Permeability and Effective Pore Pressure of Shales

Summary Laboratory-derived permeability and pore-pressure data obtained for Wellington and Pierre shales are used to describe swelling pressure, and spalling types of wellbore instability. Tests showed that increased pore pressures can lead to wellbore failure. The laboratory pore-pressure information developed displays a time-dependent swelling process followed by a Darcy type of flow. A "total aqueous chemical potential" concept is presented that describes the driving potentials that operate during both phases of flow. Experimental methods are presented to determine the "storage" of water shale during the swelling phase and the permeabilities with steady-state-flow and transient-flow techniques. Permeability values measured under effective stresses up to 8, 000 psi show the Wellington shale to have values as low as 0.30 × 10−6 md.