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The coexistence of acid gas/water/alcohol/electrolyte mixtures is common during the gas production process. Traditional methods have difficulty accurately predicting hydrate formation temperatures and pressures because of complicated molecular and ionic interactions in such mixture components. Based on the electrolyte Cubic-Plus-Association (e-CPA) equation of state (EoS) and the Parrish-Prausnitz model, this paper proposed a unified thermodynamic model to describe the multiple interactions of acid gas/water/alcohol/electrolyte components on gas, liquid, and hydrate phases by using the advantages of the e-CPA EoS to characterize the hydrogen bond association, solvation, ion electrostatic interaction, and ionic solvation. This work has achieved an accurate and flexible way to compute the hydrate formation conditions with or without acid gases, alcohols, and electrolytes. A total of 281 groups of experimental data at pressures from 0.068 MPa to 186.2 MPa are applied to validate this model. The results show that the average relative deviations (ARDs) of predicted temperatures and pressures are 0.13% and 6.52%, respectively. For nonelectrolyte systems, the ARD between the experimental and calculated hydrate formation temperatures is 0.19%, which is reduced by 0.24% and 0.34% in comparison with traditional Soave–Redlich–Kwong and Peng–Robinson EoSs. For mixtures containing alcohol and/or electrolytes, the ARD of predicted hydrate temperatures is 0.12%. Particularly, for harsh systems with ultrahigh pressure (186.2 MPa), the sour gas content of 24.52 mol%, MeOH concentration of 50% by mass, and salinity of 31% by mass, the deviations are less than 2.2 K. More cases demonstrate the performance and synergistic effect of alcohol and electrolytes in inhibiting hydrate formation.

Injecting CO2 into the hydrate reservoirs to replace CH4 is an effective method for the natural gas hydrate (NGH) exploitation and CO2 sequestration. The accurate prediction of the CH4 recovery factor under different pressures, temperatures, and CO2 injection amounts are essential for the design of the CO2 replacement process. However, conventional multiphase flash algorithms are not stable for CO2-containing hydrate system calculations because of inappropriate initial estimations on the hydrate phase fraction and composition. This paper proposed a new multiphase flash algorithm by developing a new initialization method for the hydrate phase fraction and composition as well as the solution procedure. A total of 61 sets of experimental data at pressures from 8 MPa and 16 MPa are applied to validate the new method. Results demonstrate that the new method successes in all the 61 cases, while the software PVTsim and CSMGem do not converge for 16 and 20 cases, respectively. Also, the average relative deviation between the calculated phase compositions and experimental values is less than 18 %, which represents a comparable accuracy to the above two software. Moreover, case studies are designed to research the variations of the CH4 recovery factor with the injected CO2 concentration in the hydrate reservoir. The most economical injected CO2 concentrations are discussed at specified reservoir conditions.