Thermodynamic Computation Of Phase Equilibrium Characteristics Of CO₂/Oil/Water Three-phase System

The rapid development of modern society,whether it is industrial development or daily life,the requirements for energy is increasing.The CO₂ flooding process can not only increase the crude oil recovery of mature reservoirs,but also bury a large amount of greenhouse gas CO₂ in the ground to achieve green energy extraction.Aiming at the displacement process of CO₂ in the coexisting environment of oil and water in the geological structure,this paper adopted a thermodynamic calculation method and used the Peng Robinson Equation of State?PR-EOS?model to calculate the phase equilibrium characteristics of the CO₂/oil phase system,CO₂/water phase system,CO₂/brine phase system and CO₂/oil/water three-phase system.The specific conclusions of this article are as follows:?1?Prediction of composition and miscible pressure of CO₂/oil phase system.The compositon ration of the CO₂/oil phase system determined by a practical mixing model was proposed,and the miscible pressure of the two-phase system was calculated.Taking the CO₂/C₁₆H₃₄ and CO₂/liquid paraffin oil system as examples,the two-phase composition determined by the mixing model at different temperatures and the error values of calculated miscible point with the experiment test results are good,verifying the correctness of the calculation principle and process in this paper.?2?The calculation of the solubility characteristics of CO₂/water phase was carried out by modifying the interaction coefficient and the gravity coefficient of the PR-EOS model.It was found that at the same temperature,when the pressure rises,the percent of CO₂ in the liquid phase progressively increases,but the percent of CO₂ in the vapor phase progressively drops.The trend is gentle at higher temperatures.At the same pressure,when the temperature rises,the percent of CO₂ in the liquid and vapor phase drops.The errors between the calculated solubility values and the experimental values reported in the literature are small.Both“platform area”and“crossing area”appear near the same pressure value,and the numerical error in quantification is also small.?3?Calculation of the solubility characteristics of CO₂/brine by modifying the interaction coefficient and the gravity coefficient of the PR-EOS model.It was found that at the same temperature and pressure,when the concentration of brine rises,the solubility of CO₂ in the liquid phase progressively decreases,and the solubility in the vapor phase rises.At the same temperature and brine concentration,when the pressure rises,the solubility of CO₂ in the liquid phase rises,and the solubility in the vapor phase rises first and then drops.At the same pressure and brine concentration,when the temperature rises,the solubility of CO₂ in the liquid phase drops first and then rises,and the solubility in the vapot phase rises first and then drops.The error analysis between the calculation results and experimental results shows that,compared with the modified fugacity coefficient and activity coefficient method,changing the interaction coefficient and gravity coefficient of CO₂/brine is more suitable for studying the solubility characteristics of CO₂/brine.?4?Based on the calculation results of the two-phase phase equilibrium characteristics,a calculation study of the phase equilibrium characteristics of the CO₂/oil/water three-phase system was carried out.The calculation results show that when the water content in the system is 1%,the miscible pressure point of the CO₂/C₁₈H₃₈/H₂O system is 8.8MPa at 40.6?,the miscible pressure point is 10MPa at50?and the miscible pressure point at is 13MPa at 61.2?,all higher than the miscible pressure point of the CO₂/C₁₈H₃₈ two-phase system at the same temperature.The results show that the presence of the water phase reduces the miscible pressure point of the CO₂/oil phase mixed system.