Abstract: In this work, the linear gradient theory (LGT) model, the simplified linear gradient theory (SLGT) model, the corresponding-states (CS) correlation, and the parachor method developed by the authors were extended to calculate interfacial tensions (IFT’s) of crude oil and gas condensate systems. Correlations of the model parameters were presented for pseudocomponents. The characterization procedures of Pedersen et al. and the SRK equation of state (EOS) were used to calculate vapor-liquid equilibria (VLE). To the exclusion of the near-critical region, the IFT’s calculated by all the models except the CS correlation were in good agreement with the measured IFT data for several crude oil and CO2/oil systems. The SLGT model and the parachor model perform better than the LGT model and the CS correlation. For N2 volatile oil systems, the performance of the LGT model is better than that of the SLGT model and the parachor model. For gas condensate systems, the predictions by use of the SLGT model are in good agreement with the measured IFT data. In the near-critical region, a correlation was proposed for estimations of IFT’s for CO2/oil systems, and satisfactory correlated results were obtained.

Abstract: PVT information of reservoir fluids is of great importance to reservoir engineers. In order to describe phase behavior and PVT properties using equations of state (EOS), a characterization procedure is required for true boiling point (TBP) and plus fractions of reservoir fluids. Although reasonable prediction accuracy can be obtained, the EOS parameters need to be tuned to improve the agreement between the measured properties and the calculated results.
In this work, several characterization methods have briefly been reviewed and the results of two widely used characterization procedures have been compared with the experimental data. The characterization methods can be applied to both single and multiple sample systems. For single sample systems, either an exponential or a three-parameter gamma distribution function was used to split the plus fraction into subfractions. Then the TBP fractions and split subfractions were lumped into groups (pseudo-components) by means of molar averaging, mass averaging, Qauss-Laguerre quadrature or arbitrary selection. For multiple sample systems, a method has been developed to generate the same number of pseudo-components for each sample and the same physical properties of each pseudo-component while the mole fractions of pseudo-components reflect the overall distribution of C7+ mixtures. ...

Abstract: The equation of state (EOS) for aqueous electrolyte solutions developed by Furst and Renon (1993) has been extended to predict vapor-liquid equilibria (VLE) of ternary water-methanol-salt systems. The model parameters have been determined by fitting only binary data and related to the cationic Stokes diameters. The predictions of vapor-liquid equilibria of ternary water-methanol-salt systems are in good agreement with the experimental data. Then the extended EOS has been utilized to develop a predictive method for gas hydrate formation conditions in the presence of electrolytes and methanol. The new hydrate method employs the Barkan and Sheinin (1993) hydrate model for the description of the hydrate phase, the extended EOS for the vapor phase fugacity and for the activity of water in the aqueous phase. The agreement is good between the predicted hydrate formation pressures and experimental data.

Abstract: This work extends the previously published aqueous electrolyte equation of state (AEEOS) to predict liquid-liquid equilibria (LLE) of mixed-solvent electrolyte systems. Interaction parameters between ions and organic solvents, and cations and anions were determined by fitting the experimental vapor-pressure data of binary methanol + halide electrolyte mixtures, and then correlated to the cationic Stokes and anionic Pauling diameters. The focus is on the ionic standard/reference state and the standard Gibbs energy for transferring salts from one solvent to another. The methods applied to predict LLE of several ternary water + organic solvent + salt system are to select: 1. the hypothetical ideal gas at unit mol fraction, the system temperature, and 1 bar as the ionic standard state; 2. the infinite dilution in the solvent mixture as the ionic reference state, whose activity coefficients were converted to those at the infinite dilution in pure water by the ionic standard Gibbs energy of transfer. The predicted LLE results agree well with the measured data without any adjusted parameters in fitting the ternary experimental data. The extended AEEOS is comparable to the model of Zerres and Prausnitz, but the latter requires two adjusted parameters in fitting the ternary experimental data for each ternary system.

Abstract: Hydrate formation conditions were measured for gas mixtures with concentrations of hydrogen from 5 mol% to 66 mol%. The thermodynamic models were proposed to calculate hydrate formation conditions for such systems. By treating hydrogen as a hydrate former, the Kihara parameters and the interaction coefficients between hydrogen and other less volatile hydrate formers in the Ng-Robinson hydrate model were determined from the experimental data. The proposed models were applied to the predictions of hydrate formation for multicomponent systems containing hydrogen. A good agreement was reached between the predictions and the measured data.

