Modeling of CO2-CH4-H2S-brine based on cubic EOS and fugacity-activity approach and their comparisons

Abstract

Abstract The phase behavior of CO2-CH4-H2S-brine systems is of importance for geological storage of greenhouse gases (CCS) and enhanced oil recovery (EOR). In such projects, reservoir simulations play a major role in assisting decision makings, while modeling the phase behavior of the relevant CO2-CH4-H2S-brine system is a key part of the simulation. There is a need for an equation of state (EOS) for such system which is accurate, wide application range (pressure, temperature and aqueous salinity) computationally efficient and easy for implementation in a reservoir simulator. In this work, corresponding to the CO2-CH4-H2S-brine system, mutual solubility models are established by two different ways: (1) a cubic model based on the revision of the Soreide-Whitson EOS is used for fugacity calculation of both aqueous and non-aqueous phases (ϕ-ϕ model); (2) Peng-Robinson model is used for the fugacity calculation of non-aqueous phase and Pitzer activity model is used for the aqueous phase activity calculation (ϕ-γ model). Both model can accurately reproduce the existing experimental data to high temperature (250 °C), high temperature (1000 bar for CO2 and CH4 solubility, and 200 bar for H2S), and high salinity (6 molal of NaCl in solutions). The ϕ-γ model has slightly better accuracy. Computation efficiency of the two models is investigated. The ϕ-γ model has obvious better computation efficiency, and can be a good choice for the implementation in numerical simulators.