Experimental study on CO2/Water flooding mechanism and oil recovery in ultralow - Permeability sandstone with online LF-NMR

Abstract

The investigation of the mechanism of CO2/water flooding in ultralow-permeability oil reservoir is critical to the enhanced oil recovery. In this study, a series of core flooding experiments in pore and pore-fracture cores subjected to different in situ stresses was conducted, and online LF-NMR technology was employed to dynamically monitor the multiphase flow and pore-fracture behavior. The results show that the percolation pore dominates the volumetric sweep efficiency, the capillary pressure caused by sandstone wettability controls the oil recovery in adsorption pore, and viscosity fingering and thief channeling mainly occurred in migration pore. The oil recovery is primarily contributed by the percolation pore and migration pore in CO2 flooding, and all pores averagely contribute to the oil recovery in water flooding. In addition, the oil recovery in the pore core decreased with the increase of the in situ stress (15–35 MPa) in CO2/water flooding. Compared with the increase of pore-fracture core oil recovery responding to the increased in situ stress in water flooding, the highest pore-fracture core oil recovery presented at 25 MPa followed by 35 MPa and 15 MPa in CO2 flooding. The oil saturated in fracture and fracture-connected pore was firstly displaced, and the evolved heterogeneous morphology of the residual oil was characterized by the increased fractal dimension from 1.5 to 1.7 in CO2 flooding and 1.78 to 1.82 in water flooding responding to the increased in situ stress. The pore core oil recovery rate in CO2 flooding is about 10 times than that in water flooding. The peak value of oil recovery presented at 25 MPa for CO2/water flooding, and the fracture influence is more obvious in water flooding. The findings provide significant guidance to the engineering practice of enhanced oil recovery in ultralow-permeability reservoir.