Abstract: In this work, the method for the calculation of hydrate phase equilibria proposed by Zuo and Stenby has been extended to represent incipient equilibrium hydrate formation conditions in aqueous solutions containing methanol and electrolytes. In order to predict hydrate formation conditions, the parameters of the extended Fürst-Renon electrolyte equation of state have been re-evaluated. Interaction parameters between water and methanol have been determined to match experimental hydrate formation data of methane in the water-methanol solutions. The interaction parameters between methanol and ions are associated with cationic Stokes and anionic Pauling diameters. Furthermore, cationic Stokes diameters in methanol are assumed to be a function of temperature. Coefficients of the temperature dependence have been determined by two methods. In method I, the cationic Stokes diameters in methanol are equal to those at 298.15 K. In method II, the coefficients are adjusted so that the predictions match the experimental hydrate formation data of the 80% methane + 20% carbon dioxide mixture in the presence of methanol and electrolytes.
The proposed methods have been applied to calculate hydrate formation conditions for a number of systems containing methanol and/or electrolytes. Good agreement has been reached between the calculated hydrate formation temperatures (or pressures) and the experimental data. The overall temperature deviations obtained by methods I and II are within 0.56 K and 0.70 K, respectively.

Abstract: In this paper, the extended Patel-Teja equation of state was modified to describe non-ideality of the liquid phaase containing water and electrolytes accurately. The modified Patel-Teja equation of state (MPT EOS) qwas utilized to develop a predictive method for gas hydrate equilibria. The new method employs the Barkan and Sheinin hydrate model for the description of the hydrate phase, the original Patel-Teja equation of state for the vapor phase fugacities, and the MPT EOS (instead of the activity coefficient model) for the activity of water in the aqueous phase. The new method has successfully predicted the gas hydrate formation conditions in aqueous solutions of single and mixed electrolytes. The agreement between experimental data and predictions was found to be excellent.

Abstract: In this work, the characterization procedure for plus fractions (Zuo and Zhang, 2000) was applied to predict hydrate formation conditions for crude oils. The Peng-Robinson equation of state (PR EOS) with temperature-dependent binary interaction coefficients for parameters a and b was employed for the vapor and liquid phases and the Ng-Robinson hydrate model adopted for the hydrate phase. The hydrate formation conditions were predicted for four crude oils with and without characterization. A comparison was made for the predicted results with the measured data. After characterization and matching the measured bubble point, the average absolute deviation of the predicted hydrate formation temperature is within 2.5 F. Furthermore, an algorithm for vapor-liquid-liquid-hydrate four-phase flash calculation was developed and tested. The calculation results have demonstrated that the proposed algorithm is very stable and effective.

Abstract: In this work, the vapor-liquid equilibrium (VLLE) data were obtained for three water-hydrocarbon and six water-methanol-hydrocarbon systems over the temperature range of -10 to 120 °C and the pressure range of 100 to 300 bar. The effects of temperature and the concentration of methanol on VLLE were also investigated. In an effort to correlate VLLE data using the Peng-Robinson equation of state, the method proposed by Pedersen et al. (1996) was followed to represent the phase behavior of the water-methanol-hydrocarbon systems. The results of this work indicate that the model generated in this work provides a reliable tool to describe the phase behavior for such systems.

Abstract: In this work, three thermodynamic models have been evaluated and compared to the experimental dew point data for hydrocarbon-water-methanol systems. The three models were applied to predict dew point temperatures for C1-H2O-CH4O, C2-H2O-CH4O and C3-H2O-CH4O mixtures. The DBR, SRK-MHV2 and EOS-CR models give very good and comparable predictions with AADs of 1.3, 1.2 and 1.9 K, respectively. The three models were also used to predict water dew points for nine SNG-water-methanol systems. The AADs of the predicted water dew temperature by using the DBR, SRKMHV2 and EOS-CR models are 0.9, 1.5 and 1.2 K, respectively.
The three models have comparable accuracies to predict water dew points for ternary hydrocarbon-water-methanol systems. The SRK-MHV2 model gives worse predictions for systems containing multi-hydrocarbons than for those containing a single hydrocarbon. Although the SRK-MHV2 and EOS-CR models are much more complicated, the DBR model gives relatively better results than the other two models for the systems containing multi-hydrocarbons.

Abstract: In this study, a method is proposed for calculating hydrate formation conditions in the aqueous solutions containing methanol and/or electrolytes. The proposed method employs the modified Barkan and Sheinin model (1993) for describing the hydrate phase behavior, and the extended Fürst and Renon electrolyte equation of state (1993) for calculating fugacities of hydrate formers in the vapor and liquid phases, and the activity of water in the aqueous phase.
The proposed model is tested against an extensive amount of the measured data in calculating hydrate formation conditions for the systems containing methanol and/or electrolytes. The calculated hydrate formation temperatures match the experimental data with an absolute average deviation of temperature less than 1 K. ...
An attempt is made to compare the hydrate formation predictions by a commercial software package, EQUI-PHASEHydrate with the experimental data for the systems investigated in this study. A good agreement is also reached.

