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To understand the production decline law and accurately calculate the decline rates at ultra-high water cut stages, the calculation formula is deduced using the theory of oil-water two-phase flow and the reservoir engineering method based on the relative permeability curve that reflects the nature of oil-water two-phase seepage flow. Through comparison with the actual production data in the ultra-high water cut period, the accuracy of the formula is verified and the analysis on the influencing factors of the production decline rate is conducted. The results show that with the increase of parameters such as absolute permeability, pressure difference between injection well and production well and the change rate of oil relative permeability, the production decline rate increases almost linearly. With the increase of row distance, the production decline rate decreases. The effects of absolute permeability, pressure difference between injection well and production well, and the change rate of oil relative permeability on the production decline rate are the same, while the row distance effect is the most significant. The research results indicate that the proposed formula can precisely calculate the production decline rate at the ultra-high water cut stage, and this production decline rate follows an exponential law. [ABSTRACT FROM AUTHOR]

CO 2 miscible flooding in low permeability reservoirs is conducive to significantly improving oil recovery. At present, the microscopic displacement simulation of CO 2 miscible flooding is mainly reflected in the simulation of the seepage process, but the pressure control of the seepage process is lacking, and the simulation of the characterization of CO 2 concentration diffusion is less studied. In view of the above problems, a numerical model of CO 2 miscible flooding is established, and the microscopic seepage characteristics of interphase mass transfer in CO 2 miscible flooding are analyzed by multiphysics field coupling simulations at the two-dimensional pore scale. The injection velocity, contact angle, diffusion coefficient, and initial injection concentration are selected to analyze their effects on the microscopic seepage characteristics of CO 2 miscible flooding and the concentration distribution in the process of CO 2 diffusion. The research shows that after injection into the model, CO 2 preferentially diffuses into the large pore space and forms a miscible area with crude oil through interphase mass transfer, and the miscible area expands continuously and is pushed to the outlet by the high CO 2 concentration area. The increase in injection velocity will accelerate the seepage process of CO 2 miscible displacement, which will increase the sweep area at the same time. The increase in contact angle increases the seepage resistance of CO 2 and weakens the interphase mass transfer with crude oil, resulting in a gradual decrease in the final recovery efficiency. When the diffusion coefficient increases, the CO 2 concentration in the small pores and the parts that are difficult to reach at the model edge will gradually increase. The larger the initial injection concentration is, the larger the CO 2 concentration in the large pore and miscible areas in the sweep region at the same time. This study has guiding significance for the field to further understand the microscopic seepage characteristics of CO 2 miscible flooding under the effect of interphase mass transfer., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)