Effect of hydration on fractures and permeabilities in Mancos, Eagleford, Barnette and Marcellus shale cores under compressive stress conditions

Hydration swelling was observed to generate microfractures at ambient conditions in some of earlier studies, thus the core permeability is increased. In this paper, to investigate if hydration swelling could induce fractures in shale cores under compressive stress, four shale cores (Mancos, Barnett, Marcellus and Eagleford) with various clay mineral contents and swelling properties were used. CT scan testing was used to observe fractures, and swelling strain testing was conducted to evaluate the hydration swelling properties. The permeabilities of shale cores before and after hydration were measured. In all of the four shale cores under 3000 psi isotropic confining pressure, hydration caused fractures to close rather than propagate. As a result, the permeabilities decreased significantly, up to two orders of magnitude, compared to intrinsic permeabilities in Mancos, Barnett, and Marcellus shales, and a large damage occurred in the Eagleford shale core. Clay mineral content was the main factor influencing the shale permeability damage due to hydration. When clay contents are higher than a certain percent (e.g., 15% in this paper), significant permeability damage was observed. 8% KCl solution could help recover more permeability damage than 4% KCl solution. As a result, during hydraulic fracturing, the salinity should be increased to mitigate the permeability damage caused by hydration.