Phase Behavior of Mixtures of Bitumen, DME, and Water at Conditions Relevant to Solvent-Assisted Thermal Recovery

Dimethyl ether (DME) is a solvent of potential interest for solvent-assisted steam assisted gravity drainage processes. Phase boundary data for mixtures of bitumen, DME, and water are required to define the phases present in the drainage layer that forms in these processes. Phase boundaries were measured for pseudobinary mixtures of bitumen and DME and pseudoternary mixtures of bitumen, DME, and water at temperatures from 90 to 180 °C and pressures up to 10 MPa. Saturation pressures (vapor/liquid–vapor, liquid–liquid/liquid–liquid–vapor, and aqueous-liquid/aqueous-liquid–vapor boundaries) were measured using a constant composition pressure step method. Water solubility limits (liquid/aqueous-liquid boundaries) were measured using a saturation pressure method based on the change of slope at the solubility limit. Heavy phase onsets (liquid/liquid–liquid boundaries) were measured using three methods: (1) the saturation pressure method; (2) a light phase method based on the change in asphaltene content at the onset; and (3) a titration performed on a high pressure microscope. The onset was defined as the DME content at the phase boundary. DME was confirmed to be only partially miscible with bitumen with an onset of approximately 41 wt % DME that was insensitive to temperature and pressure. The addition of DME to mixtures of bitumen and water substantially increased the saturation pressure and increased the water solubility limit by up to 3 wt %. The addition of water to mixtures of bitumen and DME had little impact on the saturation pressure and decreased the heavy phase onset by up to 2.0 wt % DME. The mixtures were found to be supercritical above the critical point of DME with critical pressures increasing as the DME content of the mixture increased. The Advanced Peng–Robinson equation of state could be tuned to match either the saturation pressures (in the subcritical region) or the onsets. However, it could not be tuned to match both the saturation pressures and onsets with the same set of binary interaction parameters. It was also inaccurate or failed to converge near and in the supercritical region.