  Two-phase flow systems in porous media have complex dynamics. It is well
established that a wide range of system parameters like viscosities and
porosity as well as flow parameters such as pressure gradient and fluid
saturation have strong impact on the dynamics. The transition from single-phase
flow to two-phase flow is a dynamical phase transition. We discuss the order of
the transition and investigate the phase diagram in parameter space, using a
network simulator for two-phase flow. A semi-empirical theory for the location
of the phase boundaries is provided.
