Experimental Research of the Characteristics of Microscopic Pore Production during CO2 Displacement in a Shale Oil Reservoir

The evaluation method for determining the pore production range during CO(2 )displacement in shale cores is established based on CO(2 )displacement experiments, core analysis using online nuclear magnetic resonance (NMR), and pore size analysis using high-pressure mercury injection. By transforming the abscissa of the NMR T-2 spectrum into pore-throat radius using the conversion formula between relaxation time and pore radius, the production behavior of crude oil in pores of different scales during CO(2 )displacement is analyzed. The target shale oil reservoir, located in eastern China, consists of two sets of salt beds. Crude oil is predominantly found in mesopores and smaller macropores. While water injection can saturate the salt and enhance seepage, the reactive areas of water imbibition are still smaller compared to those of CO(2 )injection. It is observed that the displacement efficiency of CO(2 )injection exhibits a negative correlation with the clay mineral content, with the swept pores mainly consisting of mesopores and small macropores. The proportion of crude oil produced from pores with diameters less than 0.1 mu m during CO(2 )displacement is significantly higher than that during water displacement. The displacement effect of CO(2 )in mesopores and smaller macropores improves with increasing injection pressure. Above the miscible pressure condition, the oil displacement efficiency of CO(2 )is 11% higher than that of water displacement, indicating that CO(2 )displacement is more suitable for this shale oil reservoir