A robust procedure for three-phase equilibrium calculations of water-hydrocarbon systems using cubic equations of state

Abstract

Phase equilibrium calculations are one of the main steps in the compositional simulation of hydrocarbon reservoirs, well tubing, oil and gas pipelines as well as separation units. The main phases in all hydrocarbon production systems from a reservoir to a stock tank are gas, oil, and water. In most studies, the aqueous phase is considered bulk and phase equilibrium calculations are conducted only on the oil and gas phases. In some conditions such as thermal processes, the solubility of water in other phases and also hydrocarbon components in the water phase cannot be ignored. The robustness of equilibrium calculations for a wide range of compositions, temperatures, and pressures, including near critical area and saddle point curve, is essential for rapid and stable simulation studies. In this study, we propose the three-phase equilibrium calculations in detail for water, oil and gas phases. All the handling steps and strategies to establish a robust and efficient phase stability testing and flash calculation are explained. Moreover, appropriate tolerance values for decision-makings in the algorithm are defined. We successfully present the effectiveness of our algorithm for some three-phase samples covering a wide range of composition and production conditions.