Study on the Microscopic Mechanism of Supercritical CO2 and Active Water Alternative Flooding in Tight Oil Reservoir

Tight oil reservoirs have the characteristics of low porosity, low permeability, and low saturation, making it difficult to achieve economic development through conventional water injection. This study evaluated different injection media and oil displacement methods through experiments, and explained the micro mechanisms of oil displacement under different oil displacement processes using nuclear magnetic resonance methods. Experiments showed that the supercritical CO2 flooding and supercritical CO2 and active water alternative flooding were much more useful for the low-permeability reservoirs compared with the conventional water flooding. The experiment showed that this technology can increase the recovery rate by more than 12.0%, which was 33.24% higher than conventional water injection. In addition, it can effectively improve the rapid increase in water content caused by the rapid advance of the water front during the water injection process. The results of NMR showed that the recovery efficiency of pores under different oil displacement conditions has good consistency. When the aperture varied between 0.1 µ m and 1 µ m (Type III), the utilization rate was highest, followed by Type IV (1-10 µ m), Type II (0.01-0.1 µ m), and Type I (0.001-0.01 µ m). By comparison, conventional water and CO2 alternate flooding was more effective for type III pores, increasing oil recovery by 12.58%, while active water+CO2 alternate flooding can further drive oil, increasing oil recovery by 33.24%, and greatly displacing oil in micro-pores and macro-pores.