Research on Microscopic Production Characteristics of Oil in Tight Sandstone Reservoirs by Different CO₂ Injection Methods

The efficiency of different CO₂ injection methods on oil recovery in tight oil reservoirs exhibits substantial variation. This investigation focuses on the Chang 6 Member of the Yanchang Formation, situated in the Ordos Basin, as the research subject. Two injection techniques, namely continuous CO₂ flooding and CO₂ huff-n-puff after flooding, were employed to scrutinize the microscopic production traits of oil. Five tight sandstones with comparable physical characteristics were selected for the study. A physical simulation flooding system was utilized to perform continuous CO₂ flooding and CO₂ huff-n-puff after flooding experiments at five pressures ranging from 4 MPa to 20 MPa. The NMR technology was employed to analyze the T₂ spectrum of the cores before and after the experiment to quantify the micro production traits of oil at different pore scales. The experimental findings revealed that the oil recovery of the total pore increased progressively with flooding pressure during the flooding process. The highest recovery rate of 75.29% was observed during 20 MPa CO₂ flooding. While the utilization degree of oil with large pores remained the highest, the contribution of oil decreased gradually. During the CO₂ huff-n-puff after flooding under immiscible conditions, the increase in CO₂ huff-n-puff recovery in macropores was the highest, up to 10.35%. The highest increase in oil utilization of small pores was 15.26% after miscibility, suggesting that oil in large pores was the primary contributor during the immiscible phases. After miscibility, oil in the small pores became the primary contributor. This investigation analyzes the influence of CO₂ flooding and CO₂ huff-n-puff after flooding injection methods on the micro pore oil production traits of tight sandstone reservoirs. The findings provide novel theoretical support for future field tests of CO₂ flooding and CO₂ huff-n-puff after flooding.