An Equation of State for thermodynamic equilibrium of gas mixtures and brines to allow simulation of the effects of impurities in subsurface CO2 storage

Comprehensive understanding and prediction of chemical and reactive processes during and following injection of CO2 in depleted gas reservoirs and saline aquifers is important for the assessment of the performance and impacts of planned and existing Carbon Capture and Storage (CCS) projects. Over the last decade significant improvements have been made in numerical modelling of the complex, coupled processes involved. Among the many remaining issues where progress is still called for, is the consistent simulation of impacts of gas mixtures. In particular the presence of ‘impurities’ or ‘co-contaminants’ in the injected CO2 stream that are retained from the original flue-gases, such as H2S and SO2, have the potential, upon dissolution in the pore water, to alter aqueous and water–mineral reactions. Moreover, presence of these and other injected or in situ gases (e.g. CH4) affect CO2 solubility and thermodynamic properties of the fluid and gas phases, which, in turn, impact transport processes.As an important step towards evaluation of the impact of gas mixtures on these processes, a new Equation of State (EOS) has been developed which allows accurate and efficient modelling of thermodynamic equilibrium of gas mixtures and brines over a large range of pressure, temperature and salinity conditions. Presently the new EOS includes CO2, SO2, H2S, CH4 and N2. This model is based on equating the chemical potentials in the system, using the Peng–Robinson EOS to calculate the fugacity of the gas phase. The model performs favourably with respect to existing EOS's and experimental data for single gas systems and accurately reproduces available data sets for gas mixtures. Preliminary analysis shows, amongst others, that CO2 solubility is most sensitive to CH4 admixture and least sensitive to the presence of SO2 in the injected gas.

► An EOS for gas mixtures and brines with various salts is presented.
► It performs favourably with respect to other EOS's and experimental data.
► It non-iteratively calculates the composition of the aqueous and non-aqueous phases.
► The EOS is fully documented and can be readily implemented by others.