Abstract: Associating fluids containing water and alkanols show a strong non-ideal behaviour on thermodynamic properties. Simple cubic equations of state (EOS), such as the Peng-Robinson (PR) equation, with conventional mixing rules are popular for its simplicity and easy implementation. However, it is incapable of reliably representing the phase behaviour of associating mixtures. An effort has been made in this study to develop a new model in which the non-density-dependent mixing rules are applied to the PR EOS to represent the phase behaviour of associating fluids. The proposed model takes into account of the polarity in the attractive term of the EOS by including both the conventional random mixing term and the asymmetric interaction term. The proposed model has been successfully applied to the calculation of the vapor-liquid equilibrium (VLE) and liquid-liquid equilibrium (LLE) of fluids containing water, alkanols, acid gases, and hydrocarbons. A satisfactory agreement between the predictions of the proposed model and the experimental data in the literature is reached.

Abstract: In this work, a characterization procedure for the true boiling point (TBP) and plus fractions has been developed to represent wax precipitation as well as the normal phase behavior for reservoir fluids. The paraffinnaphthene- aromatics (PNA) distribution up to C80 is estimated according to the properties of the TBP and plus fractions with one adjustable parameter. An optimized value for that parameter has been obtained, which can be treated as a constant for most crude oils and gas condensates or as a tunable parameter.
The model has been used to predict wax appearance temperatures (WAT) over a wide range of pressures and wax contents as a function of temperatures or pressures for a great number of systems. The average deviation of the calculated WAT is within 1.5 K for the systems with experimental PNA distribution data of TBP fractions, and 3.0 K for the systems without experimental PNA distribution data of TBP fractions. The results of the calculated wax formation locus and amount of precipitated wax as a function of temperatures or pressures by the proposed model are in good agreement with the measured data. Moreover, the proposed model has demonstrated the capability of simulating the retrograde phenomenon of gas condensates, which indicates that the amount of precipitated wax first decreases and then increases with pressure decrease in the vapor-liquid-solid phase region at a specified temperature.

Abstract: In this work, a solid-solution model has been developed for improving the representation of wax precipitation from petroleum fluids. The model takes into account the Poynting correction in the solid fugacity calculation. Acharacterization procedure has been developed for the true boiling point (TBP) and plus fractions. The para7n-naphthene-aromatics (PNA) distribution up to C80 is estimated according to the properties of the TBP and plus fractions with one adjustable parameter. An optimized value for that parameter has been obtained, which can be treated as a constant for most crude oils and gas condensates or as a tunable parameter.
The model has been used to predict wax appearance temperatures (WAT) over a wide range of pressures and wax contents as a function of temperatures or pressures for a great number of systems. The average deviation of the calculated WAT is within 1:5 K for the systems with the experimental PNAdistribution data of TBP fractions, and 3:0 K for the systems without the experimental PNAdistribution data of TBP fractions. The results of the calculated wax formation locus and amount of precipitated wax as a function of temperatures or pressures by the proposed model are in good agreement with the measured data. Moreover, the proposed model has demonstrated the capability of simulating the retrograde phenomenon of gas condensates, which indicates that the amount of precipitated wax >rst decreases and then increases with pressure decrease in the vapor–liquid–solid phase region at a specified temperature.

Abstract: In this work, an effort has been made to further improve the predictions of the wax formation conditions based on the previously proposed wax model (Zuo, J. Y.; Zhang, D.; Ng, H.-J. An improved thermodynamic model for wax precipitation from petroleum fluids. Chem. Eng. Sci. 2001, 56 (24), 6941-6947). The model framework consists of the three-parameter Peng-Robinson equation of state for describing the nonideality of the vapor and liquid phases and the predictive universal quasi-chemical (UNIQUAC) model proposed by Coutinho for the solid (wax) phase. The characterization procedure for plus fractions proposed by Zuo and Zhang (Zuo, J. Y.; Zhang, D. Plus fraction characterization and PVT data regression for reservoir fluids near critical conditions. SPE 64520, 2000.) has been modified and extended to reservoir fluids using high-temperature gas chromatography (HTGC) data. The wax model developed in this study has been applied to predict wax appearance temperature (WAT) and wax cut curves for a number of defined component systems, diesel fuels, and reservoir fluids. The average deviation of the predicted WAT is within 1.5 K at low and high pressures for defined component systems. For reservoir fluids, the predicted thermodynamic WAT locus tends to be much higher than the measured WAT. It is observed that a very small amount of wax precipitated in the fluids could shift the WAT to the measured WAT. The prediction of wax compositions in the solid phase is in good agreement with the experimental data. The results indicate that the proposed wax model is a useful tool to the flow assurance industry.

