Ultra low IFT or wettability alteration: What is more important for tight carbonate recovery

Recently, carboxybetaine based zwitterionic surfactants (CnDmCB) have gained attention for surfactant aided recovery processes for unconventional oil-wet reservoirs due to their high salinity tolerance, wettability alteration potential and ultra-low IFT at extremely low concentrations. Several researchers have investigated the dominant recovery mechanisms among the wettability alteration and interfacial tension (IFT) reduction during surfactant flooding in unconventional, tight, oil-wet reservoirs. Most of the previous studies carried out using spontaneous imbibition fail to answer the dominance of prominent mechanisms, especially with respect to time and location. In this paper, these research gaps are addressed through physico-chemical interactions, and microfluidic studies carried out using carboxybetaine based zwitterionic surfactants (C_nDmCB). Four zwitterionic surfactants corresponding to tertiary amines with different chain lengths of 12, 14, 16, 18 carbons were synthesized and characterized by 1H NMR. IFT measurements and rock wettability were investigated for wide range of salinities and surfactant concentrations. C_nDmCB surfactant based on its ability to alter rock wettability and reduce interfacial tension reduction was selected for representative carbonate microfluidic studies. Experimental results demonstrated the IFT of carboxybetaine surfactants reduced with the increasing carbon chain length except for C18DmCB due to its poor solubilisation in the high saline brine system. The lowest IFT was up to 4*10-3mN/m for surfactant C16DmCB under the concentration of 0.025 wt% with produced high saline brine. However, this formulation called as F1 could change the zeta potential values of limestone only mildly from 3.07 mV to -3.79 mV. F2 and F3 formulations could change zeta potential value from 10.4 mV to -6.22 mV and -8.12 mV respectively. This signifies that higher wettability alteration potential of F3 and F2 when compared with F1 formulation. The IFT of F2 formulation is also ultralow (6.6*10-3mN/m), whereas the IFT of F3 formulation is relatively higher (0.115 mN/m). The observations of microfluidic studies are significant to emphasize that at early time F2 and F3 formulation corresponded to higher imbibition rate than F1 formulation due to its higher ability to alter the rock wettability from oil-wet to water-wet. As the front propagates to far location, F1 with ultra-low IFT begins to outperform F3 by providing better microscopic displacement and quicker front propagation throughout. The amount of residual oil trapped at far location was higher for F3 formulation than F1 due to its higher IFT. The results of the study signify that significant IFT reduction is needed at later stage while good wettability alteration is important during early stage of flooding and therefore this study holds significance in selecting or designing surfactant based fluid formulation for applications in unconventional tight reservoirs.