Water adsorption and transport in shale nanopore: The impact of three-dimensional surface roughness

Investigating the effect of surface roughness on water adsorption and transport is important for hydraulic fracturing technology application in shale reservoir. In this study, based on atomic force microscope test, the roughness data of shale nanopore surface was obtained, and the shale nanopore models with different three-dimensional surface roughness were constructed. The adsorption and diffusion mechanisms of water under varying roughness and pressure conditions at 318 K were explored. The results show that with the increase in surface roughness of shale nanopore, the adsorption space for water is significantly expanded, and the peak value of adsorption density increases. Water adsorption in shale nanopore belongs to semi-confined adsorption. Water molecules adsorbed on the surface are basically only affected by one side of the matrix. The enlargement of pore size also obviously increases water adsorption density. The increase in surface roughness and pore size can improve the adsorption strength of shale pore wall through increasing the adsorption sites, broadening the free space volume, shortening the length of hydrogen bonds and enhancing the number of long hydrogen bonds. The interaction energy and isosteric heat of adsorption analysis indicate that the rough surface is more conducive to water adsorption. The horizontal diffusion coefficient of water molecule in the pore is positively correlated with the pore size and negatively correlated with the surface roughness. The adsorption potential energy of water in the pores displays a multi-peak distribution. When the pore size and roughness are small, secondary adsorption sites will be formed.