Microscopic visualization experiment on gas occurrence characteristics in CO₂ flooding

The CO₂ microscopic occurrence characteristics are significant for the study of CO₂ flooding to enhance the oil recovery. However, the form variation and occurrence characteristics of gas in CO₂ flooding have been rarely discussed so far. By using the microscopic flow simulation technology, a two-dimensional microscopic visualization model considering different pore throat characteristics was built. With the model, microscopic visualization displacement experiments were performed on CO₂ miscible flooding, CO₂ immiscible flooding, and water flooding after gas flooding, to clarify the occurrence characteristics of CO₂ under different displacement processes, displacement stages and pore throat characteristics. The results show that the occurrence characteristics of CO₂ are jointly controlled by displacement process, displacement stage and pore throat characteristics. During CO₂ flooding, the occurrence characteristics of CO₂ are mainly affected by displacement process, followed by pore throat characteristics. During gas flooding, CO₂ occurs mainly as a continuous phase. During miscible flooding, CO₂ is enriched and in a supercritical state locally, and partially dissolved in the residual oil. The phenomenon is unobvious during immiscible flooding. During water flooding after gas flooding, the occurrence characteristics of CO₂ are jointly affected by displacement process and pore throat characteristics. During miscible flooding, the low-permeability model exhibits the gas occurrence in dispersive bubble form, and as slugs in fine throats, while the high-permeability model demonstrates the gas occurrence dominantly as dense slugs, supplemented by dispersive bubble form. During immiscible flooding, the low-permeability model shows slugs in fine throats, while the high-permeability model reveals the dominance of small slugs, which are concentrated between throats. The research findings are conducive to the study of CO₂ microscopic displacement mechanism and occurrence characteristics, and of theoretical guiding significance for research on improvement of CO₂ flooding efficiency and contribution to CO₂ storage.