Effects of water flooding speed on oil recovery efficiency and residual oil distribution in heterogeneous reservoirs

To verify the effects of water flooding speed (i.e. water injection rate) on oil recovery efficiency and residual oil distribution in heterogenous reservoirs, water flooding experiments with different flooding speed are carried out in microheterogenous cores, vertically heterogenous cores, and areally heterogenous cores. In the experiments, the cores are sequentially saturated with synthetic brine and synthetic oil, and then flooded with synthetic brine with different injection rates. The results demonstrate that the increase of flooding speed shortens water-free production period and decreases oil recovery efficiency for all the three kinds of heterogenous cores, resulting in more residual oil in the cores. This is because higher flooding speed forms preferential pathways in high permeable areas, inducing inferior sweep efficiency in low permeable areas. This unfavorable effect of higher flooding speed overrides the favorable effects on the recovery of oil droplets trapped in pore throats and pore dead-ends and oil films on pore surfaces, making more residual oil left in the low permeable areas of the cores and decreasing the ultimate oil recovery efficiency. These findings could provide in-depth knowledge on the effects of water flooding speed on oil recovery efficiency and residual oil distribution.