## 3 Results and Discussion  
### IFT Measurement  
Surfactant concentrations were 0.001% wt., 0.01% wt., 0.05% wt., 0.1% wt., and 0.3% wt. as most often used in stimulation jobs. IFT curves are shown in Figure 4a-d. It can be seen that IFT curves of surfactants of different classes show different behavior types. IFT values of anionic sulfate SDS and cationic quatermized amine CTAB are lowered when CA is added if compared with aqueous solutions. The reason for this behavior was explained in [47]. This effect results from the salting-out nature of CA, dehydration of hydrophilic heads, enhancement of surfactant molecules' packing, and electrostatic attraction between anionic SDS and EDTA. However, the IFT of cationic surfactant CPB is increased when CA is added. It should be noted that solutions of CPB with CA have a slight yellow tincture with its deepness increasing with surfactant concentration. The yellow color may appear because of bromine formation; therefore, the possible mechanism behind IFT increase when CA is present is the oxidation of the surfactant by dissolved oxygen. N-alkyl pyridinium salts are often used as inhibitors of radical reactions [50] as they are radically oxidized easily. The CA molecule might be a catalyst for this reaction. The possible mechanism is shown in Figure A1. The addition of CA does not influence IFT in the case of AO, which is consistent with the reference data [36]. It seems that aminoxide is stable towards salting-out, like other amphoteric surfactants [51]; therefore, the IFT of AO is not altered. Figure 4: IFT curves of surfactant solutions and surfactant-CA mixtures. (**a**)—SDS, (**b**)—CTAB, (**c**)—CPB, (**d**)—AO. Results of experiments without surfactants are shown on OY axis. A concentration of 0.05% wt. for wettability alteration experiments is chosen, as IFT values of all solutions (except for plain SDS solution) reach a plateau at this concentration. This fact indicates that it is above critical micelle concentration (CMC). This condition is favorable for the formulation of technological fluids [52]. Plain CA is not very efficient for wettability alteration. Contact angle values lie at the same level of intermediate and oil-wet state before and after treatment; \(R/R_{0}\) value shows hydrophobization at 25 \({}^{\circ}\)C and mild hydrophilization at 120 \({}^{\circ}\)C. CA alone cannot desorb stearic acid ions because both EDTA/citric acid and stearate ions are negatively charged, which means that the surface is still covered with stearate ions after the treatment. The neutral pH value of the CA solution also does not favor the desorption of stearic acid because of its attraction to the positively charged surface, as the zero point of charge of calcite is 8-9.5 [53]. Leaching of calcium ions from the surface, which is the main mechanism of wettability alteration by CA, is also hindered by the adsorbed layer of stearic acid and short duration of the treatment. A small shift towards water-wetness at 120 \({}^{\circ}\)C according to capillary rise may be ascribed to diffusion enhancement and penetration of some part of the CA molecules through the adsorbed layer at high temperature, which results in leaching of a little portion of Ca\({}^{2+}\) ions. Nevertheless, this effect is negligible, according to contact angle measurements. SDS and its mixtures with CA are also inefficient for wettability alteration. Moreover, rock is found to be more oil-wet after treatment at 120 \({}^{\circ}\)C (\(R/R_{0}\) value is higher than unity). The inefficiency of SDS and other anionic surfactants at concentrations close to 0.05% wt. was previously described [54; 55]; however, no detrimental effects at high temperatures were observed. It seems that SDS washes away some excess stearic acid from the calcite surface at 25 \({}^{\circ}\)C and shifts it slightly towards water-wetness. The reason for strong hydrophobization at 120 \({}^{\circ}\)C is the adsorption of SDS at the surface. Three factors enhance SDS adsorption on the oil-wet calcite surface under studied conditions.