Water, CO2 and Argon Permeabilities of Intact and Fractured Shale Cores Under Stress

Abstract

As shale is the caprock over many reservoirs targeted for CO2 storage, shale permeability to CO2 has become an important concern. Measurements of this permeability need to be performed under in-situ conditions, with realistic temperatures, confining pressures and fluid pressures, and the effects of variables such as pressure, temperature, and shale moisture content need to be thoroughly addressed. Furthermore, the exposure of wet shale to dry CO2 can lead to, for example, dehydration and two-phase flow, that in turn affect permeability. This paper reports shale permeability measurements performed on two shale core plugs from Svalbard, under in-situ pressure and temperature conditions relevant for CO2 storage, using argon, CO2, and water as the permeate, and using both transient pulse and constant flow techniques. Permeability was found to be dependent mainly on effective confining pressure and on shale moisture content. The following was observed: (1) permeability decreased with increasing effective confining pressure; (2) permeability to water was lower than permeability to Ar or CO2; (3) shale moisture content had a strong inhibiting effect on the flow of Ar and CO2; (4) when a high CO2-flow was applied to a shale sample containing a hydrous pore fluid, a breakthrough effect occurred; and (5) in the presence of pore water, compaction creep can occur, causing a permanent decrease in permeability.