Method for testing nanomechanical properties of natural gas hydrate samples

A method for testing nanomechanical properties of natural gas hydrate samples, wherein a natural gas hydrate sample in an ultralow-temperature environment is measured by means of a nanoindentation system, wherein the nanoindentation system comprises:

• a glovebox and a sample stand,
• a microscope system,
• a nano positioning stage,
• a micro-nano test module and a control cabinet, wherein the micro-nano test module and the control cabinet are mounted in the glovebox,
• a milling tool, wherein the milling tool is in close contact with the sample stand and used for cutting a surface of the sample being arranged on a side of the sample stand;
• and wherein
• the nano positioning stage drives the micro-nano test module to perform indentation on the hydrate sample on a side of the sample stand;
• the micro-nano test module records a displacement and force when a nano probe is indented into the hydrate sample and transmits the displacement and force to the control cabinet in real time;
• wherein, the method for testing nanomechanical properties of natural gas hydrate samples comprises:
• step (A), constructing a dry pure nitrogen environment and a partial ultralow-temperature liquid nitrogen test condition by placing a liquid nitrogen tank containing the natural gas hydrate sample in the glovebox in advance, constructing a pure nitrogen test environment, for transferring the hydrate sample, in the glovebox, and maintaining a micro-positive pressure state in the glovebox;
• then, injecting nitrogen into the sample stand to cool the sample stand and the milling tool, and when temperatures of the sample stand and the milling tool reach a set temperature, controlling a heating system at a bottom of the sample stand to maintain the temperatures of the sample stand and the milling tool at the set temperature;
• step (B), in the dry nitrogen environment, transferring and installing the hydrate sample on the sample stand, and driving the milling tool to cut the surface of the natural gas hydrate sample to form a flat surface on an upper surface of the natural gas hydrate sample;
• step (C), performing an indentation test by:
• step (C1), testing indentation parameter reliability by selecting on the surface of the hydrate sample an initial indentation point for indenting the nano probe, indenting the nano probe into the hydrate sample at a fixed set load, recording in real time an indentation amount and the load during the indentation process of the nano probe, and plotting a displacement-load curve;
• withdrawing the nano probe, and recording a displacement-load curve during the withdrawing process;
• sequentially testing all initial indentation points in this way, and plotting all displacement-load curves in a same coordinate system;
• if the displacement-load curves of all the initial indentation points have a high degree of overlap, indicating that homogeneity of the hydrate sample and stability of the nano probe are optimal and an indentation test result is reliable;
• step (C2), testing indentation by changing the indentation load and the indentation point to obtain a variation pattern of a nanoindentation curve of the same hydrate sample with the indentation load under a same temperature condition to analyze residual strength and other characteristic parameters of the natural gas hydrate sample under different loading conditions;
• and
• step (C3), testing temperature sensitivity by changing the hydrate sample according to Step B, changing the temperature of the sample stand, and repeating step (C1) and step (C2) to obtain nanoindentation response characteristics of the hydrate sample under different temperature conditions;
• and
• step (D), after the indentation test is completed, slowly releasing the positive pressure in the glovebox, operating the micro-nano test module to return and clean the nano probe, and after the entire nanoindentation system is dried, taking out the liquid nitrogen tank containing the hydrate sample, and maintaining the low-temperature glovebox in a dry nitrogen-rich environment.