Effects of Surface Mesopores in the Newly Formed Crystalline Layer on Pore Structure Characterization of Water-Bearing Shale

Nanoscale pores are widely developed in shale and often contain moisture underground. To effectively identify high-quality shale reservoirs for natural gas, it is crucial to explore the presence of moisture in shale and understand its impact on the pore structure of shale. Currently, moisture equilibration experiments and low-pressure gas adsorption experiments are generally combined to characterize the effective reservoir space inside shale under different moisture contents. However, existing studies primarily focus on experimental data analysis but lack direct microscopic observations. In this research, on the basis of moisture equilibration experiments and low-pressure gas adsorption tests, we further employed field emission scanning electron microscopy (FE-SEM) to directly visualize and understand the effects of moisture content on the pore space of shale. The research found that with increasing moisture content, the pore volume and specific surface area of shale exhibit a decreasing trend; however, there are cases showing an inverse increase in pore volume and specific surface area, which is contrary to normal physical laws. The further investigation revealed that a crystalline layer of K2SO4 formed on the shale surface after moisture equilibration experiments at high relative humidity. The K2SO4 crystallization layer exhibits two types of morphological features: network protrusions or extensive development of mesopores. The latter crystalline layer contains numerous mesopores and is responsible for the inverse increase in pore volume and specific surface area. The newly formed crystalline layer and its surface mesopores are artificially induced during the experiments, and may result in inaccuracies when assessing the storage space of water-bearing shale. These findings emphasize the importance of considering the impact of this inorganic crystalline layer when characterizing the pore structure of porous media after moisture equilibration experiments.