Effect of Alkyl Ammonium Ionic Liquids on the Interfacial Tension of the Crude Oil-Water System and Their Use for the Enhanced Oil Recovery Using Ionic Liquid-Polymer Flooding Sivabalan Sakthivel, † Ramesh L. Gardas, ‡ and Jitendra S. Sangwai* ,† † Petroleum Engineering Program, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600 036, India ‡ Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India * S Supporting Information ABSTRACT: Crude oil recovery from matured reservoir still pose challenges due to the low efficiency of the existing enhanced oil recovery (EOR) methods. The chemical enhanced oil recovery technique is one of the potential EOR technique being used to produce trapped oil from mature reservoirs. One of the key challenge for chemical-EOR process is that the conventional surfactants does not show efficacy toward reduction in interfacial tension and oil recovery under high saline and high temperature conditions. Ionic liquids (ILs) can be one of the potential alternative for possible application in chemical-EOR due to their enhanced stability under high saline and thermal conditions. In this investigation, six different alkyl ammonium ILs and sodium dodecyl sulfate (SDS) have been investigated for their effect on the interfacial tension (IFT) of low waxy crude oil-water system (with and without salt) as a function of temperature (283.15-353.15 K) and for EOR-flooding process. Water-soluble polymer (polyacrylamide) was used as a polymer flood after SDS/ILs flood in EOR study. This combined flood is referred to as SDS/IL + polymer EOR flooding process. Several EOR flooding experiments such as, only polymer, only SDS, only IL, and SDS/ILS + polymer have been carried out. Also, the evaluation of ILs and SDS for EOR has been investigated under zero and high salinity (100 000 ppm) reservoir conditions and compared. The study also provides an insight into the effects of different cations and anions (alkyl chain length) of the ILs on the IFT of crude oil-water system and for the enhanced oil recovery operation. 1. INTRODUCTION Depleted matured reservoirs contain almost two-third of trapped oil, which remain unrecovered even after primary and secondary oil recovery methods. 1,2 Tertiary oil recovery, also referred to as the enhanced oil recovery (EOR) method, involves the implementation of advanced methods or their combinations to enable the recovery of residual oil, thus extending the economic life of the reservoir. Several EOR techniques are being widely used to improve the productivity of the reservoirs. They are: thermal steam flooding, miscible gas flooding, chemical flooding, and microbial flooding. Thermal flooding is mainly performed in heavy and extra heavy crude oil reservoirs since none of the other methods could effectively improvise the production. 3 Gas flooding is mostly preferred for light, condensate, and volatile oil reservoirs, where the gases, such as N 2 , CO 2 , and hydrocarbons can be used for injection into the reservoirs. 3 Chemical flooding methods involve the use of surfactant, polymer, and alkali which are typically used for medium to light oil reservoirs. Most of the chemicals used in these methods, such as surfactant and alkali, tune the capillary force of the trapped oil by altering their interfacial tension and wettability of the rock surfaces, thereby improving the microscopic sweep efficiency. Subsequent injection of aqueous polymer solution help to improve macroscopic sweep efficiency. Chemical enhanced oil recovery is one of the successful methods suitable for matured reservoirs. 2,4,5 Various chemicals, such as surfactants, polymers, alkali, salts, gases, acids, and organic solvents, facilitate oil recovery by means of oil dissolution, wettability alteration, reduction of interfacial tension between oil/water and/or oil/rock interfaces, and reduction of viscous fingering within the pay zone, etc. 2,5-7 One of the key factors in the investigation of chemical EOR is to understand the interfacial tension between oil-water systems in the presence of surfactant, which are acting on the trapped oil. Consequently, reduction in the interfacial tension (IFT) will enable to mobilize the residual oil by means of decreasing the capillary forces during the flooding processes. 5,8 In chemical EOR, chemicals, such as surfactants (e.g., zonyl FSE fluoro surfactant, sodium dodecyl sulfate, Triton X-100, Triton X-405, Tergitol 15-S-5, Tergitol 15-S-7, Tergitol 15-S-9, butyl-α,ω-bis(tetradecyldimethylammonium bromide) and the butyl-α,ω-bis(hexadecyldimethylammonium bromide)) and water-soluble polymers (e.g., polyacrylamide, xanthan gum/ biopolymer) are being used. However, many of the surfactants had been found to be ineffective under high temperature and high saline conditions, due to the hydrolysis and degradation of the same. 8-11 Karnanda et al. 9 observed that the aqueous solution of surfactant Triton X-100 remained as a clear and transparent solution until 60 °C, whereas the solution turned dark (completely black) at 80 °C and above which, the high temperature prevented further measurements on the IFT. It was also observed that the Triton X-100 surfactant in brine became turbid even at much lower temperature of 40 °C and Received: December 27, 2015 Revised: February 11, 2016 Published: February 15, 2016 Article pubs.acs.org/EF © 2016 American Chemical Society 2514 DOI: 10.1021/acs.energyfuels.5b03014 Energy Fuels 2016, 30, 2514-2523