Page 58 - Enhanced Oil Recovery in Shale and Tight Reservoirs

46 Enhanced Oil Recovery in Shale and Tight Reservoirs

Figure 2.38 Oil recovery factors for the cases of No Geomechanics (no stress-
dependent permeability changes, top curve), Base Gamma ¼ 5e-4 1/psi (middle curve)
and Base Gamma ¼ 10e-4 1/psi (bottom curve) (Gala and Sharma, 2018).

where k is the permeability at the effective stress s,k 0 is the permeability at
the initial stress s 0 , b is the permeability-stress exponent. Apparently
opposite to what we expect, this figure shows that the oil recovery is the
highest when the permeability does not change with the effective stress (no
geomechanics). This is because in their simulation model, the permeability is
kept at the highest value k 0 at the initial effective stress s 0 in the case of No
Geomechanics. For a fair comparison, the permeability in the case of No
Geomechanics should have been chosen at the value at the end of 5 years of
primary depletion, so that the benefit of permeability increase from the huff-
n-puff process can be shown.
The benefit of huff-n-puff injection can be seen from Fig. 2.39 in which
the ratios of oil production from the huff-n-puff cases to that from the non-
huff-n-puff case are plotted. It can be seen that the ratio from the case of No
Geomechanics is lower than those from the other two cases. In other words,
in real reservoirs where the permeability is stress-dependent, the huff-n-puff
beneficial is enhanced.

2.13 Huff-n-puff mechanisms
Many EOR mechanisms have been proposed in the literature for
huff-n-puff gas injection, including increase in reservoir pressure, volumetric