Page 382 - Enhanced Oil Recovery in Shale and Tight Reservoirs

354 Enhanced Oil Recovery in Shale and Tight Reservoirs

10 ft matrix block being 1 mD and oil-wet. The osmotic pressure reached 40
psi after 30 years imbibition. Oil started to flow from the matrix to the
fracture at 20 years. The oil recovery from the matrix reached 35% after
40 years. 5% membrane efficiency was used. Note that the maximum os-
motic pressure from their model is more than two times the average pressure
(0.128 MPa or 18.9 psi) from Neuzil and Provost’s (2009) review mentioned
earlier. Such osmotic pressure should be much lower than the capillary
pressure. To make oil be able to flow out of matrix, they used almost
zero capillary pressure for the oil-wet matrix which is presented in
Fig. 12.12. It seems that the significance of the effect of osmotic pressure
on oil recovery in a shale or tight formation is questionable. However, in
a separate paper (Fakcharoenphol et al., 2016), their simulation shows that
the osmotic effect was more significant than the capillary effect on increasing
gas rate and decreasing water production.
2þ
þ
Some shale formations are oil-wet owing to Ca /Na bridging of oil
molecules to the negatively charged clay surface. When low-salinity water
2þ
þ
invades the high-salinity zone, Ca /Na will detach from the rock surface.
As a result, the surface may become more water-wet, resulting in the in-
crease in oil relative permeability and decrease in residual oil saturation (Kur-
toglu, 2013). More possible mechanisms for low-salinity waterflooding are
summarized in Sheng (2014).
Because of osmosis, some water will be held in the high-salinity zone of a
shale or tight formation. That could be one of the reasons to explain less flow

(A) (B)
1.0 1000
Relative permeability, fraction 0.6 Oil-wet Capillary pressure, psi 600 Water-wet
0.8
800

400
0.4
0.2

0.0 Water-wet 200 0 Oil-wet
0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0
Water saturation, fraction Water saturation, fraction
Figure 12.12 (A) Relative permeability, (B) capillary pressure used in Fakcharoenphol
et al.’s (2014) model.