Page 57 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Huff-n-puff gas injection in oil reservoirs 45

1.0
0.9 T1 saturated with Wolfcamp dead oil (EF_1)
T5 saturated with water and Wolfcamp dead oil (EF_2)
0.8 T6 saturated with water and Wolfcamp dead oil (EF_3)
0.7
Liquid / Oil recovery 0.5
0.6

0.4
0.3

0.2
0.1
0.0
0 1 2 3 4 5 6 7 8 9
Number of huff-n-puff cycles
Figure 2.37 Liquid/oil recovery factors at the end of cycles with different oil
saturations.

effective as the single oil fluid system to produce fluid in the huff-n-puff
mode. This is another factor which needs to be considered when predicting
a field performance.

2.12 Effect of stress-dependent permeability
After some years of primary production, the reservoir pressure (pore
pressure) is reduced, and the formation effective stress is increased. As a result,
the formation permeability is reduced. If huff-n-puff gas injection is carried
out, the formation permeability will be increased during the huff period and
the early puff period, because the formation of effective stress is reduced owing
to the increased pore pressure. Therefore, the huff-n-puff process helps well
injectivity and productivity. Gala and Sharma (2018) evaluated this benefit
using reservoir simulation, as shown in Fig. 2.38. It shows the oil recovery
factors for the cases of No Geomechanics (no stress-dependent permeability
changes), Base Gamma ¼ 5e-4 1/psi (middle curve), and Base
Gamma ¼ 10e-4 1/psi (bottom curve). The gamma is the permeability-
stress exponent in an uploading/unloading cycle in the following equation:

k ¼ k 0 e bðs s 0 Þ (2.22)