matched the Yu and Sheng’s experiments and the
models also predict the continuous increase with the cycle.
The case H300P300 is extended from 10,950 to 32,850 days (about 90
years) to create the case H300P300ext. The cumulative oil recovery factor
keeps increasing as shown in Fig. 5.10; the oil rate decreases with time
(Fig. 5.11). An economic analysis is needed to determine the optimum
number of cycles for a real project.

5.4.4 Start time of huff-n-puff
Sanchez-Rivera et al.’s (2015) simulation data indicate that too early start of
the first huff-n-puff gas injection from the primary depletion is detrimental
to oil recovery, but too late start is disadvantageous to the net present value.