Permeability values consistently demonstrate that measurements at field scale (hundreds of meters to kilometers) show an increase of several orders of magnitude compared to values obtained at laboratory scales. This behavior suggests that in reservoirs fluids flow primarily in fractures rather than in pores. Fluid-filled fractures are capable to carry two types of dispersive slow waves: Biot wave and Stoneley guided wave, which is not described by Biot's theory. Results of numerical modeling suggest that Stoneley guided wave has overwhelmingly large amplitude compared to all other waves excited by a source in a fracture. It is likely that Stoneley guided wave is a key phenomenon which might explain observed frequency-dependence and nonlinear behavior of fluid reservoirs.