High-pressure vapor–liquid equilibria in the nitrogen–n-pentane system
Introduction
An increasing number of wells require that pressure be preserved to sustain current levels of production, which can be achieved by natural gas or nitrogen injection. Injection of nitrogen (N2) into oil reservoirs is an oil recovery technique that is currently used in certain Mexican fields to keep a high pressure in oil reservoirs and then maintain the oil extraction. Using N2 for pressure maintenance and enhanced oil recovery presents several advantages as a replacement for hydrocarbon gases also used for this purpose. Among these advantages, it is abundant, economically easy to obtain, and requires one-eighth the energy for its compression than that for an equivalent gas volume. Depending on the injection rate and pressure at wells, the cost of N2 can be as low as a quarter to a half that of the cost for natural gas.
In general, an optimal recovery strategy in enhanced oil recovery by gas (CO2, N2) injection requires of extensive knowledge of the phase equilibria and physicochemical properties inherent to the thermodynamic systems found at reservoir conditions. In the case of CO2 injection, such properties are barely known in spite of all the available field experience. This lack of knowledge is even more severe in the case of N2 injection. A literature survey of the phase behavior of N2-containing systems showed that all binary N2–hydrocarbon fluid mixtures develop, except for methane, type III phase diagrams according to the classification scheme of van Konynenburg and Scott [1]; i.e., one critical line going from the component with the lower critical temperature to an upper critical endpoint and a second critical line going from the component with the higher critical temperature to a critical point in a dense mixture at extremely high pressures. Consequently, a thorough understanding of the phase behavior of the N2 and the crude oil is essential for applications of N2 in enhanced oil recovery; e.g., the equilibrium phase diagram of N2–crude oil systems can be used to establish whether a miscible or immiscible condition will occur.
In practice, the phase behavior of these multicomponent mixtures is predicted by using equations of state. However, it is difficult to use these equations to predict correctly the complex phase behavior of N2–crude oil systems due to a lack of experimental phase equilibrium information of N2–hydrocarbon systems in a wide range of temperature and pressure. In fact, most of the information on this subject is about binary N2–hydrocarbon systems [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50] and only a few N2–hydrocarbon data are reported for ternary [10], [15], [32], [51], [52], [53], [54], [55], quaternary [32], [56], and multicomponent [57] systems. Nevertheless, as pointed out by Grausø et al. [9], many of the binary N2–hydrocarbon vapor–liquid equilibrium data reported in the literature are generally not internally consistent and mutually conflicting; i.e., there is a great deal of scatter among the experimental points.
It is deduced from above that it is necessary to carry out more experimental phase equilibrium studies at elevated temperatures and pressures of N2-containing binary systems along with experimental fitting using thermodynamic models. It will ensure the right qualitative and quantitative description of the phase behavior of N2–hydrocarbon mixtures and will provide a better understanding of phase behavior patterns that hydrocarbon mixtures develop during an enhanced oil recovery process by N2 injection. Toward this end, we have undertaken a systematic study of the phase behavior of N2–hydrocarbon mixtures at high pressures. This study is part of a research project in which phase behavior is studied for enhanced oil recovery in selected Mexican fields by N2 injection.
In this work, we report new vapor–liquid equilibrium measurements for the system N2–n-pentane over the temperature range from 344.3 to 447.9 K and pressures up to 35 MPa. Five isotherms are reported in this study, which were determined in a high-pressure phase equilibrium facility of the static-analytic type. The apparatus uses a sampling-analyzing process for determining the composition of the different coexisting phases. This sampling-analyzing system consists of a series of Rolsi™ capillary samplers [58] connected altogether on-line with a gas chromatograph that makes the apparatus very practical and accurate for measurements at high temperatures and pressures.
The experimental data obtained in our measurements were modeled using the PR [59] and PC-SAFT [60] equations of state. The mixing rules used for these equations were the classical one-fluid mixing rules. For both models, a single independent-temperature interaction parameter was fitted to all experimental data.
Section snippets
Materials
Nitrogen and helium (carrier gas) were acquired from Aga Gas (Mexico) and Infra (Mexico), respectively, both with a certified purity greater than 99.999 mol%. Pentane normal was purchased from Aldrich (USA) with a minimum purity of 99 mol%. They were used without any further purification except for careful degassing of the n-pentane.
Apparatus and procedure
The experimental apparatus (ARMINES, France) used in this work is shown in Fig. 1. It is based on the static-analytical method with fluid phase sampling, and can be
Experimental results
The N2–n-pentane system has been previously studied by several authors [13], [23], [24] at different temperature and pressure conditions. Table 1 contains a summary of the earlier results, including those presented in this paper.
The new measured equilibrium phase compositions for this binary system, temperatures, and pressures, are tabulated in Table 2. Uncertainties in the phase compositions, due mainly to errors associated with sampling, are estimated to be ±0.003 in the mole fraction,
Modeling
It is well known that mixtures formed of components that markedly differ in size and shape (e.g., N2–hydrocarbon mixtures) behave highly asymmetric so that the calculation of phase equilibria using an equation with pure-component information only is generally unsatisfactory. Thus, in order to increase the usefulness of the combining rules in the equations of state for predicting the global phase behavior of the N2–n-pentane system, we have estimated the interaction parameter for the PR [59] and
Conclusions
An experimental static-analytic apparatus with pneumatic capillary samplers has successfully been used to determine the vapor–liquid equilibria of the N2–n-pentane system over a wide temperature range from 344.3 to 447.9 K and pressures up to 35 MPa. Special care was taken to obtain representative samples of the coexisting phases for compositional analysis using gas chromatography.
New experimental vapor–liquid equilibrium data reported in this work are in good agreement with those determined by
Acknowledgments
This work was supported by the Molecular Engineering Program of the Mexican Petroleum Institute under research projects D.00182 and D.00332. The authors thank Prof. Dominique Richon from École des Mines de Paris for the design and construction of the static-analytic apparatus. G. Silva-Oliver gratefully acknowledges the National Council for Science and Technology of Mexico (CONACyT) for their pecuniary support through a PhD fellowship (117349).
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