Pressure-dependent permeability of shales

The matrix permeability of shales is of great importance in determining the behaviour of shale seals and also of shale gas reservoirs. Methods of permeability determination must take into account sensitivity to variations in confining and pore pressures. We seek to establish whether common generic patterns of behaviour exist and to establish their parameters experimentally. Pressure sensitivities of two shales are compared, but the same pattern also applies to others. They follow the general law k = A exp(- g(Pc - A Pp)) (1 + D/ Pp) in which k is permeability, Pc is confining pressure, Pp is pore pressure, A, g, a and D are empirical parameters. g and a describe the sensitivity to confining pressure and pore pressure and variations of k by more than 3 orders of magnitude can occur over the whole reservoir pressure range. Slip (Klinkenberg) flow begins to be significant at gas pore pressures below about 50 bars. Partial fluid saturation leads to a reduction in permeability, and in all cases flow is highly anisotropic. If pressure sensitivity of permeability is not taken into account, reservoir evaluations from well tests will lead to substantial overestimation of original gas in place and likely yield with time.