Abstract:

The poor physical properties and low pressure coefficient of tight oil reservoirs make it difficult to directly apply the advanced water injection development technology, which has been successfully used in low-permeability oil reservoirs, to tight oil reservoirs. The advanced CO₂ injection technology, as an emerging method for enhancing oil recovery, has received increasing attention. However, its microscopic oil displacement characteristics and enhanced oil recovery effectiveness require further investigation. Therefore, core samples from the Chang 8 member reservoir of the west 331 block in the Ordos Basin were selected. Experiments including water flooding, CO₂ flooding at different pressures, and advanced CO₂ injection flooding at different pressures were conducted using nuclear magnetic resonance (NMR) technology. The oil recovery characteristics and microscopic mobilization characteristics under different development methods were clarified. Meanwhile, a calculation model for the mobilization lower limit was established based on the capillary model, and the pore mobilization lower limits under different development methods were obtained. The experimental results showed that the oil recovery of water flooding was about 40%, with the oil primarily produced from macropores, while meso- and micropores exhibited poor mobilization. Compared with water flooding, the oil recovery of supercritical CO₂ flooding was higher and increased with the injection pressure, reaching 76% under miscible conditions. The advanced CO₂ injection flooding further enhanced the recovery rate, achieving 87% when the pressure reached 1.2 times the miscible pressure, with meso- and micropores contributing 14.1% of the recovery—approximately 1.5 times that of miscible flooding. After water flooding and non-miscible CO₂ flooding, the remaining oil was mainly in the form of continuous remaining oil, with a considerable amount still present at the core outlet end. As the CO₂ flooding pressure increased, the crude oil saturation decreased significantly, and the continuous remaining oil diminished, resulting in more isolated oil droplets. After advanced CO₂ injection flooding, the crude oil saturation further declined, and the large area of continuous remaining oil significantly reduced, primarily manifesting as isolated oil droplets and small clusters of continuous remaining oil. The mobilization lower limit of water flooding was 200 nm, and the pore mobilization lower limits of CO₂ flooding and advanced CO₂ injection flooding decreased with increasing injection pressure. Advanced CO₂ injection flooding can mobilize crude oil in 20 nm pores.

Key words: tight oil, nuclear magnetic resonance, advanced CO₂ injection, enhanced oil recovery, mobilization lower limit