  The nonlinear Forchheimer equations are used to describe the dynamics of
fluid flows in porous media when Darcy's law is not applicable. In this
article, we consider the generalized Forchheimer flows for slightly
compressible fluids, and then study the expanded mixed finite element method
applied to the initial boundary value problem for the resulting degenerate
parabolic equation for pressure. The bounds for the solutions, time derivative
and gradient of solutions are established. Utilizing the monotonicity
properties of Forchheimer equation and boundedness of solutions, a {\it priori
} error estimates for solution are obtained in $L^2$-norm, $L^\infty$-norm as
well as for its gradient in $L^{2-a}$-norm for all $a\in (0,1)$. Optimal
$L^2$-error estimates are shown for solutions under some additional regularity
assumptions. Numerical results using the lowest order Raviart-Thomas mixed
element confirm the theoretical analysis regarding convergence rates.