Abstract: The equation of state (EOS) and viscosity models are widely used for predicting phase behavior and physical properties of conventional oils, which have been found unsuited for heavy oils. In this work, we sought to determine whether EOS with appropriate characterization could be used to accurately represent the phase behavior and fluid properties of heavy oils. The heavy oils we tested had GOR and API gravity ranged from 80 to 160 scf/stb and 9 to 18 API, respectively. The reservoir temperatures ranged from 90 to 160 F. To represent PVT properties, the extended characterization procedure for plus fractions proposed by Zuo and Zhang and the Peng–Robinson EOS were applied to describe the non-ideality of vapor and liquid phases. The friction theory model was employed to estimate the viscosities of heavy oils at different temperatures and pressures. It is found that the calculated PVT properties of heavy oils agreed closely with the experimental data. The friction theory model provided a good presentation of heavy oil viscosities with parameter adjustment. The results indicate that the proposed models provide a useful tool to calculate phase equilibria and physical properties for heavy oils with good accuracy.

Abstract: The design and development of suitable processes for heavy oil or bitumen recovery from reservoirs need fundamental information on thermodynamic and transport properties of heavy oil or bitumen. Early in 1980, as per the request of Alberta Oil Sands Technology and Research Authority (AOSTRA), Dr. D.B. Robinson and his associates compiled a Data Book on the Thermodynamic and Transport Properties of Bitumens and Heavy Oils through reviewing, interpreting, and assessing numerous technical papers, books, theses, and reports (AOSTRA Technical Report, 1984). Alberta bitumen is considered as a Cn+ fraction with an average molecular weight of 544 g/mol and a specific gravity of 1.01 g/cm3.
In an attempt to develop thermodynamic models to represent fluid properties and phase behavior for heavy oil and bitumen, an effort is made in this work to apply the characterization procedure for plus fractions to predicting phase equilibria, density and viscosity of Alberta bitumen with and without solvents injection. The Peng-Robinson EOS with volume shift parameters is employed to describe the nonideality of vapor and liquid phases and to predict the fluid densities. In addition, three viscosity models, Corresponding states principle with one reference fluid (CSP1), Corresponding states principle with two reference fluids (CSP2) and the Lohrenz-Bray-Clark model (LBC) are evaluated for heavy oil and bitumen through comparison of the predictions against the experimental data.

Abstract: In this paper, a general framework of theoretical models and algorithm is developed to predict phase envelopes (saturation points) and quality lines of shale gas and oil in nanopores. The equation of state (EOS) and the modified Young−Laplace equation are used to take into consideration the effect of phase behavior and capillary pressure on phase envelopes, respectively. The Zuo and Stenby parachor model is applied to determine interfacial tensions between the vapor and liquid phases. In addition, a critical property shift of pure components is utilized to account for the impact of nanopore confinement on phase envelopes. The algorithm has proven to be robust for generating phase envelopes including critical points, cricondentherms (maximum temperatures), and cricondenbars (maximum pressures) for a variety of fluids at different compositions, vapor mole fractions (quality lines), and pore sizes. The models and algorithm are then used to explain the recently measured data of normal boiling point or bubble point temperatures for pure n-heptane in type I kerogen, binary mixtures of n-pentane + n-hexane and n-pentane + n-heptane, and a ternary mixture of n-pentane + n-hexane + n-heptane in the nanofluidic devices. For pure n-heptane in type I kerogen, with a presumption of nanopores being completely wetted by the liquid phase, the models agree well with the experimental data within a reasonable range of type I kerogen nanopore distributions in the presence of capillary pressure effect only as well as both capillary pressure and nanopore confinement effects. However, for the binary and ternary mixtures in the nanofluidic devices, the complete wettability assumption seems no longer valid. The wetting fluid−wall interaction parameter (λ) is then adjusted to match the experimental data at the nanopore radius of 5 nm. The adjusted parameters are λ = −142.2 ∼ −167.5 and λ = −14.0 for the three tested binary and ternary mixtures in the presence of capillary pressure effect only as well as both capillary pressure and nanopore confinement effects, respectively. The models provide not only good predictions at other radii but also a correct trend for the mixtures in the presence of capillary pressure effect only but a wrong trend against the experimental data in the presence of both capillary pressure and nanopore confinement effects. In addition, in the presence of both capillary pressure and confinement effects, a decrease in bubble and dew point pressures with decreasing pore radius is observed for shale gas and oil. For gas condensate mixtures, field production data show that produced liquid and gas ratios decrease even at reservoir pressures above bulk retrograde dew points. It is obvious that the model with critical property shift contradicts the field observation. More research activities in this area are required. Although in the presence of capillary pressure effect only, a decrease in bubble point pressures is estimated for shale oil, an increase in dew point pressures is predicted for shale gas with decreasing pore radius.