Article pubs.acs.org/EF
Enhanced Oil Recovery with the Ionic Liquid Trihexyl(tetradecyl)phosphonium Chloride: A Phase Equilibria Study at 75 °C Sara Lago, María Francisco,† Alberto Arce, and Ana Soto* Chemical Engineering Department, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain ABSTRACT: The enhanced oil recovery (EOR) through microemulsion flooding implies the formulation of a complex aqueous mixture containing surfactants, co-surfactants, cosolvents, or viscosity-increasing polymers, among other additives. The optimal formulation is associated with a three-phase behavior, in which the interfacial tension becomes significantly low. One parameter that greatly affects this formulation is the temperature. In this work, it has been shown that the three-phase system generated when adding trihexyl(tetradecyl)phosphonium chloride ionic liquid to a water−oil mixture remains stable in a wide range of temperatures and in the presence of salt. In contrast to conventional systems, no co-surfactant is required. This thermal stability is an interesting feature from the EOR point of view. The use of higher temperatures implies that a slightly minor quantity of ionic liquid is needed to solubilize the water and oil mixtures. Moreover, when the temperature is increased, there is an important decrease of the microemulsion−water/brine interfacial tension and a mild decrease in the case of the microemulsion−oil. The trihexyl(tetradecyl)phosphonium chloride ionic liquid was proven to be an effective surface-active agent to recover oil.
1. INTRODUCTION There is an increasing concern about the dependency of developed countries upon fossil fuel reserves as the primary energy source. Petroleum is the raw material in the production of transportation fuels, fuel oils (for heating and electricity generation), and asphalt. It is also the basis of a refining industry that produces chemicals, plastics, and synthetic materials. These materials are found in nearly every manufactured good that the occidental society handles daily. While other energy sources (as renewables) are being developed, there is an important need to recover oil, even from wells that years ago were abandoned because of their poor output. Some hydrocarbons that are trapped in non-conventional rocks or that have unconventional characteristics also constitute an unexploited potential supply of energy. Several technology breakthroughs are trying to deal with their use, but many technical challenges still remain.1 Enhanced oil recovery (EOR) methods take oil recovery one step further compared to the natural or induced energy technologies of primary and secondary recovery. As one example, the micellar polymer flooding method uses the injection of a micellar slug into a reservoir. This micellar slug contains a mixture of surfactant, co-surfactant, alcohol, brine, and oil that moves through the oil-bearing formation, releasing much of the oil trapped in the rock.2 The control of mobility is important for the success of the process. In this approach, a polymer-thickened water is injected behind the micellar slug to increase the viscosity of displacing water, decreasing in this way the water/oil mobility ratio. Many factors influence the performance of surfactant-based chemical flooding processes. These factors include the type and formulation of the surfactant, stability, phase behavior, chemical loss and adsorption, wettability, reservoir rock morphology, and heterogeneity.3 The involved phase equilibrium is a key factor in understanding the know-how of oil recovery by these © 2013 American Chemical Society
processes. Since Winsor’s pioneering work in the 1950s, it has been known that a minimum interfacial tension and a concomitant three-phase behavior of a surfactant−oil−water system occurs when the interactions of the surfactant and the oil and water phases are exactly equal.4 Ionic liquids (ILs) are substances that consist exclusively (or almost exclusively) of ions and have melting points or glasstransition temperatures below 100 °C.5 ILs have opened up a new era of solvent chemistry and chemical engineering because of their interesting properties. Unlike organic molecular solvents, they show negligible vapor pressure. They have wide liquid ranges, where they are thermally and chemically stable. They are usually non-flammable and have a wide range of solubilities and miscibilities. Moreover, they can be tuned for a specific application by switching ions, designing specific functionalities in their structures, or even mixing several ILs with complementary sets of properties. Because of all of the mentioned reasons, albeit slowly, the potential