Establishing a universal chart for dynamic performance of gas flooding in water-flooded oil reservoirs

The Tong’s Chart is widely used in water flooding performance analysis， but a similar universal chart is lacking for gas injection performance analysis， making it difficult to evaluate the effectiveness of gas flooding and related measures in engineering applications. Through the momentum conservation equation in porous media， under steady-state flow conditions， the flow velocity of gas follows Darcy’s fractional flow law， whether in miscible or immiscible flooding. The study indicates that： ① Based on the fundamental theory of gas-liquid fractional flow， and considering the comprehensive effects of capillary pressure， gravity segregation， injection-production driving forces， and reservoir heterogeneity on the stable migration of the gas-liquid interface during gas injection after water flooding in heterogeneous reservoirs， a theoretical chart with strict theoretical significance for analyzing the performance of gas injection after water flooding in heterogeneous reservoirs was derived and established； ② A method and procedure for developing corresponding universal charts for specific reservoirs were proposed. Two distinct case reservoirs—the Layer CⅢ1 of the Donghe Block， Tarim Oilfield and the 4 Oil Group in the X Oilfield， both transitioning from water flooding to gas injection—were selected for detailed comparative analysis. The application of this method achieved favorable results in predicting the performance of both miscible gas-assisted flooding in the top of the Layer CⅢ1 of the Donghe Block， Tarim Oilfield and immiscible gas injection in the top of the 4 Oil Group in the X Oilfield， demonstrating its good universality for different types of gas flooding after water flooding； ③ This chart possesses universal functions similar to those of the Tong’s Chart. It can be used to analyze gas injection performance and assess the stability of the gas-liquid interface in gas injection reservoirs， thereby providing guidance for the regulation of gas injection development.