Page 18 - Enhanced Oil Recovery in Shale and Tight Reservoirs

8 Enhanced Oil Recovery in Shale and Tight Reservoirs

2.2 Initial simulation studies of huff-n-puff gas
injection

Chen et al. (2013) are among the first who simulated the effect of
reservoir heterogeneity on huff-n-puff CO 2 injection enhanced oil recovery
in shale oil reservoirs using the UT-COMP reservoir simulator (UT Austin’s
compositional simulator). Their conclusion was that if the reservoir was
homogenous, injected CO 2 moved deep into the reservoir without much
increase in the near-well reservoir pressure, and unable to carry oil back
to the well in the production stage, resulting in a lower recovery factor
compared to that in primary recovery. In their journal publication version
(Chen et al., 2014), they concluded that the effect of reservoir heterogeneity
was to expedite the decline of recovery rate in the production stage, leading
to a reduced final recovery factor; the final recovery factor in the huff-n-puff
was lower than that in the primary recovery because the incremental
recovery in the production stage was unable to make up the production
loss in the huff and shut-in stages.
Sheng (2015d) further analyzed Chen et al.’s (2014) data and results. In
their models, the huff-and-puff process was from 300 to 1000 days; the
injection pressure was 4000 psi, and the bottom hole producing pressure
was 3000 psi. Sheng (2015d) believed that their result was due to the low
production history and the low injection pressure. To support the argument,
Sheng used a simulation model to mimic Chen et al.’s injection pressure,
injection and production history. The model results showed that the oil
recovery factor at 1000 days from the huff-and-puff process was 2.94% which
was lower than 3% from the primary depletion. Thus, Chen et al.’sobserva-
tion was repeated by Sheng’s model. However, the model showed that the oil
recovery factors at the end of 30, 50, and 70 years from the huff-n-puff
process were all higher than those from the primary depletion, when the in-
jection pressure of 7000 psi was used. Therefore, Chen et al.’s results were
caused by the low injection pressure of 4000 psi which was lower than the
initial reservoir pressure of 6840 psi. The injection pressure in the high-
pressure reservoir should be raised to show the EOR potential of huff-n-puff.
Wan et al. (2013a) independently proposed huff-n-puff gas injection
during almost the same time as Chen et al. did the above-mentioned
work. Their simulation results showed that a significant increase in oil
recovery could be obtained from huff-n-puff gas injection. After that,
extensive experimental and numerical studies have been carried out in
their research group. Some of those studies combined with other studies
published in the literature are discussed next.