Effects of oil recovery rate on water-flooding of homogeneous reservoirs of different oil viscosity

Abstract

Based on physical simulation of water-flooding homogeneous reservoirs, the water-flooding characteristics of homogeneous reservoirs with different oil viscosity are examined at different oil recovery rate. Reservoirs with low-viscosity (<5 mPa·s) oil can be evenly swept, with thick streamline. With increasing oil recovery rate, water rush weakens along the reservoir bottom and sweeps the reservoir more evenly in the vertical direction; and the sweep efficiency difference between top and bottom of the reservoir decreases. In high-rate development of the low-viscosity oil reservoir, the water-free recovery percent is significantly higher than that in low-rate development, and the rising velocity of water cut is lower than that under low-rate development, which proved that such reservoirs are suitable for high-recovery-rate development. For reservoirs with medium-high viscosity (5−50 mPa·s) oil, the injected water fingers significantly in the water-flooding process, with thin streamline, the coverage is not swept completely, especially in area between streamlines, the sweep efficiency difference between top and bottom is great. As the oil recovery rate increases, the streamline becomes thinner, the coverage becomes more incomplete, and the sweep efficiency of top and bottom both decreases. Medium to high-viscosity oil reservoirs developed at high rate have a short water breakthrough time, and the recovery percent in the water-free period is much lower than that in low rate development, and the rising velocity of water cut is higher than that under low-rate development, so high-rate development is not adaptable for medium-high viscosity reservoirs. Key words: water-flooding experiment, homogeneous reservoir, oil viscosity, oil recovery rate, water-flooding law, injected water sweep characteristics, high-rate development adaptability