Oil Displacement Characteristics of High-Pressure CO2 Miscible Flooding and Optimization of Dynamic Adjustment Measures at Different Development Stages

Oil displacement characteristic curves provide an effective means of evaluating displacement performance. However, the displacement characteristics of high-pressure CO2 miscible flooding remain insufficiently understood. Dynamic adjustment measures and parameter optimization methods for different development stages also require further improvement. This study accounts for the compressibility of CO2, crude oil, and rock and uses seepage theory to construct characteristic charts for high-pressure CO2 miscible flooding. Existing stage classification methods are compared, and an improved method is developed by incorporating bottom-hole pressure variations. An integrated analytic hierarchy process (AHP)–entropy weight–technique for order preference by similarity to ideal solution (TOPSIS) method is then employed to optimize adjustment measures and their operating parameters. The method identifies the combinations that provide the best oil displacement and CO2 storage performance at different development stages. The results show that the actual curve of Well X1 in a representative well group in Block F of Shengli Oilfield bends upward and noticeably crosses the characteristic chart curves. Its development performance is therefore expected to deteriorate. The upward crossing is more pronounced for Well 8, which exhibits poor development performance. The improved method divides the entire CO2 flooding process into four stages: gas-free oil production, early gas channeling development, late gas channeling development, and storage. Injection–production coupling provides poorer oil displacement and CO2 storage performance than chemical profile control and plugging or water-alternating-gas flooding at all stages. Nevertheless, its low cost and operational flexibility support continued application. Water-alternating-gas flooding (WAG) gives the best performance at every stage. The optimal combination comprises 25 WAG cycles, a 4-month alternation period, a gas injection intensity of 3.33 t/(d·m), and a water–gas ratio of 3:1. These findings support performance evaluation and adjustment measure optimization for high-pressure CO2 miscible flooding.