  We study two-phase displacement via the steady compression of an air
reservoir connected to an oil-filled capillary tube. Our experiments and
modelling reveal complex displacement dynamics depending on compression rate
and reservoir volume that, for large reservoirs, depend on a single
dimensionless compressibility number. We identify two distinct displacement
regimes, separated by a critical value of the compressibility number. While the
subcritical regime exhibits quasi-steady displacement after an initial
transient, the supercritical regime exhibits burst-like expulsion.
