Micromechanical tests of tetrahydrofuran hydrate using atomic force microscope

Abstract

The mechanical properties of micro- or nano-scale hydrate are of great significance to reveal the interaction between hydrate and sediment skeleton in pores under the external loads and to evaluate the macroscopic mechanical behavior of hydrate deposits in nature. In this paper, the tetrahydrofuran (THF) hydrate samples are analyzed by the atomic force microscope and the silica (SiO2) microspheres colloidal probe with a diameter of 5 μm. The relationship between the press-in depth, contact time, and contact force is obtained at the temperature in the range from -30℃ to -10℃ and at driving speed of 0.5-20 μm/s. The adhesion force between SiO2 microsphere and THF hydrate is also measured. The result shows that the plastic deformation of the THF hydrate occurs during the indentation, and the phase transformation induced by the indentation can further enhance this plastic behavior. At the same temperature and contact force, a greater press-in depth was observed at a lower indentation rate or higher temperature. The THF hydrate exhibited greater plasticity and lower strength. The yield stress of the THF hydrate has a threshold, which can be one of the important reasons for strain hardening and strain softening of macroscopic hydrate sediments. According to the modified power-rate rheological (PLR) viscoelastic model, the viscoelasticity of a THF hydrate is significant at a low driving rate and relatively high temperature. The adhesion force between the microsphere and THF hydrate was caused by the liquid-like layer and decomposed liquid on its surface, and it was 1.1-2.5 μN, which was closely related to the contact area before they separated.