Experimental and Modeling Study of the Phase Behavior of CO2-Hydrocabon-Water Mixtures

Experimental and Modeling Study of the Phase Behavior of CO₂-Hydrocabon-Water Mixtures

Saif Zahir Al Ghafri, Esther Forte, Amparo Galindo, Geoffrey C. Maitland, and J. P. Martin Trusler*

Qatar Carbonates and Carbon Storage Research Centre, Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ

* Corresponding author e-mail: m.trusler@imperial.ac.uk.

1. Introduction

Experimental phase-behavior data for CO₂-hydrocarbon-water mixtures play a fundamental role in reservoir engineering and reservoir management, wherein phase equilibrium data at reservoir conditions are required for determination of the in-place volume and recovery factor. These data are also key inputs to reservoir simulations, used for evaluating reservoir development plans, and are required for the interpretation of well-test data and in the design of the surface facilities and processing plants. Knowledge of phase behavior for these mixtures is also crucial in carbon capture and storage (CCS) and enhanced oil recovery (EOR) processes where the miscibility of carbon dioxide with hydrocarbons plays an essential role.  Furthermore, measuring the mutual solubility of water and hydrocarbon fluids and the water content of hydrocarbon gases and non-hydrocarbon gases such as CO₂ is necessary to assess corrosion problems caused by moisture condensation in pipelines or process equipment, and to determine the dosage of chemical inhibitor required to avoid hydrate crystal formation which may block pipelines, equipment and instruments.

Due to the complexity of oil mixtures, simpler systems that may represent certain characteristics of the real ones are often studied. Despite their importance as a simple model for reservoir fluids, experimental data for CO₂-hydrocarbon-water ternary mixtures are scarce and limited to VLE measurements in most cases. Although much effort has been dedicated to the development of integrated modeling approaches for the prediction of phase behavior and other thermophysical properties of mixtures, experimental data are still required to tune the interaction parameters of the models or to assess the predictive capability of models. Therefore, any new measurements on ternary mixtures of the type CO₂ + hydrocarbon + water is a valuable addition to the literature especially in the three phase, VLLE, region.

<>2. Experimental

In this work the phase equilibria of (n-heptane + carbon dioxide + water) and (methane + carbon dioxide + water) mixtures have been measured. The n-heptane system was chosen as being a representative model for (light oil fraction + carbon dioxide + water, while the methane system is considered representative of (natural gas + carbon dioxide + water). The phase equilibrium measurements were carried out using a high-pressure quasi-static-analytical apparatus that was previously described previously [1]. The apparatus relies on recirculation of two coexisting phases using a two-channel magnetically operated micro-pump with sampling and on-line compositional analysis by gas chromatography. The maximum operating temperature and pressure of this apparatus are 423 K and 45 MPa, respectively. The apparatus was calibrated and validated by means of comparison with the published available literature data of the systems (n-heptane + carbon dioxide) and (water + carbon dioxide).  

In this work, new experimental data have been measured for the system (n-heptane + carbon dioxide + water) under conditions of three-phase equilibria. Compositions of the three coexisting phases have been obtained along five isotherms at temperatures from (323.15 to 413.15) K and at pressures up to an upper critical end point at which the heptane-rich and the carbon dioxide-rich phases become critical. In addition, new experimental data have been measured for the system (methane + carbon dioxide + water) under conditions of two-phase vapor-liquid equilibrium, three-phase vapor-liquid-liquid equilibrium, and four-phase vapor-liquid-liquid-hydrate equilibrium. Compositions of three coexisting fluid phases have been obtained along eight isotherms at temperatures from (285.15 to 303.5) K and at pressures up to either the upper critical end point (at which the carbon dioxide liquid-rich and the gas-rich phases become critical) or up to the hydrate formation locus. Compositions of coexisting vapor and liquid phases have been also obtained along three isotherms at temperatures from (323.15 to 423.15) K and pressures up to 20 MPa. The quadruple curve along which hydrates coexists with the three fluid phases was also measured.

<>3. Modeling

The experimental data obtained for these ternary mixtures have been compared with the predictions of the statistical associating fluid theory for potentials of variable range (SAFT-VR) in which pure-component model parameters are usually obtained by fitting the theory to the experimental values of the vapor pressure and saturated liquid densities [2] and unlike binary interaction parameters are usually determined by comparison to mixture data. In this work, we used the SAFT-VR parameters reported previously in the work of Míguez et. al [3] for the system (methane + carbon dioxide + water), while the parameters used for the system (n-heptane + carbon dioxide + water) were obtained in our own analysis. Furthermore, a detailed study of the ternary mixtures was carried out based on comparison with available data for the constituent binary subsystems. In this way, we analyzed the observed effects on the solubility when the third component was added.

<>References

1.        Forte, E.; Galindo, A.; Trusler, J. P. M., J. Phys. Chem. 2011, 115, 14591.

2.        Gil-Villegas, A.; Galindo, A.; Whitehead, P. J.; Mills, S. J.; Jackson, G.; Burgess, A. N., J. Chem. Phys. 1997, 106, 4168–4186.

3.        Míguez, J. M.; dos Ramos, M. C.; Piñeiro, M. M.; Blas, F. J., J. Phys. Chem. 2011, 115, 9604.

Acknowledgment

We gratefully acknowledge the funding of QCCSRC provided jointly by Qatar Petroleum, Shell, and the Qatar Science and Technology Park, and their permission to publish this research.

Keywords: Phase behavior; modeling; SAFT, hydrocarbon; water, methane, heptane, carbon dioxide.