On instability during immiscible displacement in fractures and porous media

Existing linear theories for the onset of instability during immiscible fluid displacement in fractures and porous media are generalized to account for arbitrary fracture, mcan flow, and fluid interface orientations. The generalized analysis shows that contrary to common belief, the interface is not perpendicular (and hence incipient fingers do not extend in parallel) to the mean direction of displacement except in special cases.