Interface light-scattering on a methane–decane system in the near-critical region at 37.8 °C (100 °F)
Introduction
A typical natural gas condensate is a single-phase fluid at original reservoir conditions. During production the reservoir pressure decreases. When the reservoir pressure falls below the dew point pressure – either near the wellbore or throughout the reservoir – the reservoir fluid goes from the single-phase state to the two-phase state. Optimal production of hydrocarbons in the two-phase region requires knowledge of the phase behaviour and fluid properties, for example gas and condensate viscosities and densities and the interfacial tension (IFT) between the two phases. The IFT is particularly important because the temperature of natural gas-condensate reservoirs may be relatively close to the critical temperature of the fluid, in which case the IFT can range over many orders of magnitude as the reservoir pressure decreases from the dew point pressure.
The development of refined predictive modelling tools for the IFT of such gas condensates has attracted interest in recent years. For examples, we refer to Llovell et al. [1] and literature cited there. Validation of theoretical models requires reliable experimental data on simpler model systems. Over the years, IFT data have been published for a number of binary mixtures of methane and one of the lower n-alkanes, e.g. ethane [2,3], propane [4,5], butane [6,7], pentane [[8], [9], [10]], hexane [11,12], heptane [8,12,13], octane [14], nonane [15], and decane [8,9,16,17]. Binary mixtures of this homologous series are the simplest model systems that can reproduce essential properties of natural gas condensates. However, despite extensive data for such systems on IFT (and other fluid phase and flow properties), data in the near-critical region are still scarce. There are only a few examples of reported IFT values below 1 × 10−2 mN m−1 [5,6,9] and no value below 1 × 10−3 mN m−1.
In this work our aim is to extend data into the near-critical region for one such model system using the experimental method of interface light-scattering. In this method, one measures the time correlation function of the intensity of light scattered from thermally exited waves at the interface. Such correlation functions contain information on the fluid properties governing the wave propagation. The relevant fluid properties in our work are the IFT, the difference between the liquid and gas densities and the sum of liquid and gas viscosities. The method has capacity for IFT measurement down to 1 × 10−5 mN m−1 [18] and is very well suited for work at elevated temperature and pressure [19].
We want to study a binary system with critical pressure within the typical range of natural gas-condensate systems, consisting of methane and a long-chained n-alkane. A mixture of a short- and long-chained n-alkane is known as an asymmetric binary n-alkane mixture. Such systems have been given special attention in recent years from a modelling perspective, e.g. Refs. [20,21]. Our aim is to take measurements in the near-critical region to determine the critical scaling exponent of the IFT and that of the difference between liquid and gas density. We want to select a system with a composition for which we can find relevant experimental data in the literature to compare with. Such data we intend to extend significantly towards the critical point.
Reamer et al. [22] have reported volumetric data at critical isotherms of several different methane–decane compositions. One of these – with 0.4980 wt fraction methane – has according to these workers, critical temperature 37.8 °C (100 °F). Following Reamer et al. [22], this composition has later been studied by several other workers [6,8,9]. We have selected the same system and composition and we can therefore compare with experimental results in the literature for the following quantities:
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IFT – reported by Stegemeier [9] and Amin and Smith [8]
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Liquid and gas density – reported by Reamer et al. [22] – from which we can calculate the difference of liquid and gas densities
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Liquid and gas viscosity – reported by Gozalpour et al. [6] – from which we can calculate the sum of liquid and gas viscosities
We stress that all three parameters referred to above – the IFT, density difference and viscosity sum – impact each individual measured intensity correlation function. Accordingly, it may be possible to extract information on all these three parameters from the same light scattering data. In this work we are able to do this in a large part of our pressure range. That is very useful but may also cause the data analysis to be challenging. The data reported in this work for IFT, density difference and viscosity sum, are based on analysis of about 1000 correlation functions, some of which have been tedious both to measure and to analyse. For reviews of the method, we refer to monograph edited by Langevin [23], to Earnshaw [24] and to Cicuta and Hopkinson [25].
Section snippets
Principle of the interface light-scattering method
IFT measurement by laser-light scattering takes advantage of microscopic roughening of the interface by random molecular motion in the adjacent fluid phases. The roughening, which is position- and time-dependent is very small with vertical displacement typically of the order of nanometres. Mathematically – by use of Fourier analysis – the roughness can be described as a superposition of sinusoidal interfacial waves with different wavelengths Λ. Each sinusoidal interfacial wave corresponds to a
Fluids and composition
The fluids used were methane and n-decane. The methane was specified by supplier (Praxair) to have purity > 99.995 mol%. The n-decane was specified by supplier (Fluka) to have purity > 99.8 mol%. The chemicals were used as received, without further purification.
All parts of the high-pressure cell used for the light-scattering measurements were thoroughly cleaned by various solvents (methanol, hexane, acetone) before the parts were assembled. After the cell was assembled, it was evacuated to
Critical point and composition
The main objective of our work has been to measure the interfacial tension γ, the difference of liquid and gas densities, Δρ, and the sum of liquid and gas viscosities, Ση, as function of pressure for temperature 37.80 °C (100 °F) and also determine critical scaling exponents for γ and Δρ. That is reported in Chs. 4.2, 4.3 and 4.4. To be able to determine critical scaling exponents, according to critical scaling theory [35], γ and Δρ must be presented as function of pressure p in terms of
Conclusion
We have by use of interface light scattering measured interfacial tension (IFT) as function of reduced pressure Δpr ≡ | p − pc |/pc at temperature 37.8 °C (100 °F) in the pressure range 4 × 10−5 < Δpr < 5 × 10−2. The IFT values range from 1.6 × 10−1 mN m−1 to 1.4 × 10−5 mN m−1. Our minimum IFT value is about two orders of magnitude lower than reported previously for any binary gas–liquid system. From the same light scattering data, we have also in the range 4 × 10−5 < Δpr < 1 × 10−2 extracted
Notes
The authors declare no competing financial interest.
Acknowledgement
Professor Johan S. Høye is thanked for very valuable discussions.
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