Page 28 - Enhanced Oil Recovery in Shale and Tight Reservoirs

18 Enhanced Oil Recovery in Shale and Tight Reservoirs

Tovar et al. (2014) used preserved sidewall cores (1in. diameter) under
confinement. The cores were soaked in CO 2 at 1600 psi and 3000 psi

and 150 F for several days. Production of oil was achieved by increasing
the system pressure above a set pressure (similarly to puff period). After the
1 hour of production, the system pressure was maintained 100 psi below
the set pressure again (similarly to a huff and soak period). The production
was carried out twice a day. The oil recovery was between 18% and 55%
of the original oil in the cores.
Alharthy et al. (2015) used their solvent soaking process (not huff-n-puff)
and found that 95% oil was achieved by CO 2 for Middle Bakken cores and
up to 40% for Lower Bakken cores. Note that the core diameters were
1.1 cm and the lengths were 4.4 cm. Other solvents like methane,
methane-ethane mixture, and nitrogen were also used.

2.4 Effect of core size

Further to the preceding initial studies and experimental verification,
many more experimental and simulation studies have been performed. The
results are summarized and discussed next.
In the preceding verification experiments, very small cores were used so
that high oil recovery was obtained. In real reservoirs, matrix is much larger.
Therefore, the experimental results cannot directly be applied to reservoirs.
The effect of core size needs to be studied.
Li and Sheng (2016, 2017a) did an experimental study about the effect of
core size on gas huff-n-puff using two groups of cores from the Wolfcamp
formation in West Texas. The first group contained core plugs with the
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same length of 2 inches but different diameters of 1 , 1.5 ,2 ,3 , 3.5 ,
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and 4 . The second group core plugs had the same diameter of 1.5 inches
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but different in lengths of 1 ,2 , 2.75 , and 3.5 . The injection pressure
was 2000 psi. Methane was used. All the experiments were performed at

the temperature of 95 F in an oven. The huff-n-puff experiments were con-
ducted following the procedures described in Section 2.3.2.
Fig. 2.10 shows the oil recovery factors for different diameters but the
same length of 2 inches. It is understandable that as the diameter was
increased, the surface-to-volume ratio was decreased, the diffusion area
and flow area were relatively low, and the pressure gradient (dp/dr) became
lower. Thus, the resultant oil recovery became lower.
Fig. 2.11 shows the oil recovery factors for different lengths but the same
diameter of 1.5 inches. It shows that the oil recovery factors were not quite