Abstract: Low-permeability reservoirs are generally developed by injecting water. In the process of waterflooding development, due to the heterogeneity of the reservoir and the microscopic randomness of the pore structure, it is common to cause fingering phenomenon, and it would become serious with continuing water injection. To make reasonable development decisions for low-permeability reservoirs, it is necessary to accurately understand the variation law of viscous fingering and its influencing factors. A viscous fingering model was established with a starting pressure gradient, for waterflooding low-permeability reservoirs, and a concept of small fingering tumors was introduced. The migration distance of small fingering tumors was used to characterize fingering change. After solving the model, the effects of permeability, density difference, formation dip, waterflooding rate and the ratio of oil-to-water viscosity were analyzed on viscous fingering. The results show that, in the process of waterflooding low-permeability reservoirs, the presence of starting pressure makes fingering phenomenon serious, but gravity can effectively slow down the phenomenon; the greater the permeability, density difference and formation dip, the less serious the fingering phenomenon; the greater the waterflooding rate and the ratio of oil-to-water viscosity, the more serious the fingering phenomenon. In order to slow down the fingering, it is suggested to inject water at the bottom of the reservoir, and the waterflooding rate should be kept reasonable.

Key words: low-permeability reservoir, waterflooding, viscous fingering, gravity, mathematical model