Page 256 - Enhanced Oil Recovery in Shale and Tight Reservoirs

238 Enhanced Oil Recovery in Shale and Tight Reservoirs

(Feng and Xu, 2015). In carbonate reservoirs, the adsorbed negative com-
pounds of carboxylic acids and positive heads of cationic surfactant form
ion-pairs and the carboxylic acids are carried away by ion-pairs.
Austadand hisworkers(e.g.,StandnesandAustad,2000; Austadand Stand-
nes, 2003) believed that cationic surfactants form ion-pairs with adsorbed
organic carboxylates of crude oil and stabilize them into the oil, thus they
were able to desorb organic carboxylates from the carbonate rock surface
and change the rock surface to more water-wet. The mechanism of ion-pair
formation is schematically described in Fig. 9.15. Due to electrostatic forces,
the cationic monomers will interact with adsorbed anionic materials from
the crude oil. Some of the adsorbed material at the interface between oil,
water, and rock will be desorbed by forming an ion-pair by the cationic
surfactant and the negatively charged adsorbed material, mostly carboxylic
groups. This ion-pair complex is termed “cat-anionic surfactant,” and it is
regarded as a stable unit. In addition to electrostatic interactions, the
ion-pairs are stabilized by hydrophobic interactions. The ion-pairs are not
soluble in the water phase but can be dissolved in the oil phase or in the
micelles. As a result, water will penetrate the pore system, and oil will be
expelled from the core through connected pores with high oil saturation in
a so-called countercurrent flow mode. Thus, once the adsorbed organic
material has been released from the surface, the chalk becomes more
water-wet. As the wettability alteration step was slow and was the dominant
mechanism, the movement of the altered water-wet front was slow using the
cationic surfactant (Kumar et al., 2008).
When positively charged organic matter adsorbs to the siliceous surface,
it will interact with negatively charged heads of anionic surfactant to form
ion-pairs; then the wettability is altered from oil-wet to water-wet (Alvarez
and Schechter, 2017). They reported that anionic surfactants changed the
wettability of negatively charged siliceous cores to water-wet better than

Figure 9.15 Mechanism of wettability alteration from oil-wet to water-wet by ion-pairs.

Large squares represent carboxylate groups, eCOO , small squares represent other po-
þ
lar components, and circles represent cationic ammonium group, eN (CH 3 ) 3 (Austad
and Standnes, 2003).