We construct an ensemble distribution to describe steady immiscible two-phase
flow of two incompressible fluids in a porous medium. The system is found to be
ergodic. The distribution is used to compute macroscopic flow parameters. In
particular, we find an expression for the overall mobility of the system from
the ensemble distribution. The entropy production at the scale of the porous
medium is shown to give the expected product of the average flow and its
driving force, obtained from a black-box description. We test numerically some
of the central theoretical results.Comment: 23 pages, 9 figure