Thermodynamic modeling and correlations of CH4, C2H6, CO2, H2S, and N2 hydrates with cage occupancies

Abstract The present work focuses on developing a framework for accurate prediction of thermodynamic conditions for single-component hydrates, namely CH4, CO2, N2, H2S, and C2H6 (coded in MATLAB). For this purpose, an exhaustive approach is adopted by incorporating eight different equations of states, namely Peng–Robinson, van der Waals, Soave–Redlich–Kwong, Virial, Redlich–Kwong, Tsai-Teja, Patel, and Esmaeilzadeh–Roshanfekr, with the well-known van der Waals–Platteeuw model. Overall, for I–H–V phase region, the Virial and van der Waals equation of state gives the most accurate predictions with minimum AAD%. For Lw–H–V phase region, Peng–Robinson equation of state is found to yield the most accurate predictions with overall AAD of 3.36%. Also, genetic programming algorithm is adopted to develop a generalized correlation. Overall, the correlation yields quick estimation with an average deviation of less than 1%. The accurate estimation yields a minimal AAD of 0.32% for CH4, 1.93% for C2H6, 0.77% for CO2, 0.64% for H2S, and 0.72% for N2. The same correlation can be employed for fitting phase equilibrium data for other hydrates too. The tuning parameter, n, is to be used for fine adjustment to the phase equilibrium data. The findings of this study can help for a better understanding of phase equilibrium and cage occupancy behavior of different gas hydrates. The accuracy in phase equilibria is intimately related to industrial applications such as crude oil transportation, solid separation, and gas storage. To date, no single correlation is available in the literature that can accurately predict phase equilibria for multiple hydrate species. The novelty of the present work lies in both the accuracy and generalizability of the proposed correlation in predicting the phase equilibrium data. The genetic programming generalized correlation is convenient for performing quick equilibrium prediction for industrial applications.

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Experimental and Theoretical Studies on the Fluid Properties Required for Simulation of Thermal Processes

Society of Petroleum Engineers Journal ◽

10.2118/8393-pa ◽

1981 ◽

Vol 21 (05) ◽

pp. 535-550 ◽

Cited By ~ 8

Author(s):

S.T. Lee ◽

R.H. Jacoby ◽

W.H. Chen ◽

W.E. Culham

Keyword(s):

Experimental Data ◽

Carbon Dioxide ◽

Phase Equilibrium ◽

Equation Of State ◽

Phase Equilibria ◽

Crude Oil ◽

Thermal Recovery ◽

Phase Equilibrium Data ◽

Equilibrium Data ◽

Robinson Equation

Abstract Experimental phase equilibrium data are presented for three reservoir oils at conditions approximating those encountered in in-situ thermal recovery processes. The fluid systems involved consist of three major groups of components: flue gas, water, and crude oil. Data were measured at temperatures from 204.4 to 371.1°C (400 to 700°F) and pressures from 6996.0 to 20785.6 kPa (1,000 to 3,000 psia). Experimental phase equilibrium data were used to develop a correlation of binary interaction coefficients of crude-oil fractions required for the Peng-Robinson equation of state. Phase equilibrium data predicted using the Peng-Robinson equation of state, using our interaction coefficients, are compared with experimental data. Generally, the Peng-Robinson equation of state predictions were in close agreement with the experimental data. Effect of feed gas/oil ratio and water/oil ratio on the equilibrium coefficients was examined through the Peng-Robinson equation of state. A study on the feasibility of representing the crude oil by only two fractions was made also. This study includes a procedure for lumping the crude-oil fractions and examples showing the importance of mixing rules in determining the pseudo critical properties of lumped fractions. Introduction The steady growth of commercial thermal recovery processes1 has created a need for basic data on phase equilibria that involve water and hydrocarbons ranging from methane to high boiling-point fractions. The in-situ thermal recovery processes often are operated at pressures above 6800 kPa (1,000 psia) and temperatures above 200°C (400°F). Experimental data and theoretical correlations on phase equilibria approximating these systems are virtually nonexistent. Early work by White and Brown2 dealt with high boiling-point hydrocarbon phase equilibria. However, the highest pressure studied was 6894.8 kPa (1,000 psia) and the lightest component was pentane. Poettmann and Mayland,3 on the basis of an empirical correlation,4 constructed charts of equilibrium coefficients, or K values, as functions of pressure and temperature for various boiling-point fractions. But the maximum pressure studied was 6894.8 kPa (1,000 psia). Later, Hoffmann et al.5 studied phase behavior of a gas-condensate system with the highest pressure reaching 20 684.3 kPa (3,000 psia) but the highest temperature investigated was only 94.2°C (201°F). In 1963, Grayson and Streed6 reported experimental vapor/liquid equilibrium data for high-temperature and high-pressure hydrocarbon systems. They also extended the Chao-Seader correlation to cover the higher temperature ranges. However, the. major light component in Grayson and Streed's system was hydrogen. Recently, because of the increasing activity in carbon dioxide flooding processes, the phase equilibria of systems involving carbon dioxide and crude oil has received attention. Simon et al.7 studied phase behavior and other properties of carbon-dioxide/reservoir-oil systems. Shelton and Yarborough8 examined phase behavior in porous media during carbon dioxide or rich-gas flooding. No extensive data on equilibrium coefficients were reported in those papers, and the temperature ranges (out of physical reality) were below 93.5°C (200°F). None of these papers surveyed included water as a component.

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