A comparative study of the permeability enhancement in coal and clay-rich shale by hydraulic fracturing using nano-CT and SEM image analysis

The low permeability of unconventional reservoirs is a key petrophysical parameter that restricts the seepage and migration of methane in micro-channels. Coal and shale have a high content of clay minerals, and swelling clay minerals lead to permeability reduction due to their unique water sensitivity and swelling mechanism, which is an important factor affecting the production efficiency of methane in reservoirs. This work begins with visual two-dimensional (2D) and three-dimensional (3D) characterization of the pore-fracture network in coal and clayrich shale and emphasizes the root cause of the different changes in coal and clay-rich shale permeability by hydraulic fracturing. The results showed the increase of pore and throat radius of coal by hydraulic fracturing is significantly higher than that of clay-rich shale, indicating that high vitrinite and low clay content contribute to brittle deformation, while high clay and low vitrinite content account for ductile deformation. Hydraulic fracturing can form more effective micro-channels and increase shale permeability by nearly two orders of magnitude. The permeability enhancement of clay-rich shale is apparently better than that of coal by hydraulic fracturing. After fracturing, the average loss ratio of permeability of clay-rich shales is 35.2%, which is much higher than the average loss ratios measured on coal (11.1%), suggesting that the abundant water provided by hydraulic fracturing leads to the absorption and swelling of clay minerals in shale. Swelling clay minerals cause persistent adsorption and filling of H2O molecules between layers, forming colloidal suspensions in the microchannels, further blocking the migration pathway of methane molecules in clay-rich shale.