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. This is a common phenomenon in viscous oil waterflooding, polymer flooding, gas flooding and carbon sequestration. The objective of this research is to model the effect of wettability on viscous fingering. To do this, 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. A new and simpler viscous finger number for water-wet systems defined as: [mathematical expression] was proposed and a better correlation of experimental results was obtained compared with previously proposed viscous finger number. In strongly oil-wet systems, a viscous finger number defined as [mathematical expression] is proposed and satisfactory correlation with both breakthrough and 1 PV cumulative recovery is obtained. Micromodel experiments were conducted in a novel 2.5-D micromodel setup. The absence of oil imbibition in oil-wet systems due to the high viscosity of oil film was observed. An effective-fingering model for oil-wet system was proposed. Oil-wet unstable coreflood experiments were history matched and satisfactory match to both recovery and pressure drop was obtained. Power-law correlations are developed between history matched model parameters and physical parameters such as the viscous finger number N [subscript vf]. This allowed accurate prediction of recovery and pressure drop of unstable displacements and is validated against unstable coreflood experiments. Fine-grid intermediate scale simulation was conducted with different degree of heterogeneity in the domain to systematically adjust the effective-fingering model parameter (shape factor) in coarse-grid simulation to match the result of fine-grid simulation. A power-law correlation between the shape factor and correlation length, Dykstra-Parsons coefficient and viscosity ratio was obtained. This correlation can be used to upscale the interaction between viscous fingering and channeling