Permeability and Effective Pore Pressure of Shales

Abstract

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. Introduction An understanding of water movement into shales is important because water movement results in the development of swelling stresses and pore-pressure increases that can lead to rock failure. Many researchers have studied water movement into shales; however, little is known about the mechanisms involved. Difficulties arise during such studies because of the extremely low permeability of shales and the influence of the charged clay surfaces on aqueous pore pressures. Water movement into formations often is thought to occur only when the wellbore hydraulic pressure exceeds the hydraulic pressure of the formation. We suggest that a more fundamental approach is needed to describe water movement into shales, an approach that describes water movement with the total aqueous chemical potential in the wellbore and the formation. For either the wellbore or the shale water, the total aqueous chemical potential depends not only on the hydraulic pressure of the water, but also on temperature, ionic strength, and particle surface charges. For brevity, these factors are referred to here collectively as "total aqueous potential" and are measured in psi. Terms used in the chemistry literature for the total aqueous chemical potential include "total free energy of the water" and "escaping tendency of the water." Water will move from the wellbore into a shale any time the total aqueous potential of the shale is less than the total aqueous potential of the wellbore fluid. A technique used to determine the various components that contribute to the total aqueous potentials for the wellbore fluid and shale was described recently.