Page 219 - Enhanced Oil Recovery in Shale and Tight Reservoirs

202 Enhanced Oil Recovery in Shale and Tight Reservoirs

8.10 Effect of high-pH water
High-pH water is typically sodium hydroxide and sodium carbonate
solution used in conventional EOR projects. Several EOR mechanisms
are proposed in the literature.
(1) A high-pH solution reacts with crude oil to generate surfactant in situ so
that surfactant-related mechanisms can be expected (Sheng, 2011).
(2) Emulsification and entrapment of oil improves the sweep efficiency of
the high-pH solution (Johnson, 1976).
(3) Emulsification and entrainment of oil improves the recovery of residual
oil (Johnson, 1976).
(4) Wettability is altered from oil-wet to water-wet or water-wet to oil-wet
(Johnson, 1976).
(5) When alkaline and surfactants are injected together, their synergy will
reduce surfactant adsorption and the microemulsion phase behavior of
the in-situ generated surfactant is improved (Sheng, 2011).
(6) Alkalies also react with divalents like calcium and magnesium to give pre-
cipitates and deposit on higher-permeability channels, switching the
subsequent water to lower-permeability zones where more oil remains.
As mentioned earlier, the high-pH solutions resulted in the highest
recovery in spontaneous imbibition and forced displacement in tight sili-
ceous reservoirs, probably due to lower interfacial tension and wettability
alteration to more water-wet. In this section, we focus on the interactions
of alkaline solutions with shale or tight rocks.
Fig. 8.25 shows the Barnett shales after being immersed in alkaline solu-
tions of different NaOH concentrations. With higher NaOH concentrations,
the samples became more dissolved and more fragmented. These results

Figure 8.25 Barnett shale samples after being immersed in (A) 0.1 wt% NaOH water,
(B) 2 wt.% NaOH water, and (C) 2 wt.% NaOH and 2 wt.% of KCl water.