Particle size effect on water vapor sorption measurement of organic shale: One example from Dongyuemiao Member of Lower Jurassic Ziliujing Formation in Jiannan Area of China

Shale formations generally contain a certain amount of water, and the occurrence of water can strongly affect the free gas content and gas storage capacity within shale. Although some studies have conducted water vapor adsorption tests to understand the water adsorption behavior and water-shale interactions, surprisingly the influence of grain size on water vapor sorption of shale is poorly understood. In this work, water vapor adsorption experiments on one Dongyuemiao shale from Ziliujing Formation in Jiannan Area, with different particle sizes (8-12 mesh, 20-35 mesh, 35-80 mesh, 80-200 mesh, and > 200 mesh) are conducted over a wide relative humidity (RH) range (5%-95%) using a gravimetric method. The influence of particle size on water vapor adsorption measurement is investigated and the optimal particle size is suggested for water vapor experiment. Results show that the maximum uptake of water vapor adsorption is smaller in larger particle sized sample, which is related to the variation of accessible pores. Monolayer adsorption capacity obtained from Guggenheim-Anderson-de Boer modelling tends to increase as the particle size increases, suggesting a stronger water vapor adsorption potential. Comparative studies show that 20-35 mesh is suggested to be the optimum particle size for comparative purpose. The quantity of adsorption on the primary and secondary sites is comparable or equals at a RH range of approximately 60%-80%. When RH value is smaller than 60%, the quantity of water vapor adsorption on the primary site dominates, while adsorption uptake on the secondary site plays a dominant role when RH value is greater than 80%. When particle size increases, water vapor adsorptions on the primary sites increases slightly, while a decrease trend is observed for water vapor adsorption on secondary sites.