  We compute the fluid flow time-correlation functions of incompressible,
immiscible two-phase flow in porous media using a 2D network model. Given a
properly chosen representative elementary volume, the flow rate distributions
are Gaussian and the integrals of time correlation functions of the flows are
found to converge to a finite value. The integrated cross-correlations become
symmetric, obeying Onsager's reciprocal relations. These findings support the
proposal of a non-equilibrium thermodynamic description for two-phase flow in
porous media.
