Secondary Oil Recovery Using Graphene-Based Amphiphilic Janus Nanosheet Fluid at an Ultralow Concentration Dan Luo, †,‡,§ Feng Wang, † Jingyi Zhu, § Lu Tang, † Zhuan Zhu, ∥ Jiming Bao, ∥ Richard C. Willson, ‡,⊥ Zhaozhong Yang,* ,§ and Zhifeng Ren* ,† † Department of Physics and TcSUH, University of Houston, Houston, Texas 77204, United States ‡ Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204, United States § State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, China ∥ Department of Electrical and Computer Engineering, University of Houston, Houston, Texas 77204, United States ⊥ Tecnologico de Monterrey, Departamento de Biotecnologıa e Ingenierıa de Alimentos, Monterrey, Nuevo Leon 64849, Mexico * S Supporting Information ABSTRACT: Nanofluid of graphene-based amphiphilic Janus nano- sheets produced high-efficiency tertiary oil recovery at a very low concentration (0.01 wt %). The more attractive way is to use nanofluid during the secondary oil recovery stage, which can eliminate the tertiary stage and save huge amounts of water, especially at times when the price of oil is low. Here, we continue to report our findings on the application of the same nanosheets in secondary oil recovery, which increased oil recovery efficiency by ≤7.5% at an ultralow concentration (0.005 wt %). Compared with nanofluids of homogeneous nano- particles, our nanofluid achieved a higher efficiency at a much lower concentration. The nanosize dimension of this two-dimensional carbon material improves transport in rock pores. After single-side surface hydrophobization of oxidized graphene with alkylamine, the partial restoration of the graphitic sp 2 network was detected by Raman, ultraviolet−visible, etc. The amphiphilic Janus nature of nanosheets led to their unique behavior at toluene−brine interface. Oil immersion testing clearly showed the change in the shape of the droplet. The three-phase contact angle decreased from 150° to 79°, indicating the change in the wettability of the solid surface from oleophilic to oleophobic. On the basis of the measured three-phase contact angles, the interfacial tension in the presence of the nanosheets was further calculated and was lower than the interfacial tension without the nanosheets. These interfacial phenomena can help residual oil detach from the solid surface, which contributes to the improved oil recovery performance. 1. INTRODUCTION The conventional production of crude oil from wells in oil fields generally has three stages. The primary stage utilizes the natural pressure difference between the wells and the reservoirs. The improved, or secondary, stage uses water flooding to continually supply reservoir energy. The enhanced, or tertiary, stage uses chemical flooding (polymer, surfactant, polymer/ surfactant, alkali/polymer/surfactant, etc.) to reduce the interfacial tension and control the mobility ratio of the fluids. 1 Currently, oil recovery using chemical methods is strongly limited by the low price of crude oil, the potential pollution of underground water, and harsh reservoir conditions. Flooding with nanofluids (solutions containing dispersed nanoparticles) to improve or enhance oil recovery in reservoirs has attracted a growing amount of attention as a promising alternative to chemical flooding. 2−6 Recently, we reported a high tertiary oil recovery factor of 15.2% using a simple nanofluid (containing only nanoparticles) of graphene-based amphiphilic Janus nanosheets at a low concentration (0.01 wt % nanosheet loading), representing a substantial improvement over chemical methods in terms of a lower cost and greater environmental sustainability. 7 The high performance was possibly due to the unique interfacial behavior of these two-dimensional amphi- philic Janus nanosheets at the oil−water interface. We found that the formation of climbing and interfacial films at different conditions may lead to oil displacement, which is different from existing mechanisms for homogeneous nanoparticles, such as oil−water interfacial tension reduction, 8,9 rock surface wett- ability alteration, 10−12 and production of a structural disjoining force. 13−16 Received: June 9, 2017 Revised: September 5, 2017 Accepted: September 7, 2017 Published: September 7, 2017 Article pubs.acs.org/IECR © XXXX American Chemical Society A DOI: 10.1021/acs.iecr.7b02384 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX