Effect of wettability on immiscible viscous fingering in porous media

When a less viscous fluid displaces an immiscible fluid of higher viscosity in a porous medium, the displacement front may be unstable and lead to viscous fingering. The goal of this paper is to study the effect of wettability on viscous fingering. The injection fluid and the porous media (geological outcrop cores) are varied to achieve injection of strongly wetting, weakly wetting and strongly non-wetting fluids. The effects of permeability and flow velocity are also studied at constant wettability. Unstable displacements are conducted in 2D micromodels to allow visualization of pore-scale mechanisms. Viscous fingering is more severe for strongly non-wetting injection fluid than for strongly or weakly wetting injection fluids. For wetting fluid injection, decreasing permeability leads to more severe fingering and lower recovery. Increasing flow rate leads to lower recovery. Micromodel experiments show that imbibition of the wetting fluid out of the fingers dampens the growth of fingers. The viscous fingering number, N-vf = N-c mu(2)(r)(D-2/K) can be used to correlate recovery for wetting fluid injection. For strongly non-wetting fluid injection, permeability does not have a significant effect on fingering and recovery. Also, flow rate has the opposite effect compared with wetting fluid injection. N-vf = N-c(-1)mu(2)(r)(D/L)(2) can be used to correlate recovery in the presence of viscous fingering for non-wetting fluid injection. Micromodel experiments suggest that imbibition of the wetting fluid into the fingers is virtually non-existent due to high viscosity of the wetting fluid.