Zhenhao Duan

An improved model calculating CO2 solubility in pure water and aqueous NaCl solutions from 273 to 533 K and from 0 to 2000 bar

Abstract A thermodynamic model for the solubility of carbon dioxide (CO 2 ) in pure water and in aqueous NaCl solutions for temperatures from 273 to 533 K, for pressures from 0 to 2000 bar, and for ionic strength from 0 to 4.3 m is presented. The model is based on a specific particle interaction theory for the liquid phase and a highly accurate equation of state for the vapor phase. With this specific interaction approach, this model is able to predict CO 2 solubility in other systems, such as CO 2 –H 2 O–CaCl 2 and CO 2 –seawater, without fitting experimental data from these systems. Comparison of the model predictions with experimental data indicates that the model is within or close to experimental uncertainty, which is about 7% in CO 2 solubility.

An equation of state for the CH4-CO2-H2O system: I. Pure systems from 0 to 1000°C and 0 to 8000 bar

An equation of state (EOS) for the CH4-CO2-H2O system covering a wide T-P range has been developed. In this article the new EOS is presented and applied to the pure endmembers. The equation is similar to that of Lee and Kesler (1975) and contains fifteen parameters. It is used with a mixing rule in the following article to provide a thermodynamic model for the mixed system. Though the parameters are evaluated from the PVT data in the temperature range from 0 to 450°C for CH4, from 0 to 1000°C for CO2 and H2O, and for pressures from 0 to 3500 bar, comparison of this EOS with a large amount of experimental data in the pure systems indicates that predictions for temperatures and pressures from 0 to 1000°C and 0 to 8000 bar (or slightly above) are very nearly within experimental uncertainty. The EOS can describe both the gas and the liquid phases of the endmember systems with similar accuracy. Fugacity coefficients are derived and compiled. In this paper mixing is considered using ideal mixing based on the endmember fugacities (Amagats rule). It is shown that such an approach leads to quite accurate predictions for high temperatures and low pressures.

The prediction of methane solubility in natural waters to high ionic strength from 0 to 250°C and from 0 to 1600 bar

Abstract A model for the solubility of methane in brines (0–6 m) for temperatures from 0 to 250°C and for pressures from 0 to 1600 bar (or slightly above) is presented. The model is based on Pitzer phenomenology for the liquid phase and a highly accurate equation of state recently developed for the vapor phase. Comparison of model predictions with experimental data indicates that they are within experimental uncertainty. Most experimental data sets are consistent within errors of about 7%. Although the parameters were evaluated from binary and ternary data, the model accurately predicts methane solubility in much more complicated systems like seawater and Salton geothermal brines. Application to fluid inclusion analysis is discussed. Minimum trapping pressures are calculated given the composition and homogenization temperature.

Equation of state for the NaClH2OCO2 system: prediction of phase equilibria and volumetric properties

An equation of state (EOS) has been developed for the NaClH2OCO2 system which consistently predicts various properties including PVTX, immiscibility or phase equilibria, solubilities, and activities with an accuracy close to that of experimental data from 300 to about 1000°C and 0–6000 bar with NaCl concentrations to about 30 wt% of NaCl (relative to NaCl + H2O) or to about 50 wt% with less accuracy. The EOS predicts that excess volumes can be over 30% of the total volume under some T-P conditions. Adding NaCl to the H2OCO2 system dramatically increases the T-P range of immiscibility. The immiscibility field is minimal around 400–500°C. Above or below this temperature, it expands for a constant pressure. A moderately saline brine can evolve into a very saline brine by phase separation at high temperatures. The presence of NaCl can substantially decrease the activity of H2O and increase that of CO2, thus affecting decarbonation and dehydration reactions. Compared to the EOS of Bowers and Helgeson (1983a), the EOS of this study is more reliable in the calculation of volumetric properties particularly in the low pressure range. In addition, BH EOS cannot predict phase equilibria.

An equation of state for the CH4-CO2-H2O system: II. Mixtures from 50 to 1000°C and 0 to 1000 bar

An equation of state (EOS) for mixtures in the CH4-CO2-H2O system has been developed. The model is based on the highly accurate endmember EOS presented in the previous article and on an empirical mixing rule. The mixing rule is based on an analogy with high order contributions to the virial expansion for mixtures. Comparison with experimental data indicates that the mixed system EOS can predict both phase equilibria and volumetric properties for the binaries with accuracy close to that of the experimental data for a temperature range from 50 to 1000°C and a pressure range from 0 to 1000 bar (or to 3000 bar with less accuracy). For temperatures below the critical point of water, there is very little PVTX (density) data. However, even for temperatures for which sufficient data exists we found that parameterization from PVTX data alone did not lead to a free energy that would accurately predict liquid-vapor equilibria. On the other hand, using this data alone we obtain a free energy that predicts both liquid-vapor equilibria and the PVTX properties of the binaries with roughly experimental accuracy. There are very few data inside the ternary. However, the mixing rule contains third order parameters, which require evaluation from ternary mixtures. For the single temperature for which we have data, the adjustment of one parameter gives good prediction of phase equilibrium in the ternary. The resulting EOS predicts that the presence of a small amount of CO2 can significantly affect the solubility of CH4. The application of this EOS to the study of fluid inclusions is discussed. The presence of CH4 in CO2rich fluid inclusions can significantly affect the predicted trapping pressure.

