Experimental study on water/CO2 flow of tight oil using HTHP microscopic visualization and NMR technology

The visualization of the transport and distribution of multiphase fluids through images is essential for understanding the mechanisms involved in the utilization of crude oil. In this study, we employed microscopic visualization and nuclear magnetic resonance technologies to monitor oil distribution and investigate fluid flow during water and supercritical CO2 flooding of tight core. The microscopic visualization experiment results indicate that during water flooding, significant viscous fingering occurred due to capillary pressure and an unfavorable mobility ratio, resulting in substantial volumes of contiguous residual oil and blind-end residual oil. In contrast, CO2 miscible flooding operates through interdependent interactions, facilitating the efficient extraction and displacement of residual oil, with an oil recovery of 93 % and a reduction in residual oil saturation of approximately 30 %. The NMR results demonstrate that the oil recovery of water flooding oil is 28.06 %, which is considerably lower than that of CO2 flooding, and the crude oil primarily comes from the macropores. We observed that the oil recovery of CO2 immiscible flooding, near-miscible flooding and miscible flooding yielded 41.35 %, 59.3 % and 66.22 %, respectively. The interaction between oil and CO2 resulted in a significant increase in oil recovery of macropores, approximately double that of immiscible flooding. As increasing injection pressure of CO2 flooding, the flow characteristics transition from capillary finger flow to a network flow, reducing residual oil saturation. This transition transforms significant volumes of contiguous residual oil into isolated oil droplets and smaller fragments, while alterations in water distribution characteristics influence CO2 flow. These findings provide valuable insights for optimizing displacement mechanisms and enhancing oil recovery in tight oil.