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

Acquiring a comprehensive understanding of the phase behaviour of mixtures of crude-oil with carbon dioxide and water is a key input for reservoir engineering in processes of enhanced oil recovery and geological storage of carbon dioxide. To gain an insight, given the very complex nature of crude-oil mixtures, the study of simpler systems is of interest. In this work the system (propane + carbon dioxide + water) has been chosen as a model (light oil fraction + carbon dioxide + water) mixture. Phase equilibrium measurements have been carried out using a quasi-static-analytical high-pressure apparatus that was validated on the system (n-decane + carbon dioxide) in comparison with literature data, and used to study the system (n-decane + carbon dioxide + water) [E. Forte, A. Galindo, J. P.M. Trusler, The Journal of Physical Chemistry B 115 (49) (2011) 14591–14609]. In the present work, new experimental data have been measured for the system (propane + carbon dioxide + water) under conditions of three-phase equilibria. Compositions of the three coexisting phases have been obtained along four isotherms at temperatures from 311 to 353 K and at pressures up to the upper critical end points where the propane-rich and the carbon dioxide-rich phases become critical. The experimental data obtained for the ternary mixture have been compared to the predictions obtained with the statistical associating fluid theory for potentials of variable range (SAFT-VR). The phase behaviour of each pair of binary subsystems has been calculated using this theory and, where applicable, a modification of the Hudson and McCoubrey combining rules has been used to treat the systems predictively. Furthermore, a detailed analysis of the phase behaviour of the ternary mixture has been carried out based on comparison with available data for the constituent binary subsystems, as well as with the previous findings for the ternary (n-decane + carbon dioxide + water). Such comparison is useful to examine the effect that adding a third component has in the mutual solubility of each pair. Remarks relevant to reservoir processing are also highlighted.

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► Three-phase equilibria data are provided for (propane + carbon dioxide + water) from 311 to 353 K.
► Upper critical end points at these conditions are identified and measured.
► The experimental results compare favourably with predictions of the SAFT-VR equation of state.
► A presence of propane decreases the solubility of carbon dioxide in the aqueous phase.