Page 35 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Huff-n-puff gas injection in oil reservoirs 25

12 MHz and a TE of 0.114 ms. Their experiments showed that the recovery
of each cycle from the slow experiment was about twice that in each corre-
sponding cycle from the fast experiment.
In their fast experiment, when the samples were immediately removed
from the pressure vessel to a desiccator to cool down to the room temper-
ature, the fluid in the sample could not come out any more, as both the
pressure and the temperature were low at room conditions. Based on our
experience, it takes time for the fluid in a tight core to come out. While
in the slow experiment, the samples were kept in the high temperature

(150 F), and the relatively higher pressure was maintained for 1 h. During
this 1 h, a lot of fluid came out based on our experience.

2.6 Effect of soaking time
It is easy to predict that if soaking time is longer, injected gas has more
time to diffuse into the matrix and dissolve into the oil, therefore, more oil
can be produced in each cycle. Such results have been confirmed in the liter-
ature (Gamadi et al., 2013; Yu and Sheng, 2015; Li et al., 2016). Fig. 2.16 is
an example (Yu et al., 2016a), in which an Eagle Ford outcrop sample was
used. The soaking pressure was 1000 psi. This pressure was depleted in
0.05 h during the puff period. Nitrogen was used. This example showed
that at the same sequential cycle number, the oil recovery was higher as
the soaking time was increased. It also showed that when the soaking
time was short from 0.025, 3e12 h, increasing soaking time significantly

Figure 2.16 Soaking time effect on huff-n-puff oil recovery.