Page 59 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Huff-n-puff gas injection in oil reservoirs 47

Figure 2.39 Ratios of oil production from the huff-n-puff cases to that from non-huff-n-
puff case (Gala and Sharma, 2018).

swelling, viscosity reduction, relative permeability hysteresis, miscibility, gas
extraction, gas solubility, and diffusion. But few are quantified.
Swelling effect stores the energy during the huff period, and it provides
the driving force in the puff period. Swelling also increases the oil volume so
that oil saturation and thus oil relative permeability is increased. Liu et al.
(2005) reported that the swelling factor in the huff period is higher than
that in the puff period at the same pressure. During the huff period, the
injected gas is in a continuous phase, whereas the gas may lose some conti-
nuity during the puff period. Some gas is trapped. This may result in the rela-
tive permeability hysteresis by which gas relative permeability is reduced
during the puff period (imbibition process). As gas diffuses into the oil phase,
oil viscosity is reduced. This mechanism may be important for heavy oil but
not for light oil. Experiments show that less soaking time leads to higher oil
recovery within the same operation time (Yu et al., 2016a), although longer
soaking time makes the oil recovery in a single cycle higher (Gamadi et al.,
2013; Yu and Sheng, 2015). Many simulation results show that without
soaking, the oil recovery is the highest with a fixed operation time (e.g.,
Li et al., 2016; Fragoso et al., 2018a). These imply that diffusion effect
cannot be significant in improving oil recovery. Experiments showed that
higher injection pressure resulted in miscibility and more oil could be pro-
duced (Li et al., 2017b). Next, solvent soaking mechanism is discussed in
more detail.
Hawthorne et al. (2013) believed that the flow in shale and tight reser-
voirs is dominated by the flow in fractures, and the oil displacement mech-
anisms in conventional reservoirs do not apply. Based on that, they proposed