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https://doi.org/10.2118/169082-ms
Copy DOIPublication Date: Apr 12, 2014 |
Abstract Modeling of enhanced / improved oil recovery processes that takes into account mass transfer between phases depends on the correct prediction of thermodynamic properties and composition of phases in equilibrium. In a steam flood process, the mutual water / hydrocarbons solubility is a function of the temperature, pressure and fluid composition of the system and may not be negligible at flooding conditions. The most used equations of state (EoS) in the petroleum industry fail to accurately correlate saturation properties of polar substances that self-associate through hydrogen bonding and as a consequence, do not calculate the distribution of the components among equilibrium phases precisely. In this paper, we present the development of the Association Peng-Robinson and Association Soave-Redlich-Kwong equations of state. The proposed equations of state are composed of two parts, one physical (the original cubic equation of state model) and one chemical (an empirical chemical reaction term which accounts for the self-association of a component) and can be used to model systems in equilibrium that contain one associating component such as water or alcohol. In the extended equations of state which includes the self-association chemical reaction, the degree of the molar volume polynomial is increased from its normal value of three to six but, in general, there are only three positive roots. The chemical part of the extended EoS includes three parameters that can be adjusted to match data, the entropy and enthalpy of the association chemical reaction and one free parameter. The fugacity calculations for the extended EoS can be split into physical and chemical parts, where the physical part has exactly the same form as the original equation of state fugacities. In order to estimate the new parameters of the proposed equations of state, the differences between the experimental and calculated saturation pressure and saturated liquid molar volume of pure water are minimized using a Particle Swarm Optimization (PSO) algorithm. The equations of state presented here enhance the original ones through the addition of a chemical part to deal with self-associating polar components. The average relative deviation between the experimental and calculated saturated data using the association forms of the Peng-Robinson and Soave-Redlich-Kwong equations of state are smaller than those obtained from the original models. The chemical reaction approach is robust and improves the prediction of thermodynamic equilibrium properties of self-associating pure components by adding only three adjustable parameters, where two of them have a clear physical meaning (self-associating reaction enthalpy and entropy).
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