Studies on multi-phase equilibrium separation of hydrocarbon/water systems

Abstract

Equations of State (EOS) have been used successfully in compositional simulators to describe phase behavior of reservoir crude and gas condensates without water. The three-phase behavior of water/reservoir crude oils both at reservoir and steam flooding temperature has not yet been successfully predicted by an EOS. Recent publications show that incorporation of the association concept into simple cubic equations such as Peng-Robinson EOS significantly improves phase behavior prediction of water/hydrocarbon and nonhydrate systems, but there are still some numerical difficulties in the pre-diction of three phase flash equilibria. In this thesis we presented, an efficient procedure that will help us eliminate common numerical difficulties in predicting three-phase equilibria for systems containing water, and simplify the programming technique. This scheme checks the existence of three-phase flash at given reservoir conditions, and if three-phase flash (vapor-oleic-aqueous) doesn't exist, it automatically switches to check correct type of two-phase flash (vapor-oleic, vapor-aqueous, or oleic-aqueous). This convergence scheme is used in an EOS-simulator (VLLESIM), which uses nine different EOS's and van der Waals mixing rules, to accurately describe multi-phase equilibrium separation of hydrocarbon/water systems. VLLESIM will be validated with literature data and experimental results obtained from two different experimental setups, for different multi-component hydrocarbon/water systems, for hydrocarbon component varying from C6 to C20. PVT-VLLE apparatus can provide on-line compositional analysis and phase volumes of all equilibrium phases for temperatures up to 350 'F and an Isochoric Steam Distillation Cell (ISDC) can provide only vapor phase compositions, but it can be used up to 500 'F. VLLESIM will then be used to study the effect of temperature, pressure and molecular weight of hydrocarbon components on multiphase equilibria of different hydrocarbon/water systems. Detailed analysis is also provided in this thesis, of the effect of increasing concentration of different hydrocarbon components on the size of the three-phase region for ranges of temperature and pressure mostly prevalent under reservoir conditions.