Wettability Alteration in Carbonates using Zirconium Oxide Nanofluids: EOR Implications Ali Karimi, † Zahra Fakhroueian, ‡ Alireza Bahramian,* ,†,‡ Nahid Pour Khiabani, † Jabar Babaee Darabad, § Reza Azin, ∥ and Sharareh Arya ⊥ † Department of Chemical Engineering, University of Tehran, Tehran 11365-4563, Iran ‡ Institute of Petroleum Engineering, P.O. Box 11155-4563, University of Tehran, Iran § ACECR, Branch of Tehran University, Tehran, Iran ∥ Department of Chemical Engineering, School of Engineering, Persian Gulf University, Bushehr 7516913798, Iran ⊥ NIOC Research & Development, Negar St., Valiasr St., Tehran, Iran ABSTRACT: Wettability alteration is an important method to increase oil recovery from oil-wet carbonate reservoirs. Chemical agents like surfactants are known as wettability modifiers in carbonate systems. However, the effectiveness of these agents can be increased by the addition of chemicals such as polymers, ionic materials, and nanoparticles. The impacts of nanoparticles on the wettability of carbonate systems have not been reported yet, and it is still in its infancy. In this work, the effect of ZrO 2 -based nanofluids on the wettability alteration of a carbonate reservoir rock was experimentally studied. Several nanofluids were made composed of ZrO 2 nanoparticles and mixtures of nonionic surfactants. The effect of nanofluids on the wettability of carbonate samples were investigated by measuring the contact angles, and it was shown that designed nanofluids could significantly change the wettability of the rock from a strongly oil-wet to a strongly water-wet condition. Scanning electron microscopy (SEM) images and X-ray Diffraction (XRD) data verify adsorption of nanoparticles on the rock and formation of nanotextured surfaces. Moreover, this paper reports the quick imbibitions of ZrO 2 nanofluids into oil-wet core plugs saturated with stock tank oil. The results show that a considerable amount of oil can be quickly recovered by free imbibitions of the nanofluids into the core plugs. A theoretical approach is also presented to explain the wettability alteration by formation of composite nanotextured surfaces. 1. INTRODUCTION Many of the world’s largest oil fields are found in carbonate reservoirs characterized by, low-permeability, fractured, and oil- wet rocks. 1,2 Oil recovery from fractured carbonate reservoirs is limited to exploitation of oil trapped into the low permeable formations. 3−5 After the primary production period, water- flooding is applied as a relatively cheap 6 and qualified choice to enhance oil recovery. Most carbonate reservoirs are preferen- tially oil-wet, 7−9 and therefore, the recovery of oil from these reservoirs by waterflooding techniques is relatively low. The presence of fractures can reduce the sweep efficiency of the injected fluid, and consequently, it can lower the amount of recovered oil. Considering the significant amount of oil in place in fractured oil-wet carbonate reservoirs, there is a huge interest to improve the oil recovery from these systems. The efficiency of the waterflooding process can be enhanced using some chemical agents such as surfactants, solvents, and pH controllers. When added to the injecting water, these chemicals can modify the wettability of the rock surface to more water wetting or significantly reduce the interfacial tension between reservoir oil and the injected fluid. Surfactants are mainly employed to increase the capillary number through lowering interfacial tension between oil and water, and polymers are utilized to improve the mobility ratio and sweep efficiency. 10,11 The polymer solution significantly affects the oil and water flow rates, and consequently, it sweeps a larger fraction of the oil reservoir than water or surfactant solution alone. Introducing inorganic alkaline chemicals such as sodium hydroxide or sodium carbonate can enhance the oil recovery by reducing interfacial tension and spontaneous emulsification, which results from in situ surfactant generation or by wettability alteration. Numerous experimental works have been published discussing the role of wettability in various aspects of oil recovery. 12−15 In several investigations, the wettability of porous media was changed by chemical treatment. 1−4,6,16−24 Wettability alteration by chemical treatment is an ongoing field of research motivated by academic and industrial interests 25 and plays an important role in almost all oil recovery techniques. 16 The alteration of reservoir rock wettability by surfactants has been deemed as an important mechanism for the recovery of oil from fractured carbonate reservoirs, and the use of ionic surfactants for changing the wettability of carbonates from oil-wet to water-wet conditions is well documented in the literature. 17−24 The mechanism is believed to be pair-ion making and desorption of organic compounds from the rock surface by surfactants. The rate of change in wettability process depends, however, on the surface activity of the surfactant. The surface activity can be increased by the addition of other chemicals such as polymers and polyelec- trolytes. 26 The interaction between surfactant and polymers can increase the surface activity of the surfactants, resulting in Received: September 28, 2011 Revised: December 23, 2011 Published: January 2, 2012 Article pubs.acs.org/EF © 2012 American Chemical Society 1028 dx.doi.org/10.1021/ef201475u | Energy Fuels 2012, 26, 1028−1036