Experimental and Molecular Modeling Study of the Three-Phase Behavior of (n-Decane + Carbon Dioxide + Water) at Reservoir Conditions

Abstract

Knowledge of the phase behaviour of mixtures of oil with carbon dioxide and water is essential for reservoir engineering, especially in the processes of enhanced oil recovery and geological storage of carbon dioxide. However, for a comprehensive understanding, the study of simpler systems needs to be completed. In this work the system (n-decane + carbon dioxide + water) was studied as a model (oil + carbon dioxide + water) mixture. To accomplish our aim, a new analytical apparatus to measure phase equilibria at high pressure was designed with maximum operating temperature and pressure of 423 K and 45 MPa, respectively. The equipment relies on recirculation of two coexisting phases using a two-channel magnetically operated micro-pump designed during this work, with sampling and on-line compositional analysis by gas chromatography. The apparatus has been validated by comparison with published isothermal vapour-liquid equilibrium data for the binary system (n-decane + carbon dioxide). New experimental data have been measured for the system (n-decane + carbon dioxide + water) under conditions of three-phase equilibria. Data for the three coexisting phases have been obtained on five isotherms at temperatures from (323 to 413) K and at pressures up to the point at which two of the phases become critical. The experimental work is 1 complemented here with a theoretical effort in which we developed models for these molecules within the framework of the statistical associating fluid theory for potentials of variable range (SAFT-VR). The phase behaviour of the three binary subsystems was calculated using this theory and, where applicable, a modification of the Hudson and McCoubrey combining rules was used to treat the systems predictively. The experimental data obtained for the ternary mixture are compared to the predictions of the theory. Furthermore, a detailed analysis of the ternary mixture is carried out based on comparison with available data for the constituent binary subsystems. In this way, we analysed the observed effects on the solubility when the third component was added.

Experimental and molecular modelling study of the three-phase behaviour of (propane+carbon dioxide+water) at reservoir conditions

“…We compare our experimental findings with the predictions of SAFT-VR [74,75]. We use this method as it has already been shown in a number of studies [4,66,68,69,71,72,[76][77][78] to provide an accurate description of the phase behaviour of mixtures consisting of alkanes, CO 2 and water, for wide ranges of thermodynamic conditions. Finally, where possible, the agreement or deviations from published binary experimental data are further studied to analyse the effect of adding a third component and a third coexisting phase in the phase equilibria of the system.…”

“…The importance of understanding the miscibility and, in general the phase behaviour, of carbon dioxide with oil in the context of both CO 2 -EOR and CO 2 storage processes was reviewed in our previous work [4]. In summary, the CO 2 injected is likely to form two phases with the existing oil: a CO 2 -rich and an oil-rich phase.…”

“…In that study, mainly VLE was measured at temperatures between (473.15 and 573.15) K and pressures between (10 and 30) MPa, but also one experimental point was reported in the threephase equilibrium region at 473.15 K and 20 MPa [11]. The system (n-decane + carbon dioxide + water) was the focus of our earlier work [4], where three-phase vapour-liquid-liquid equilibrium (VLLE) data were measured at temperatures from (323 to 413) K and pressures from (1 to 18) MPa. The remaining studies found in the literature concern lighter alkanes.…”

“…In our previous manuscript [4], we discussed the necessity of having a reliable sampling technique to analyse aqueous mixtures of this type involving also components of different volatility. This was tackled with the design of an experimental apparatus with re-circulation, micro sampling and on-line chromatographic analysis.…”

“…Associating theories such as the statistical associating fluid theory (SAFT) [52,53] prove advantageous for modeling these highly non-ideal mixtures. Previous applications of associating theories to model (alkane + water) systems [54][55][56][57][58][59][60][61][62][63][64][65] and (carbon dioxide + water) [66][67][68][69][70][71] were reviewed in detail in our previous manuscript [4], to which the reader is referred for a complete analysis. Certain challenges were highlighted in these systems, such as the difficulty of modelling mutual solubilities of phases of very different nature, as represented by large variations between the dielectric constant of each phase and/or dipole moment of the molecules.…”