Experimental Investigation on the Movability of Water in Shale Nanopores: A Case Study of Carboniferous Shale From the Qaidam Basin, China

The movability of water in shale nanopores intensely affects the transport and storage of other fluids and plays an important role in the geological water cycle. To investigate the movability of water, three shale samples were used to conduct flow experiments using the steady state and step-by-step depressurization method. The experimental results show that the movability of water strongly depends on the pressure gradient. Two water-related mechanisms, shear fluidity and electroviscous effect, are primarily responsible for the difference in movability between free water and bound water. The flow of water in these samples deviates from Darcy's law, and the threshold pressure gradient (TPG) is observed. When the pressure gradient is higher than the TPG, water starts to flow, and the permeability increases with increasing pressure gradient. Based on the analysis of the solid-liquid interfacial forces, the water in the shale was quantitatively classified into four types (L1, L2, L3, and L4) according to its movability. The calculation results indicate that most of the water, which could be up to 74.46-89.73%, is immovable under the experimental conditions. The proportion of movable water increases with increasing pressure gradient, and the relation can be expressed by a quadratic function. The moving ratio was defined as the ratio of the volume of water involved in flow to the total pore volume. The correlation between the moving ratio and permeability is a power function with exponents ranging from 1.16 to 1.43, indicating that water permeability is sensitive to the moving ratio. Key Points The TPG was investigated and discussed, combining the step-by-step depressurization experimental method and DLVO theory The distribution and proportion of water with four different movability are analyzed based on the solid-liquid interfacial forces The increase of moving ratio of water in shale nanopores with the pressure gradient leads to an increase in water permeability