Abstract
Abstract An improved phase-partitioning model is proposed for the prediction of the mutual solubility in the CO2-brine system containing Na+, K+, Ca2+, Mg2+, Cl-, and SO42-. The correlations are computationally efficient and reliable, and they are primarily designed for incorporation into a multiphase flow simulator for geology- and energy-related applications including CO2 sequestration, CO2-enhanced geothermal systems, and CO2-enhanced oil recovery. The model relies on the fugacity coefficient in the CO2-rich phase and the activity coefficient in the aqueous phase to estimate the phase-partitioning properties. In the model, (i) the fugacity coefficients are simulated by a modified Peng-Robinson equation of state which incorporates a new alpha function and binary interaction parameter (BIP) correlation; (ii) the activity coefficient is estimated by a unified equilibrium constant model and a modified Margules expression; and (iii) the simultaneous effects of salting-out on the compositions of the CO2-rich phase and the aqueous phase are corrected by a Pizter interaction model. Validation of the model calculations against literature experimental data and traditional models indicates that the proposed model is capable of predicting the phase-partitioning behaviors in the CO2-brine system with a higher accuracy at temperatures of up to 623.15 K and pressures of up to 350 MPa. Using the proposed model, the phase diagram of the CO2+H2O system is generated. An abrupt change in phase compositions is revealed during the transfer of the CO2-rich phase from vapor to liquid or supercritical. Furthermore, the preliminary simulation shows that the salting-out effect can considerably decrease the water content in the CO2-rich phase, which has not been well experimentally studied so far.
Highlights
The preliminary simulation revealed that the heat exploitation efficiency was decreased by 27% in a CO2 geothermal system due to CO2 dissolution and mineral precipitation [25]
The equilibrium constant of CO2 in the aqueous phase (KCO2), which can be simplified as Henry’s constant at low pressure; (2) the relative activity coefficient including the effect of temperature and pressure for pure water, and the effect of electrolytes; and (3) specific volume which accounts for the effect of pressure
(3) The model parameters in the fugacity model of the aqueous phase are determined by the experimental data of CO2 solubility in the aqueous phase
Summary
CO2-water/brine is one of the most important and commonly encountered systems [1,2,3] in CO2 sequestration [4,5,6,7,8,9,10,11], CO2-enhanced oil recovery (EOR) [2, 12, 13], CO2enhanced geothermal systems (EGS) [14, 15], global CO2tracing [16,17,18,19], and so on. Assembled from 21 literature experimental studies, a databank of CO2 solubility containing 508 pieces of data was developed by Akinfiev and Diamond [42] They proposed an accurate φ-γ model with a valid range of 0-100 MPa and below 100°C, but the model is not accurate enough for estimating the water content in the CO2-rich phase. Affected by the scope of the experimental database used in model development, the model calculations in other brines were not as accurate as those in NaCl solutions [45] This model cannot be used for accurately estimating the water content in the CO2-rich phase. There are three major improvements compared to the traditional models Both the CO2 solubility in the aqueous phase and water content in the CO2-rich phase can be accurately estimated at temperatures. The salting-out effect of Na+, K+, Ca2+, Mg2+, Cl-, and SO42- on the composition of both the aqueous phase and the CO2-rich phase is considered
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