Investigating the Effects of Temperature, Pressure, and Paraffin Groups on the N 2 Miscibility in Hydrocarbon Liquids using the Interfacial Tension Measurement Method Ali Zolghadr, Masoud Riazi, Mehdi Escrochi, and Shahab Ayatollahi* ,† Enhanced Oil Recovery (EOR) Research Centre, School of Chemical and Petroleum Engineering, Shiraz University, P.O. Box 7134851154, Shiraz, Iran * S Supporting Information ABSTRACT: In this study, interfacial tension measurement (IFT) is utilized to assess the impact of temperature, pressure, and paraffin type on a nitrogen injection process as an efficient enhanced oil recovery method. The pure and equilibrium densities of oil in contact with nitrogen are examined to find IFT behavior and gas solubility in oil. The minimum miscible pressure (MMP) of different systems has been measured using the vanishing interfacial tension technique. The experimental results show that IFT decreases linearly with pressure, with two different slopes. The results indicate that IFT values decrease linearly with temperature at different pressure conditions. The obtained IFT values for (hexadecane + N 2 ) and (diesel fuel + N 2 ) systems are close to each other. The variation in IFT of nitrogen-paraffin systems by pressure shows a similar slope to that of the N 2 and oil mixture (diesel fuel) system. The MMP of different systems was observed to decrease with increasing temperature. The results of this work show that nitrogen injection would be an effective enhanced oil recovery process in high-pressure and high-temperature oil reservoirs. 1. INTRODUCTION Enhanced oil recovery (EOR) processes are vital for the energy industry since oil consumption and its price have increased sharply in the past decades. Gas injection is one of the most effective methods recommended for enhanced oil recovery purposes. 1-3 The injected gas displaces oil in a reservoir under miscible or immiscible conditions. 4 Miscibility is obtained if there is no interface between the two phases involved (i.e., zero interfacial tension (IFT)) or if the two phases can be mixed together in each other at any ratios. 5 Both of these two processes depend mainly on reservoir conditions, displacing gas and oil type. Nitrogen injection has been rapidly developed as an efficient EOR process. 6,7 Nitrogen could be economically collected through a separation process from air or through production from underground natural reservoirs. Under certain field development conditions, nitrogen injection could also be economically favorable compared to other gases such as carbon dioxide and hydrocarbon gases. These characteristics distin- guish nitrogen as a very cost-effective EOR agent to be injected into oil reservoirs. 8 Nitrogen injection is usually applied as an immiscible displacement process, because of the high minimum miscible pressure (MMP) of typical crude oils with nitrogen. In other words, a miscible displacement process by nitrogen takes place at high pressure conditions. Also, the N 2 injection is used only for light hydrocarbon systems. 7 Thus, high pressure oil reservoirs with light hydrocarbons would be suitable for miscible nitrogen injection process. 7 Accurate determination of MMP is one of the crucial tasks that should be carried out for a precise prediction of the performance of such processes. Different theoretical and experimental approaches have been suggested for the prediction and measurement of MMP. 9-11 The well-known experimental techniques to measure MMP are slim tube and rising bubble methods. A new experimental method, known as the vanishing interfacial tension (VIT) technique, has been developed and utilized to determine the miscibility conditions of various crude oil-N 2 systems. 12,13 The advantages of this method are that it is cost-effective and it takes less time to measure MMP compared to other methods. The VIT technique is based on the IFT measurement between the reservoir oil and injected gas at different pressure conditions. It is well-known that IFT decreases as pressure increases. The pressure at which IFT approaches zero is defined as MMP. This can be obtained by extrapolating the IFT vs pressure to find the pressure matching to the zero IFT value. 13 It should be noted that the MMP is a key important parameter, since the oil recovery reaches its maximum value at this condition. Because of different temperatures and oil composi- tions in the reservoirs, the N 2 injection process enhances oil either through miscible or immiscible displacement. It is recognized that during immiscible conditions the interfacial tension between crude oil and N 2 decreases linearly as the pressure increases until it reaches the miscible condition at which oil recovery is critically improved. 14 It was previously explained 15 that the existence of a paraffin group in a crude oil has an essential role in determining IFT values between CO 2 and crude oil. The slope of IFT versus pressure also reduces as temperature increases. However, to the best or our knowledge, the impact of paraffin groups on (N 2 + oil) ’s IFTs at different temperatures and pressures has not yet been investigated. In this study, the role of temperature and the impact of paraffin groups in the oil phase on IFT and MMP are examined. Received: April 23, 2013 Revised: June 22, 2013 Accepted: July 2, 2013 Published: July 2, 2013 Article pubs.acs.org/IECR © 2013 American Chemical Society 9851 dx.doi.org/10.1021/ie401283q | Ind. Eng. Chem. Res. 2013, 52, 9851-9857