Carbonate reservoirs typically exhibit significant heterogeneity, high temperature and high salinit ies. Surfactant and polymer selection for such reservoirs is a challenging process where extensive laboratory investigations are required to reach optimum concentration and formulation.
In this work, we have evaluated a number of anionic, cationic, nonionic, and amphoteric surfactants (total 28 surfactants) and four different polymers for high temperature and high salinity carbonate reservoirs. Surfactants used were mainly: alkyl sulfonate, olefin sulfonate, alcohol ethoxylate, carboxyl betaine, ethoxylated fluorocarbon, amine oxide-based fluorocarbon, carboxy betaine, propoxylated alcohol, linear alcohol ethoxylate, and alkyl benzene sulfonate. Surfactants were evaluated using a series of evaluation steps which include but are not limited to evaluation of compatibility, phase behavior, interfacial tension, adsorption, rheology, and core-scale performance.
Almost all anionic and cationic surfactants were either incompatible or thermally unstable. None of the anionic surfactants (sulfate, alcohol propoxylate and ethoxylate, phosphate and sulfonate), provided by various companies were thermally stable at harsh reservoir conditions. Most of the surfactants from amphoteric and nonionic classes were thermally stable and compatible at harsh reservoir conditions. Fluorinated amphoteric surfactants showed very good thermal stability at 90oC but their IFT was relative ly high (>1 mN/m). Similarly, fluorinated non-ionic surfactants showed good thermal stability but relative ly high IFT (>1 mN/m). Hydrocarbon surfactants were compatible, thermally stable and showed lower IFT values (10-2 mN/m). For core flooding experiments, fluorinated amphoteric surfactants and hydrocarbon surfactants were selected. Although fluorinated amphoteric surfactants showed relatively high IFT, they were selected due to good thermal stability, lower adsorption and synergistic viscosity enhancement with the selected polymers.
The extensive screening of surfactants and polymers for high temperature and high salinity carbonate reservoirs improved our understanding of chemical selection for such conditions. This study could be used as a guideline in future research for selection of surfactants and polymers.