Page 29 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Huff-n-puff gas injection in oil reservoirs 19

60%
50%
Oil Recovery Factor 30% 1 inch(EXP)
40%

1.5 inches(EXP)
20%
3 inches(EXP)
10% 2 inches(EXP)
3.5 inches(EXP)
4 inches(EXP)
0%
0 1 2 3 4 5 6 7 8 9
Number of Injection Cycles
Figure 2.10 Oil recovery factors for cores of different diameters but the same length.

60%
50%
Oil Recovery Factor 40%

30%

20%
1 inch(EXP)
1.5 inches(EXP)
10%
2.75 inches(EXP)
3.5 inches(EXP)
0%
0 1 2 3 4 5 6 7 8 9
Number of Injection Cycles
Figure 2.11 Oil recovery factors for cores of different lengths but the same diameter.

different, because the surface-to-volume ratio did not change, the diffusion
area and flow area were not changed, and the pressure gradient (dp/dr) was
the same when the length was changed.
The above experiments show that the oil recovery factor from a huff-n-
puff gas injection varies with the core size. It can be predicted that it
will vary with the matrix size in the field scale. To be able to use the
experimental data, an upscale method is needed. Li and Sheng (2017b)
proposed a curve of the oil recovery versus a dimensionless time for
different sizes:

0:000264 kt
t D ¼ (2.17)
2
fmc t ðL Þ p 2 D
where t D is the dimensionless time; k is the permeability in mD; t is the
operation time in hours; f is the matrix porosity; m is the oil viscosity in cP;