Page 264 - Enhanced Oil Recovery in Shale and Tight Reservoirs

246 Enhanced Oil Recovery in Shale and Tight Reservoirs

and Mohanty (2014) tested surfactants called ethoxylated aliphatic amine,
trade name Ethomeen by AkzoNobel. At neutral to high pH, Ethomeen
is a triamine with ethoxylate (EO) groups. In acidic brine, it may be proton-
ated and become a cationic surfactant. EO group usually increases the
hydrophilicity of a surfactant at high temperatures. Tests showed that
Ethomeen T/25 is a good candidate for wettability alteration in harsh dolo-
mite conditions.
Rock type may affect selection of surfactants. Feng and Xu (2015)
showed that for carbonates with shale oil with a higher total acid number
(TAN), a cationic surfactant is better than an anionic surfactant; for sand-
stones with shale oil having a higher total base number (TBN), an anionic
surfactant is better than a cationic surfactant.
Alvarez et al. (2018) reported that the cationic surfactants changed the oil
wetness of carbonate surfaces of Wolfcamp and Eagle Ford rocks to water-
wetness more than anionic surfactants, due to the electrostatic interactions
between its positively charged heads and the negatively charged oil com-
pounds, mostly acid compounds, attached to positively charged carbonate
surfaces present in both Wolfcamp and Eagle Ford rocks. Oil molecules
attached to the rock surface are stripped and moved to the oil phase, so
that the rock wettability is changed to less oil-wet. Table 9.4 presents the
data of IFT reduction and wettability alteration by different surfactants used
in the different cores. When the anionic, nonionic, and cationic surfactants
were used, the carbonate cores (both Wolfcamp and Eagle Ford) had highest
oil recovery factors (47.3% and 9.0%, respectively), when the cationic surfac-

tant solution had lowest final contact angles (38.1 and 34.3 , respectively).
A separate set of measurements of contact angles, zeta potentials, and IFTs
of the surfactant solutions are shown in Figs. 9.26e9.28, respectively. Inter-
estingly, the absolute values of zeta potential for the cationic solution are
lower than those for the anionic one and anionic/nonionic solutions, not
consistent with the contact angle values (if consistent, the absolute values
should be higher, as others reported (Liu and Sheng, 2019)). The IFTs of
cationic solution are higher than those of anionic and anionic/nonionic solu-
tions, resulting in the highest final capillary pressures, and the cationic solution
had higher oil recovery. It seems that a low wetting angle and an intermedi-
ately high IFT, which leads to the high positive capillary pressure, are favor-
able to oil recovery spontaneous imbibition.
However, the Wolfcamp siliceous cores had higher oil recovery factors by
spontaneous imbibition (33.9% and 28.5% in Table 9.4), when the anionic

surfactant solution had lower final contact angles (57.4 and 32.4 ), as shown