FULL-SCALE PORES AND MICRO-FRACTURES CHARACTERIZATION BY INTEGRATED FE-SEM, GAS ADSORPTION, NANO-CT AND MICRO-CT METHODS

Micro- and nano-pores and micro-fractures in shale reservoirs provide storage spaces and main permeability pathways for migration of natural gas molecules, which directly determines the accumulation and dissipation of shale gas. However, investigations about the pore-fracture structure of shales were mainly focused on nano-submicron pores, while the contribution of micrometer-scaled pores and micro-fractures to pore system of shale reservoirs lack sufficient attention. Here, using a combination of field-emission scanning electronic microscopy (FE-SEM), gas adsorption (N₂ and CO₂) and CT scanning (Nano-CT and Micro-CT), the Longmaxi shale cored from JYA well, in the Fuling area, was selected to comprehensively evaluate the characteristics of the pore-fracture structure at different scales. The results show that the Longmaxi shale samples mainly contain organic pores, inorganic pores, and micro-fractures. The forms of organic pores are diverse in shapes including slits, irregular polygons, discrete distributed sponge-shaped and ellipses. The shale pore diameters range from 0.305 nm-98.5 μm, mainly between 0.305-1.5 nm, 2-10 nm and 1-2 μm. The proportion of micro-pore (0.305-2 nm), meso-pore (2-50 nm), macro-pore (50 nm-2 μm) volume account for 33.59%, 36.28%, and 14.04% of the total pore volume, respectively, and that of micro-fracture (2-98.5 μm) is 16.09%. Moreover, shale permeability and specific surface area were calculated to be 0.005 mD and 39.44 m²/g, respectively, based on the full-scaled pore splicing. About 92% of specific surface area is contributed by pores with diameter of 0.305-5 nm. Micro-fractures connect with each other and form net-shaped structure with great connectivity, and their permeability account for more than 90% of the total permeability. Micro-pores and small meso-pores in shale reservoirs provide large adsorption space for adsorbed gas, while micro-fractures enhance the seepage capability of shale gas, which are favorable for shale gas accumulation and production. The integration of CO₂ and N₂ adsorption, Nano-CT and Micro-CT scanning studies provided an effective way to the comprehensive characterization of complex pore-fracture system in unconventional tight shale reservoirs, which might be helpful for accurate evaluation of engineering sweet spots and reduce exploration risks.