  We develop a novel `moving capacitor' dynamic network model to simulate
immiscible fluid-fluid displacement in porous media. Traditional network models
approximate the pore geometry as a network of fixed resistors, directly
analogous to an electrical circuit. Our model additionally captures the motion
of individual fluid-fluid interfaces through the pore geometry by completing
this analogy, representing interfaces as a set of moving capacitors. By
incorporating pore-scale invasion events, the model reproduces, for the first
time, both the displacement pattern and the injection pressure signal under a
wide range of capillary numbers and substrate wettabilities. We show that at
high capillary numbers the invading patterns advance symmetrically through
viscous fingers. In contrast, at low capillary numbers the flow is governed by
the wettability-dependent fluid-fluid interactions with the pore structure. The
signature of the transition between the two regimes manifests itself in the
fluctuations of the injection pressure signal.
