Calculation of thermodynamic properties of water by the CPA equation of state

Abstract An appreciable amount of formation water exists inevitably in the porous media of reservoirs, but the existence of water is often neglected in the usual calculation of phase equilibrium. The commonly used equation of state is simple, but may have large deviations in estimating some thermodynamic properties. The existing equation of state, which has been modified, is not very suitable for the system containing polar substances (e.g. hydrocarbons, water, and alcohols). In this paper, the CPA (Cubic-Plus-Association) equation of state which considers both physical and association interactions between molecules was used to calculate the thermodynamic properties of water under the saturated and unsaturated states. Moreover, the disadvantages of the commonly used equation of state in the calculation were analyzed. The following results were obtained. First, the commonly used equation of state can be used to effectively calculate the saturated vapor pressure of water, but it presents certain differences in calculating water density and enthalpy, and the enthalpy calculation is not affected by volume modification. Second, the average absolute/relative deviations between the thermodynamic properties of water calculated by the CPA equation of state and the experimental data are approximately 1%. For the system containing polar substances, the CPA equation of state is preferred in calculating the thermodynamic properties. The accurate estimation of the thermodynamic properties of water is fundamental in identifying the fluid state in the water-bearing gas reservoirs, and it is of great significance in the development and production of such gas reservoirs.