Polymer-surfactant mixtures in aqueous solutions present unique rheological and interfacial properties that promote their applications in chemical flooding. The objective of this study is to investigate the interaction between anionic polyacrylamides and cationic surfactants in different temperature and salinity conditions and the potential application of the polymer-surfactant mixtures in carbonate reservoirs. Cationic surfactants were selected owing to low adsorption on carbonate rocks. Compatibility tests of polymer-surfactant mixtures were conducted in brine with different salinities to study the interaction between anionic polymers and cationic surfactants in the presence of salts. The effect of cationic surfactants on polymer viscosity at different temperatures was investigated.

The compatibility of the mixtures of the cationic surfactants and the anionic polymers was significantly improved in high salinity injection water (with a total dissolved solid of 57,670 mg/L), compared with the compatibility in deionized water. This is attributed to the shielding of polymer and surfactant charges by the salts, which diminishes the electrostatic interaction between the chemicals. Rheological measurements indicated that the polymer viscosity increased in the presence of the cationic surfactant CAS-S or CAS-B. This effect was decreased at 90˚C. Other cationic surfactant CAS-1 or CAS-3 slightly increased the polymer viscosity at 25˚C and significantly decreased the viscosity at 90˚C. These observations can be explained based on the surfactants self-assembly. At room temperature, CAS-1 and CAS-3 form spherical micelles while CAS-S and CAS-B form wormlike micelles. The entanglement of the polymers with wormlike micelles explains the observed viscosity enhancement. At 90˚C, wormlike micelles became shorter which weakens this viscosity enhancement effect. In conclusion, the charges and self-assembly structures of surfactants play an important role in the performance of polymer-surfactant mixtures that should be taken into account in the design of optimal formulations. This work provides the insight of interaction between anionic polymers and cationic surfactants with different self-assembly structures for the potential application in improving oil production.

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