## 4 Conclusions  
We carried out an experimental investigation of how CA, surfactants, and their mixtures alter carbonate rock wettability. CA consisted of EDTA (0.4 M) and citric acid (0.05) at pH 6.7. This composition is applicable for well stimulation in hot carbonate reservoirs. Four surfactants of three different classes (anionic, cationic, and amphoteric) were studied. Surfactant concentration of 0.05% wt. for the rock treatment was selected on the base of IFT screening. This concentration is higher than the CMC of the surfactants' mixtures with CA for all surfactants under study. Next, we examined how CA, surfactants, and CA-surfactant mixtures affect the wettability of stearic-acid-treated model carbonate rock. Treatments were performed at 25 \({}^{\circ}\)C and 120 \({}^{\circ}\)C for 3 h, mimicking well stimulation conditions involving CA. It is observed that CA alone cannot alter the wettability of model rock, which means that it cannot break stearate-calcite ionic bonds under conditions of medium pH and short contact time. SDS and its mixtures with CA are ineffective too. Moreover, SDS is detrimental to the rock wettability at 120 \({}^{\circ}\)C as it increases oil-wetness. Three factors--high temperature, the presence of organic coverage, and salting-out EDTA ions--suggest this outcome. Cationic (CTAB and CPB) and amphoteric (AO) surfactants are more effective at wettability alteration. Mixtures of cationic surfactants and CA underperform plain surfactants at 25 \({}^{\circ}\)C because of partial electrostatic screening and surface-reaction-limited dissolution. However, the situation changes at 120 \({}^{\circ}\)C, and a synergistic effect between CA and surfactant is observed. The AO and CA mixture performs better than pure AO at both temperatures. We suggested possible mechanisms involving ionic pair formation, diffusion of the ionic pair to the bulk volume, solubilization of stearate ions in the surfactant's micelles, CA-induced Ca\({}^{2+}\) leaching on the freed patches of the surface, and spontaneous desorption of stearate due to increased negative charge. AO was selected as the most promising CA additive. Shifting the rock's wettability state towards oil-wetness significantly reduces plain CA's dissolution ability. Yet, adding 0.05% wt. AO restores this capacity. Contact angle investigation shows that this phenomenon is related to the rock's wettability alteration towards the water-wet state. The final experiment with oil-aged rock demonstrates that a mixture of CA and AO surfactant is able to alter the rock's wettability, rendering this surfactant-CA mixture suitable for practical field use. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**R Value**} & \multicolumn{3}{c}{**Temperature of the Treatment, \({}^{\circ}\)C**} \\ \cline{2-3}  & **25 \({}^{\circ}\)C** & **120 \({}^{\circ}\)C** \\ \hline Before ageing & & 0.73 \\ After ageing & & 1.62 \\ CA & 0.47 & 0.39 \\ CA-surfactant mixture & 0.89 & 0.51 \\ Surfactant & 2.45 & 1.13 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Wettability alteration of reservoir rock. **Author Contributions:** Conceptualization: T.I.Y., L.F.D., L.A.M. and M.A.S. ; Data Curation: T.I.Y. and L.F.D. ; Funding: Acquisition: L.A.M. and M.A.S. ; Investigation: T.I.Y., L.F.D. and D.N.K. ; Methodology: L.F.D., T.I.Y. and D.N.K. ; Validation: T.I.Y. and L.F.D. ; Visualization: T.I.Y. ; Writing--original draft: T.I.Y. ; Writing--review and editing: T.I.Y., L.F.D., L.A.M. and M.A.S. All authors have read and agreed to the published version of the manuscript. **Funding:** This work was supported by the Ministry of Science and Higher Education of the Russian Federation under agreement No. 075-15-2022-300 dated 18.04.2022 within the framework of the development program for a world-class Research Centre \"Efficient development of the global liquid hydrocarbon reserves\". **Acknowledgments:** We express profound gratitude towards NIIPAV, Ltd. for sharing surfactant samples with our research group.