SPE-170828-MS Accelerated Oil Droplet Separation from Produced Water Using Magnetic Nanoparticles Saebom Ko, Valentina Prigiobbe, Chun Huh, and Steven Bryant, University of Texas at Austin; Martin V. Bennetzen and Kristian Mogensen, Maersk Oil Research and Technology Centre Copyright 2014, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in Amsterdam, The Netherlands, 27–29 October 2014. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright. Abstract The treatment of highly stable small oil droplets in produced water is challenging for offshore production, where platform space is constrained, because their long residence time requires large equipment volumes. The use of magnetic nanoparticles (MNPs) to remove dispersed droplets is a promising alternative due to their quick response to external magnetic field, allowing easy separation of oil droplets on which MNPs were attached from water. The goal of this study is to prove the concept of magnetically separating oil droplets from produced water using surface-coated MNPs. Batch-scale experiments were performed and they showed that droplets in 5 wt. % of decane-in-water emulsions, which have negative surface charges, were successfully separated from water using cationic surfactant-coated MNPs, with decane removal efficiency of 85 to 99.99%, depending on the experiment conditions. Anionic surfactant-coated MNPs did not remove oil droplets, indicating that the electrostatic attraction between emulsions and MNPs control the attachment of the MNPs to the droplet surface. The settling velocity of a droplet coated with MNPs was derived by extending classical theory to account for the magnetic force as well as buoyancy and drag forces. Under the applied experimental conditions and considering the geometry of the oil-water-MNP system, velocity calculations show that the droplet settles spontaneously when a magnetic field is applied. Otherwise the MNP-coated oil droplets (MNP-droplets) would naturally migrate upwards due to buoy- ancy. The velocity of a single MNP-droplet is strongly dependent on the intensity of the magnetic field and it changes up to three orders of magnitude within the height of tested sample of approximately 3 cm. Introduction During oil recovery processes, a large volume of water is produced, as much as 20 times of the oil, and the treatment of the produced water for re-use or safe disposal is generally one of the largest oil-field operating expense. One major component of the produced water treatment is the removal of the highly stable dispersed oil. U. S. Environmental Protection Agency (USEPA) requires a zero oil discharge for all produced water during the onshore production. For offshore production, USEPA limits the discharge of oil and grease in produced water to a daily maximum of 72 mg/L and a 30-day average of 48 mg/L (USEPA, 2004). However, about 60% of offshore platforms in Gulf of Mexico are believed not to be able to reach USEPA discharge requirement (Thoma et al. 1999). The Convention for the Protection of Marine