Interplay of viscosity and wettability controls fluid displacement in porous media

Direct numerical simulations are used to elucidate the interplay of wettability and fluid viscosities on immiscible fluid displacements in a heterogeneous porous medium. We classify the flow regimes using qualitative and quantitative analysis into viscous fingering (low M, M being the viscosity ratio-described as ratio of fluid viscosity of injected by defending phases), compact displacement (high M), and an intermediate transition regime (M approximate to 1). We use stability analysis to obtain theoretical phase boundaries between these regimes, which agree well with our analyses. At the macroscopic (sample) scale, we find that wettability strongly controls the threshold M (at which the regimes change). At the pore scale, wettability alters the dominant pore-filling mechanism. At very small M (viscous fingering regime), smaller pore spaces are preferentially invaded during imbibition, with flow of films of invading fluid along the pore walls. In contrast, during drainage, bursts result in filling of pores irrespective of their size. As M increases, the effect of wettability decreases as cooperative filling becomes the dominant mechanism regardless of wettability. This suggest that for imbibition at a given contact angle, decreasing M is associated with change in effective wetting from neutral-wet (cooperative filling) to strong-wet (film flow).