Research On Several Problems In Numerical Simulation Of CO₂ Flooding In Low Permeability

The application of CO2 flooding in low permeability reservoir can not only improve oil recovery,but also effectively realize the storage of the greenhouse gas CO2.Thus,multiple benefits can be gained,such as in economy,environment and et al..In order to improve the accuracy,efficiency and stability of numerical simulation for CO2 flooding,this thesis focuses on several relevant issues,including pseudo-component combination,selection strategy of primary variables and influence of oil-gas capillary force.Based on the study of different combination schemes for pseudo components,it is found that the combination schemes for light components have greater impacts on the bubble point line,while the the combination schemes for heavy components have greater impacts on the dew point line.The influence of single light component on the bubble point line is found to be C1>C2>C3>C4>C5>C6,so it is suggested that the light component C1 should be treated as a pseudo component alone when it is low in content.Two selection strategies for primary variables are studied in the thesis,and their different performances during the numerical simulation of the full implicit solution of the compositional model are focused.In the strategy A,the primary variables changes in different phase areas,but in the strategy B,the primary variables keep unchanged.There are some significant differences between these two strategies about the calculation efficiency when solving the second type variables.When the Kvalue-based compositional model is employed,the application of the strategy A will gain faster calculation speed and better stability than the strategy B.Numerical tests show that the computation efficiency of strategy A is about 32%higher in 1D case,and about 27%higher in 2D case than strategy B.When the EoS-based compositional model is employed,the calculation efficiency and total time consumption are nearly equivalent,and no significant difference can be detected between these two strategies.Furthermore,the effect of the oil-gas capillary force on the calculation of the phase equilibrium in binary or multi-component systems is studied.It is found that the existence of oil-gas capillary force will lower the bubble point line,narrow the two-phase area,and make the phase equilibrium tend to liquid phase.The numerical simulation results indicate that,as the total mole fraction of CO2 decreases,the temperature decreases and the pore diameter decreases,the capillary force increases and the influence on the phase equilibrium becomes more obvious.In addition,the influence of capillary force on CO2 flooding is also studied.Numerical examples show that the existence of oil-gas capillary force will increase the oil saturation near the displacement fronts,thus promoting the oil flow to the production well,which is beneficial to CO2 flooding.Compared with the case without considering the capillary force,the difference is quite significant.This implies that the influence of capillary force should be fully considered in the practical numerical simulation of CO2 flooding in low permeability reservoir.