Flash Calculation and Phase Stability Analysis of Reservoir Gas-Water System—Implication for Extracting Dissolved CH4 by CO2 Injection

Abstract

Abstract Geologic formations with abnormally high pressure and temperature are capable of storing huge amounts of methane, the production of methane while storing CO2 in aquifer could help offset the cost of CO2 capture and sequestration. The effect of dissolved CO2 in the water-rich phase on the total methane recovery from a CH4-saturated aquifer is still not clear, due to the lack of reliable equation of state to model water-containing reservoir gas systems. Modeling vapor-liquid phase equilibria of water-containing reservoir gas systems is previously considered a challenge for the cubic equation of state models. A concise and reliable phase behavior model for compositional reservoir simulation is presented that uses a modified Wong-Sandler mixing rule with Non-Random-Two-Liquid (NRTL) model to perform flash calculation and stability analysis for gas-water systems (CH4-H2O, CO2-H2O, CO2-CH4-H2O, etc) at reservoir temperatures and pressures. The proposed model is able to handle both strongly polar fluid system and hydrocarbon fluid system under the same thermodynamic framework. The model performance for the CH4-H2O and CO2-CH4-H2O systems was validated by a large amount of experimental data. As for the CH4-H2O system, the average absolute deviation of model calculated phase composition from the experimental data is around 5% for the gas phase and 7% for the aqueous phase. The model was used to simulate the two processes of CH4 recovery by CO2 injection: 1) the forward multiple-contact process; and 2) the backward multiple-contact process. The results showed that the forward multiple-contact process dominates CH4 recovery by CO2 injection. The maximum CH4 recovery factor (MRF) from CH4-saturated water by CO2 injection is approximately 50% to 70% and it is achieved only within a narrow temperature range (350 to 370K), regardless of pressure. The multiple-contact phase behavior simulation showed that, in typically reservoir pressures and temperatures (20 to 160 MPa, and 300 to 470K), 5 to 12 mole CO2 may be needed to recovery 1 mole of CH4 from CH4-saturated water.