Experimental study on stress-dependent multiphase flow in ultra-low permeability sandstone during CO2 flooding based on LF-NMR

Abstract

Oil production performance in the unconventional reservoirs is significantly affected by the flooding agent and occurrence environment. In this study, the in-situ stress and displacement pressure effects are considered, the response of CO2/Oil and pore structure was monitored by online low field nuclear magnetic resonance (LF-NMR). The results show that the pore structure was composed of adsorption pore (AP＜3 m s), percolation pore (3 m s＜PP＜30 m s), and migration pore (30 m s＜MP). The stress sensitivity of adsorption pore is more obvious than that of percolation pore and gradually weakened responding to the increase of in-situ stress. The increased in-situ stress narrowed and closured pore throats and limited the migration of CO2/Oil, and the pore transformed from gas-channeling type into homogeneous type and the hydraulic conductivity of CO2 and oil decreased non-linearly responding to the pore shrinkage. The percolation pore and migration pore dominate the oil recovery, and the decreased MP resulted in the non-linearly and dynamic reduction of oil recovery. The higher relative permeability was presented and the end-point oil saturation non-linearly increased with the increasing in-situ stress. The increased displacement pressure effectively improved the oil recovery and dynamic gas channeling, and the incremental oil recovery was primarily contributed by the PP and MP due to the increased CO2 swept area. The positive effect of displacement pressure on oil recovery gradually reduced with the increase of in-situ stress. The findings provide a significant insight on the application of CO2-EOR in ultra-low permeability reservoirs.