PREDICTION OF UNUSUAL RETROGRADE CONDENSATION IN MODEL RESERVOIR FLUIDS

Vapour-liquid-liquid equilibrium experiments were conducted with model reservoir fluids comprising the binary pair ethane + n-decylbenzene t o which nitrogen, methane, ethylene or carbon dioxide was added. These ternary mixtures were found to exhibit retrograde condensation of the heavier liquid phase, over narrow ranges of pressure, temperature and composition. The experimental data are modelled qualitatively with th e Peng-Robinson equation of state in conjunction with a three-phase fl ash routine (CMGPROP). Relatively large pressure ranges are predicted for this type of retrograde condensation when trace amounts of the hea vy hydrocarbon and nitrogen or methane are present. The maximum pressu re range predicted for the ternaries with nitrogen and methane arises during the transition of the shape of the liquid-liquid-vapour zone fr om an oval to a tear drop shape.