Evaluation Method for Miscible Zone of CO2 Flooding

Miscible zone, the area swept by injected CO₂ and its in-situ crude oil miscible with injected CO₂, great effect the oil recovery of CO₂ miscible flooding in reservoir. But there are little published researches on the evaluation method of CO₂ flooding. In this paper, the potential affective factors of miscible zone in CO₂ flooding are analyzed by theory. Dimensionless parameters are used to characterize the potential affective factors. In order to evaluate the miscible zone of CO₂ flooding accurately, published MMP prediction model and reservoir numerical simulation are combined to build its evaluation model in CO₂ flooding based on Box-Behnken design (BBD). And the evaluation method is proposed. The results show that the effect of miscible zone includes less miscibility ability and inhomogeneous pressure distribution. The former is caused by less reservoir pressure and impure injected gas. The latter is caused by inhomogeneous permeability and unreasonable development mode. MMP changes with injected CO₂ volume. The average absolute errors of continue CO₂ flooding and water-alternate-gas flooding calculated by proposed evaluation method are 4.2% and 4.5% respectively. The miscible zone shows a trend of decrease after the first increase with increasing volume of injected CO₂. And the main affective factor of continue CO₂ flooding and water-alternate-gas flooding all are the relative critical temperature of injected gas. The result indicates that the proposed evaluation method can be used to forecast the miscible zone of CO₂ flooding. And the result shows a scientific guidance for further study on CO₂ miscible flooding.