applications of these compounds are becoming an industrial reality.6 With regard to the petroleum industry, the properties of ILs as solvents can be promising from the production and refining stage of crude oil to the manufacture processes of high-value petrochemical products.7 With regard to the oil recovery, ILs have been successfully used at laboratory scale to recover bitumen from oil sands.8−11 Oil or tand sands compose a significant proportion of the world’s known oil reserves. Traditionally, the recovery of bitumen from these sands involves significant amounts of water, high energy consumption, and processing aids (historically NaOH). Painter et al.8−10 have recently shown that several imidazolium ILs together with a nonpolar solvent, such as Received: June 19, 2013 Revised: August 23, 2013 Published: September 17, 2013 5806
dx.doi.org/10.1021/ef401144z | Energy Fuels 2013, 27, 5806−5810
Energy & Fuels
Article
toluene, can affect this separation at ambient temperatures. However, successive extraction steps are required. The obtained bitumen is essentially free from mineral fines. Another application of the ILs in the petroleum recovery refers to the demulsification of water/oil emulsions formed in oil wells when natural surfactants are present.12,13 This separation is required before the oil refining. Surface-active ILs have been proven to be very useful in dehydrating light, heavy, and ultraheavy crude oils. Microwave irradiation significantly increases the efficiency of the process. Besides these applications, we have recently advanced that surface-active ILs can be useful in EOR.14 In our paper, it was shown that the IL trihexyl(tetradecyl)phosphonium chloride, [P6 6 6 14]Cl, without the use of any co-surfactant, drastically reduces the water/oil interfacial tension and increases the aqueous phase viscosity. Both effects are promising features for the micellar polymer tertiary method in oil recovery. Systematic core flooding experiments with another surfactant IL, 1dodecyl-3-methylimidazolium chloride, were also carried out, and found results also support the possibility of ILs to be used in this application.15 It is well-known that the temperature plays a crucial role in the phase behavior of binary and ternary mixtures as well as their physical properties, including surface and interfacial tension. For this reason, it is a parameter that must be taken into account in the optimal formulation for EOR. In this work, as a proof of concept, the influence of changing the temperature in phase behavior, viscosity, and interfacial tension is explored for the use of [P6 6 6 14]Cl as a promising surface-active IL for oil recovery.
Figure 1. Experimental procedure for the characterization of the phase behavior of water + IL + oil mixtures as described in this work.
Table 1. Gas Chromatograph Operation Conditions for the Analysis of the Ternary System Water + [P6 6 6 14]Cl + Dodecane column detector carrier gas injector
oven
type flux type temperature temperature split rate injection volume temperature program
HP-FFAP constant flux of 1 mL/min TCD 230 °C He 230 °C 200:1 2 μL 80 °C (3 min) → 100 °C/min until 200 °C (1 min)
The interfacial tension of the existent phases was measured using a Krüss K11 tensiometer thermostatted with a Julabo F12 cryogenic thermostat. The Wilhelmy plate method was adopted for this purpose. A specially adapted platinum plate with cylindrical shape was used to carry out more reliable measurements using lower amounts of sample compared to a conventional plate. The method described above has an estimated uncertainty of ±0.1 mN m−1.The densities of the homogeneous phases were measured by means of an Anton Paar DMA 5000 densimeter with viscosity correction and self-control of the temperature. The uncertainty in the density measurement is ±10−5 g cm−3. Kinematic viscosities were determined using micro Ubbelohde viscometers. The flow time measurement was performed by Lauda Processor viscosity system PVS1. A constant temperature over each measurement was mantained using a Lauda clear-view thermostat D 20 KP with a through-flow cooler DLK 10. The uncertainty for the dynamic viscosity determination is estimated to be ±0.5%.
2. EXPERIMENTAL SECTION 2.1. Materials. A simulated reservoir fluid was prepared as a mixture of oil, water (or brine), and surfactant. The oil was simulated as n-dodecane (Merck, ≥99 wt %). Prior to its use, it was washed with fresh bidistilled water at least 3 times and passed through a column of alumina to remove the impurities. The IL [P6 6 6 14]Cl was obtained from Cytec Industries, Inc. (96−97 wt %). Prior to use, this IL was dried under high vacuum (
