Abstract:
The CO
2 microscopic occurrence characteristics are significant for the study of CO
2 flooding to enhance the oil recovery. However, the form variation and occurrence characteristics of gas in CO
2 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 CO
2 miscible flooding, CO
2 immiscible flooding, and water flooding after gas flooding, to clarify the occurrence characteristics of CO
2 under different displacement processes, displacement stages and pore throat characteristics. The results show that the occurrence characteristics of CO
2 are jointly controlled by displacement process, displacement stage and pore throat characteristics. During CO
2 flooding, the occurrence characteristics of CO
2 are mainly affected by displacement process, followed by pore throat characteristics. During gas flooding, CO
2 occurs mainly as a continuous phase. During miscible flooding, CO
2 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 CO
2 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 CO
2 microscopic displacement mechanism and occurrence characteristics, and of theoretical guiding significance for research on improvement of CO
2 flooding efficiency and contribution to CO
2 storage.