PHASE-BEHAVIOR OF RESERVOIR FLUIDS .4. MOLECULAR-WEIGHT DISTRIBUTIONS FOR THERMODYNAMIC MODELING

Phase behavior predictions for reservoir fluids containing bitumen (heavy oil) and supercritical components (e.g. CO2) are sensitive to bitumen characterization. Huang and Radosz (1991) recently explored and documented one element of characterization related to molecular lumping. Another important element of characterization is related to molecular weight distributions and their effects on phase equilibrium predictions. This paper documents evaluation of bitumen molecular weight distributions on the basis of continuous and discrete versions of Perturbed Hard Chain theory. Distributions derived from Gas Chromatography Simulated Distillation (GCD) and from Gel Permeation Chromatography (GPC) are found internally consistent but systematically different. The GPC distributions are wider than the corresponding GCD distributions by a factor of three. Discrete GCD models, but not continuous models based on GCD or GPC, are found to predict simultaneously the bitumen solubilities in CO2 and the molecular weight distribution of the CO2 extract.