A general equation of state for supercritical fluid mixtures and molecular dynamics simulation of mixture PVTX properties

Abstract A general Equation of State (EOS), which we previously developed for pure nonpolar systems, is extended to polar systems such as water and to mixtures in this study. This EOS contains only two parameters for each pure component and two additional parameters for each binary mixture (no higher order parameters are needed for more complicated mixture systems). The two mixing parameters can be eliminated for nonaqueous mixtures with a slight loss of accuracy in both total mole volume and in excess volume (or nonideal mixing). Comparison with a large amount of experimental PVTX data in pure systems (including H2O) and in the mixtures, H2O-CO2, CO2-N2, CH4-CO2, and N2-CO2-CH4 results in an average error of 1.6% in density. Comparison with commonly used EOS for supercritical fluids shows that the EOS of this study covers far more T-P-X space with higher accuracy. We believe that it is accurate from supercritical temperature to 2000 K and from 0 to 25,000 bar or higher with an average error in density of less than 2% for both pure members and mixtures in the system H2O-CO2-CH4-N2-CO-H2-O2-H2S-Ar and possibly with additional gases. Comparison with the published simulated data suggests that this EOS is approximately correct up to 300,000 bar and 2800 K. We also simulated the PVTX properties of a number of supercritical fluid mixtures using molecular dynamics (MD) simulation. These results and those of other authors are well predicted by the EOS of this study.

An optimized molecular potential for carbon dioxide

An optimized molecular potential model for carbon dioxide is presented in this paper. Utilizing the established techniques of molecular-dynamics and histogram reweighting grand canonical Monte Carlo simulations, this model is demonstrated to show excellent predictability for thermodynamic, transport, and liquid structural properties in a wide temperature-pressure range with remarkable accuracies. The average deviations of this new model from experimental data for the saturated liquid densities, vapor densities, vapor pressures, and heats of vaporization are around 0.1%, 2.3%, 0.7%, and 1.9%, respectively. The calculated critical point is almost pinpointed by the new model. The experimental radial distribution functions ranging from 240.0 to 473.0 K are well reproduced as compared to neutron-diffraction measurements. The predicted self-diffusion coefficients are in good agreement with the nuclear-magnetic-resonance measurements. The previously published potential models for CO2 are also systematically evaluated, and our proposed new model is found to be superior to the previous models in general.

Molecular dynamics simulation of PVT properties of geological fluids and a general equation of state of nonpolar and weakly polar gases up to 2000 K and 20,000 bar

The PVT properties of CH[sub 4] from 30-360 cm[sup 3] mol and roughly from 273-2,000 K and from 100-20,000 bar have been simulated by molecular dynamics using Lennard-Jones potentials. The simulated results compare with data within 1.5% in volume. Using these simulated values and experimental PVT data, an equation of state (EOS) was developed. Because of the choice of potential, a simple scaling generalizes the EOS to predict the supercritical PVT properties of CO[sub 2], N[sub 2], CO, H[sub 2], O[sub 2], and Cl[sub 2] within an average error of about 1.5%.

A model to predict phase equilibrium of CH4 and CO2 clathrate hydrate in aqueous electrolyte solutions

Abstract A thermodynamic model to predict phase equilibrium of methane and carbon dioxide hydrate in aqueous electrolyte solutions is presented. Using the Pitzer model to account for the variation of water activity due to electrolytes and dissolved gas in aqueous solutions, we extended the model based on ab initio molecular potential developed recently by us for the CH4-H2O and CO2-H2O binary systems to the CH4 (or CO2)-H2O-salts system. Comparison of the model with extensive experimental data indicates that this model can accurately predict the phase equilibrium of CH4 hydrate and CO2 hydrate in various electrolyte solutions (such as aqueous NaCl, KCl, CaCl2, NaCl + KCl, NaCl + CaCl2 solutions, and seawater) from zero to high ionic strength (about 6 m) and from low to high pressures.

Equations of state for the NaCl-H2O-CH4 system and the NaCl-H2O-CO2-CH4 system: Phase equilibria and volumetric properties above 573 k

Abstract An equation of state (EOS) based on thermodynamic perturbation theory is presented for the NaCl-H 2 O-CH 4 system. This equation consistently reproduces PvTX properties and phase equilibria with an accuracy close to that of data in the temperature, pressure and concentration ranges from 648 K to 873 K, 0 to 2500 bar and up to 2.37 mol % NaCl. Good agreement with recent ternary immiscibility data from 673 K to 873 K suggests that the EOS may provide accurate predictions for NaCl concentrations as high as 40 mol %. We could not find any experimental data above 873 K that can be used to validate the predictions of the EOS inside the ternary. However, parameters for the mixed ternary system were established from parameters evaluated for pure and binary systems and accurate combination rules. Therefore, predictions in the ternary should be reliable to the high temperatures and pressures where the EOS for the lower order systems are valid (about 1300 K and 5000 bar). Using the same combining approach, an EOS for the quaternary NaCl-H 2 O-CO 2 -CH 4 is constructed on the basis of parameters from our earlier model for the NaCl-H 2 O-CO 2 system and the present NaCl-H 2 O-CH 4 model. This suggests that predictions of the quaternary EOS are reliable also to about 1300 K and 5000 bar.