Page 136 - Enhanced Oil Recovery in Shale and Tight Reservoirs

122 Enhanced Oil Recovery in Shale and Tight Reservoirs

parameters are needed such as equipment cost, operational cost, royalty tax,
and interest rate. These parameters are very case-specific. It will be very diffi-
cult to use this parameter to work out general optimization criteria. Alter-
natively, oil recovery factor is chosen as the objective function.
Compared with gas flooding, huff time, puff time, and soaking time are
important parameters. They are closely related to injection and production
pressures, and injection and production rates. First, pressures and rates are
addressed. As discussed in the preceding chapters, injection pressure should
be as high as possible. Generally, this pressure is set to be the initial reservoir
pressure which is 7800 psi in this model. A higher pressure may fracture the
reservoir. This is a typical practice for pressure maintenance. The maximum
injection rate for the whole fractured horizontal well is set to be 9 MMSCF/
D. For this model, we only simulate a half-fracture for a 15-stage well. Thus,
the maximum gas rate in the model is 300 MSCF/D. Sheng and Chen
(2014) showed that a higher oil recovery is obtained if a lower bottom-
hole flowing pressure (BHFP) is used, even though the flowing pressure is
lower than the bubble point pressure. Thus, the minimum bottom-hole
flowing pressure is set at 500 psi. The maximum producing oil rate is
1500 STB/D or 50 STB/D in the model. The maximum producing gas
rate is 9 MMSCF/D or 300 MSCF/D in the model. Before gas injection,
the primary depletion is extended from 1.2 years to about 3 years
(1000 days) at the minimum flowing pressure of 500 psi. Huff-n-puff
injection is continued until 10,950 days (total about 30 years). The injected
gas is methane.

5.4 Optimization criteria
Based on the above principles, optimum huff time, puff time, and
soaking time are determined, and the number of cycles is discussed.

5.4.1 Optimum huff time and puff time
In the literature, huff time, puff time, and soaking time in conventional and
tight field projects range from days, months, to years (Kurtoglu, 2013;
Shoaib and Hoffman, 2009; Wang et al., 2010). Oil recovery factors at
different huff and puff times are presented in Table 5.4. When the huff
time is increased from 100 days (Case H100P300) to 300 days
(Case H300P300) but the same puff time of 300 days is kept, the oil recovery
factor increases by 6.15% from 15.05% to 21.2%, indicating the oil recovery
is very sensitive to the huff time. Note in the case name H100P300, H means