Study on typical temperature effect mechanism of multi-component coal during low-temperature thermal expansion

Liquid Nitrogen (LN2) cold shock on coal reservoir is a promising technology, and its complex fracture networks are directly related to the temperature effect of each component in coal caused by temperature change. Among them, the typical temperature effects include the expansion difference of different mineral particles, the evaporation of pore water and the volume expansion of gas heated, and the phase transition of water into ice. The above-mentioned effects will all occur in the process of cold shock and temperature returning of coal samples, and further influence the generation of fractures. In this paper, the relationship between minerals, moisture and porosity of six kinds of coals in the range of -30 degrees C-40 degrees C and the overall thermal expansion coefficient of coal samples is explored by using low-temperature thermal expansion coefficient tester, low-field nuclear magnetic resonance tester, XRD diffraction analyzer, CT scanner and muffle furnace industrial analyzer. It is found that the main minerals in coal are generally lower than the thermal expansion coefficient of the whole coal. The moisture and porosity of coals are proportional to the overall thermal expansion coefficient of coal samples. Among them, the porosity of small holes accounts for the highest proportion, and its internal moisture and gas have the greatest influence on the overall thermal expansion of coal, which indicates that the temperature effect of pore water evaporation and gas volume expansion in pores plays an important role in the process of coal temperature change.