Hydrate Equilibria for Binary and Ternary Mixtures of Methane, Propanes Isobutane, and n-Butane: Effect of Salinity
- S.G. Paranjpe (U. of Alaska) | S.L. Patil (U. of Alaska) | V.A. Kamath (U. of Alaska) | S.P. Godbole (U. of Alaska)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1989
- Document Type
- Journal Paper
- 446 - 454
- 1989. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 1.6.9 Coring, Fishing, 4.6 Natural Gas, 4.3.1 Hydrates, 5.9.1 Gas Hydrates, 4.2 Pipelines, Flowlines and Risers
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This paper provides new experimental data on the phase behavior of gas hydrates for several binary and ternary mixtures of methane, propane, isobutane, and n-butane. After the molecular Kihara parameters for n-butane in the thermodynamic model were tuned, the predictions were in good agreement with the experimental data. The role of n-butane in hydrate formation is explained, and the effect of salinity on the phase behavior of binary mixtures of propane/n-butane is determined.
Knowledge of hydrate-forming conditions is essential for (1) identification of natural-gas-hydrate occurrences in the Arctic and subsea environments and an assessment of natural gas trapped in these resources; (2) prevention of hydrate formation in natural gas pipelines, gas-handling equipment, and production wells; (3) prediction of natural-gas recovery performance from hydrate reservoirs; and (4) study of the effect of hydrates on oil/gas production in reservoirs containing oil and gas along with hydrates. Several studies have been done on the measurement and prediction of hydrate-forming conditions for various pure gases and their binary mixtures. The early experimental work provided a foundation on which most of the current theoretical work has relied. Hydrate crystallographic and theoretical studies have greatly increased the understanding of the nature of hydrates and the ability to predict conditions under which they form. Detailed reviews of early work can be found in Refs. 14 through 16.
A renewed interest in the phase behavior of hydrates developed because of potential discoveries of natural-gas hydrates in Arctic regions. Recently, there have been indications that natural gas in hydrate form often contains large concentrations of heavier components, such as propane, isobutane, and n-butane. Kamath et al. and Holder et al. showed that the composition of heavier components of natural gas strongly influences the hydrate stability in earth. The enclathration of such relatively large molecules tends to stabilize the hydrate-crystal structure strongly, thereby lowering the equilibrium pressure considerably.
Gas Hydrate Phase Behavior
In the water/gas systems that form gas hydrates, six different phases can be present at different conditions: vapor (V), water-rich liquid (L1), ice (I), hydrocarbon-rich liquid (L2), Structure I hydrates (HI), and Structure II hydrates (HII). Although it is possible for ternary gas mixtures to have all six phases present simultaneously at equilibrium, extensive measurements have been conducted for three-phase (V/L1/H or V/I/H) and four-phase (V/L1/L2/H or V/L2/I/H) conditions. Only one study has been reported for five-phase (V/L1/L2/HI/HII) conditions for ternary mixtures of methane, ethane, and propane.
Some of the important questions one must answer to understand hydrate phase behavior are (1) what is each component's role in forming hydrates: (2) what are the gas/water interactions and how are they affected by the size of the gas molecules; (3) what type of crystal structure is formed by hydrates of gas mixtures, and (4) why do some mixtures--such as propane/H2S and methane/propane--display azeotropic behavior and others do not.
Experimental data for systems containing isobutane have been studied. Wu et al. studied three- and four-phase conditions for methane/isobutane mixtures. Uchida and Hayano and Rouher and Barduhn studied mixtures of isobutane and n-butane. No data can be found in the literature on the V/L1/L2/H conditions for binary mixtures of propane and isobutane or ternary mixtures of methane, propane, and isobutane. Establishment of V/L1/L2/H loci for both these systems was one objective of this study.
Another objective was to understand the role of n-butane in hydrate formation. Wilcox et al. reported forming hydrates of n-butane but could not reproduce these results in subsequent experiments. Scheider and Farrar also reported lack of success in forming hydrates of pure n-butane. Uchida and Hayano and Rouher and Barduhn studied hydrate formation in isobutane/n-butane and concluded that gas mixtures containing less than about 70 mol% isobutane do not form hydrates. Ng and Robinson studied methane/n-butane mixtures and reported that below 16 deg. C [61 deg. F], n-butane in methane/n-butane mixtures does participate in hydrate formation. Recently, John and Holder measured V/L2/I/H and V/I/H curves for methane/n-butane mixtures. Their results show that n-butane stabilizes hydrates of Structure II and that the equilibrium pressure decreases with increasing n-butane mole fraction in vapor. Kamath et al. studied V/I/H isotherms for propane/n-butane mixtures and observed that an increase in the n-butane mole fraction in the vapor phase increases equilibrium pressures. Because of the two different behaviors of n-butane in methane/n-butane and propane/n-butane mixtures, our study was aimed at understanding the role of n-butane in ternary mixtures of methane, propane, and n-butane.
Many methods to predict the phase behavior of hydrates have been suggested. One of the simplest is the K-value method of Katz. In this method, hydrate equilibrium is treated like vapor/solid equilibrium with a constant, Kvs,i, given by
where yi=vapor-phase mole fraction of ith gas component and xi=corresponding hydrate-phase mole fraction. van der Waals and Plateeuw developed a fundamental thermodynamic model of hydrate equilibrium based on their crystal structures. Several modifications have been made in this model by various investigators to improve the accuracy of predictions.
To calculate hydrate equilibria, the following equation is solved:
where ni = number of cavities of Type i per water molecule in the hydrate phase; Mj, yj = fugacity coefficient and mole fraction of Component j in tie vapor phase, respectively; and delta mu c = chemical potential difference between occupied hydrate and empty (hypothetical) hydrate lattices. The dependence of delta mu c on p and T is described by Holder. Cij in Eq. 2 is Langmuir constant of the jth gas species in the ith cavity and is obtained by considering a smoothed-cell-potential function. The ideal smoothed-cell Langmuir constant, C*, is
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