Permeability of shaly sands

The permeability of a sand shale mixture is analyzed as a function of shale fraction and the permeability of the two end-members, i.e., the permeabili ty of a clay-free sand and the permeability of a pure shale. First, we deve lop a model for the permeability of a clay-free sand as a function of the g rain diameter, the porosity, and the electrical cementation exponent. We sh ow that the Kozeny-Carman-type relation can be improved by using electrical parameters which separate pore throat from total porosity and effective fr om total hydraulic radius. The permeability of a pure shale is derived in a similar way but is strongly dependent on clay mineralogy. For the same por osity, there are 5 orders of magnitude of difference between the permeabili ty of pure kaolinite and the permeability of pure smectite. The separate en d-members' permeability models are combined by filling the sand pores progr essively with shale and then dispersing the sand grains in shale. The perme ability of sand shale mixtures is shown to have a minimum at the critical s hale content at which shale just fills the sand pores. Pure shale has a sli ghtly higher permeability. Permeability decreases sharply with shale conten t as the pores of a sand are filled. The permeability of sand shale mixture s thus has a very strong dependence on shale fraction, and available data c onfirm this distinctive shale-fraction dependence. In addition, there is ag reement (within 1 order of magnitude) between the permeabilities predicted from our model and those measured over 11 orders of magnitude from literatu re sources. Finally, we apply our model to predict the permeabilities of sh aly sand formations in the Gulf Coast. The predictions are compared to a da ta set of permeability determination made on side-wall cores. The agreement between the theoretical predictions and the experimental data is very good .