Fluid Flow Characteristics of CO ₂ Miscible Gas Flooding in Reservoirs under Constant-Pressure Boundary Conditions

Gas injection is a crucial method for enhancing oil recovery in current oil reservoir development. Among these methods, the CO2 miscible gas flooding technology holds significant potential for widespread application. During the gas flooding process, the area near the injection well typically exhibits circular fluid flow characteristics. Moreover, dynamic changes in the bottom-hole pressure of the injection well are essential for well test research and the analysis of gas injection development parameters. In this study, the analytical solution for the two-zone radial composite model near the miscible gas flooding injection well is solved based on previous research. The type curve of dimensionless log–log bottom-hole pressure dynamic response characteristics under constant-pressure boundary conditions is obtained, and the effects of eight key parameters, including constant-pressure boundary conditions, the mobility ratio of the transition-zone fluid to crude-oil zone, and the variation index in the transition zone, are systematically analyzed. A numerical simulation model for miscible gas flooding in CO2 is also established. By comparing the analytical solution with numerical simulation results under identical conditions, it is found that both approaches are in strong agreement. The study reveals that factors such as oil–gas viscosity ratio, reservoir permeability, porosity, and gas injection rate significantly affect the dimensionless pressure and pressure-derivative curves, which ultimately affect the gas flooding sweep range and bottom-hole pressure. This study provides a foundation for optimizing the design of gas injection development parameters, interpreting well test data for miscible flooding injection wells, and conducting radial composite pressure dynamic analysis.