Experimental and Modeling Study of the Phase Behavior of (Synthetic Crude Oil + CO2)

Experimental and Modeling Study of the Phase Behavior of (Synthetic Crude Oil + CO₂)

Saif Zahir Al Ghafri, 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

A quantitative understanding of the phase behavior of CO₂-hydrocarbon mixtures at reservoir conditions is essential for the proper design, construction and operation of carbon capture and storage (CCS) and enhanced oil recovery (EOR) processes. In order to model these processes quantitatively, it is necessary to know the phase behavior and physical properties of the mixtures formed between CO₂ and reservoir fluids under the conditions prevailing in the reservoir and, for this purpose, compositional thermodynamic models are required. Unfortunately, the phase behavior and other thermophysical properties of petroleum fluids are very difficult to predict a priori. Accordingly, there is a great deal of interest in developing improved models and computational packages to predict the phase behavior and thermodynamic properties of such mixtures with the least inputs of experimental data.  In pursuit of this objective, a wide variety of approaches have been developed, ranging from empirical and semi-empirical equation of state (EoS), such as cubic equations, to molecular-based models such as the Statistical Associating Fluid Theory (SAFT). Nevertheless, experimental data are still required (and are in fact vital) to tune the interaction parameters of such models, to help in developing predictive approaches, and to assess the predictive capabilities of all models. While equilibrium data for binary CO₂-hydrocarbon mixtures are plentiful, equilibrium data for ternary and multi-component CO₂-hydrocarbon, CO₂-brines and CO₂-hydrocarbon-brines mixtures are limited, especially for systems containing heavy hydrocarbons and/or hydrocarbons other than alkanes. Therefore providing new experimental data for multi-component mixtures at reservoir conditions is of great value.

<>2. Experimental Design and Measurements

In this work, a new experimental apparatus was designed and constructed to measure the phase equilibria of systems containing CO₂ and hydrocarbons at reservoir temperatures and pressures. The apparatus involved a high-pressure high-temperature variable-volume view cell, with wetted parts fabricated from Hastelloy C-276, driven by a computer-controlled servo motor system, and equipped with a sapphire window for visual observation. Two calibrated syringe pumps (Quizix, model Q5210) with a maximum service pressure of 70 MPa were used for quantitative fluid injection. A low-dead-volume pressure transducer (Model DF2, DJ Instruments Ltd) was used for the pressure measurements while the cell temperature was controlled by a heating jacket, and the temperature was measured by means of a Pt100 sensor. The phase behavior of the mixture in the cell was observed with the aid of a CCD camera operating with both front and rear illumination. The maximum operating pressure and temperature of the entire setup were 40 MPa and 473.15 K, respectively. The apparatus was validated by comparison with published isothermal vapor-liquid equilibrium data for the binary system (CO₂ + heptane).

In this work, we report experimental measurements of the phase behavior of mixtures of CO₂ with a synthetic crude oil. The composition of the synthetic oil was chosen to match the physical and chemical properties of a bottom-hole crude oil sample from a Qatari field. The ‘dead' oil contained a total of 17 components, each of between 5 and 30 carbon atoms, including alkanes, branched-alkanes, cyclo-alkanes, and aromatics. Solution gas (0.81 methane + 0.13 ethane + 0.06 propane) was added to obtain live synthetic crudes with various gas-oil ratios (GORs).  Phase equilibrium measurements are reported for the ‘dead' oil and also for ‘live' oils, with GORs of 58 and 200 [1], under the addition of CO₂. The measurements were carried out at temperatures of (298.15 K, 323.15 K, 373.15 K and 423.15 K) and at pressures up to 36 MPa and include vapor-liquid, liquid-liquid and vapor-liquid-liquid equilibrium conditions.

<>3. Modeling

Traditional EoS models contain large numbers of adjustable binary parameters which require for their determination not only a compositional analysis but also experimental phase equilibrium data on each binary sub-system. In this work, we tested the capabilities of a recently-developed predictive model [2] based on the Peng-Robinson 78 EoS [3]. In this approach, the binary parameters are obtained from a group contribution scheme. Careful attention was paid the critical constants and acentric factor of high molar-mass components for which these properties are subject to significant uncertainty. Since the mixture also involved several light substances with critical temperatures lower than the some or all experimental temperatures, we also investigated the use of different alpha functions in the PPR78 EoS that have been developed specifically for super-critical components. The results showed that the model can predict the vapor-liquid equilibria of CO₂-hydrocarbon mixtures with reasonable accuracy without the need to regress binary parameters against experimental data.

References and notes

1.      The GOR is the ratio of the volumes of gas and liquid when the mixtures is flashed at standard conditions of T = 288.15 K and p =101.3 kPa.

2.      Vitu, S.; Privat, R.; Jaubert, J.-N.; Mutelet, J. Supercrit. Fluids 45 (2008) 1-26

3.      D.Y. Peng, D.B Robinson, Ind. Eng. Chem. Fundam. 15 (1976) 59-64.

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; CO₂; hydrocarbon; PPR78; variable volume cell, SAFT, heptane, toluene.