Study On Seepage Law And Parameter Optimization Of CO₂ Foam In Low Permeability Reservoirs

CO₂ foam flooding,as a rapidly evolving new tertiary recovery technology,combines the advantages of CO₂ flooding with foam flooding.It not only can improve the displacement efficiency through misciblity,but also delay the gas channeling during the displacement process.Therefore,CO₂ foam flooding possess widely application prospects in low permeability formation with high heterogeneity.Currently,there is lack of recognition and research on the CO₂ foam percolation mechanism and mathematical characterization.It is necessary to study the seepage law of CO₂ foam flooding in low permeability reservoirs,to provide a theoretical basis for the large scale onshore field tests.Based on the fractional-flow theory,a novel model of CO₂ foam flooding in two cases,which are an FCM gasflood with foam injection and an FCM gasflood with foam,with surfactant dissolved in supercritical CO₂.We derive the fractional-flow solution for the foam displacement and find the water-saturation distribution.Also,we present numerical simulations using the same model to verify the solutions obtained from fractional-flow methods.By analyzing the fractional-flow solution with different foam quality and surfactant distribution,we get the best injection condition.Then,based on the mechanism of bubble formation and collapse,numerical simulation of CO₂ foam flooding in low permeability reservoirs has been carried out,to analyze the development and predict the performance of CO₂ foam flooding in heterogeneous reservoirs.The key influencing factors on the performance of CO₂ foam flooding were selectd via the simulation models with the grey relational analysis method.Finally,based on the characteristics of geology and fluid in the practical field,the numerical simulation models used to simulate CO₂ foam flooding in low permeability reservoirs are developed.By using genetic algorithm and orthogonal experimental design,the injection factors were optimized.The results show that CO₂ foam flooding can help to control profile in heterogeneous reservoirs,which is beneficial to improve the oil recovery of the middle and low permeable layers.For the ground pre-generated foam,the optimal water fraction for the foam maintains the gas front slightly ahead of the foam front.Mixed injection method is better than the ground pre-generated and surfactant gas injection method.As the starting time of foam flooding is postponed,the ultimate recovery first increases slightly and then decreases sharply.The injection rate and gas-liquid ratio have a similar regular pattern.With increasing surfactant concentration or foam slug size or front surfactant slug size,the ultimate recovery increases rapidly to reach a plateau.With increasing crude oil viscosity,the ultimate recovery decrease rapidly.Also,with increasing variation coefficient of permeability,the improved oil recovery first rise and then reduce.The multi-factors affect the ultimate recovery of foam flooding to different extents in descendent order of foam slug size,gas-liquid ratio,surfactant concentration,starting time of foam flooding,front surfactant slug size and injection rate.The front slug size and injection rate were optimized by genetic algorithm,and the foam slug size,gas-liquid ratio,surfactant concentration and starting time of foam flooding were combinatorial optimized by adopting the orthogonal experiment design.Compared with water flooding program,the optimed foam flooding could enhance oil recovery by 9.60%.