Experimental study on fracture effect on the multiphase flow in ultra-low permeability sandstone based on LF-NMR

Abstract

Multiphase flow affected by fractures is essential for enhanced oil recovery in ultralow-permeability reservoirs. In this study, CO2/water flooding of intact and fractured cores was performed under in-situ stress, and low-field nuclear magnetic resonance was employed to monitor the pore structure evolution and multiphase flow. The adsorption pore (AP), percolation pore (PP), and migration pore (MP) were determined in the ultralow permeability sandstone, and the PPs and MPs contributed to oil recovery during CO2 flooding. Three-scale pores contributed to oil production during water flooding. Compared with water flooding, CO2 flooding had a considerably higher average recovery efficiency, which was associated with pronounced gas channeling and lower ultimate oil recovery. In CO2 flooding, the oil production in PPs and MPs depended on the pressure difference, whereas that in APs was determined by oil swelling. But for water flooding, oil migration in the AP and MP was dominated by capillary imbibition and pressure difference, respectively. The fracture enhanced the oil recovery during CO2 flooding by increasing the CO2 sweep area in the PPs and MPs. It also improved oil recovery during water flooding based on the increment of the oil-water flow area in the APs and MPs. The mass transfer in CO2 flooding between the fracture and the matrix occurred through the PP and MP connected to the fracture, controlled by the pressure difference, number of PP and MP connected to the fracture, and oil swelling capacity. For water flooding, the oil-water exchange in the fracture-matrix system was dominated by number of pores connected to the fracture, hydrophilic ability of the rock, and pressure difference. These results can be used for enhancing oil recovery in fractured ultralow-permeability reservoirs.