Abstract In order to study the evolution law of surrounding rock microstructure of underground coal gasification, and to reveal the principle from the change of mineral composition and organics in rock, the sandstone nearby the gas transmission channel was used as the tested rock. In this paper, the rock was heated at 300, 400, and 500 °C, simulating the heat convective environment of the gas transmission channel. The mineral, organic functional groups, and microstructure of sandstone transformation products were detected by XRD, FTIR, and SEM. It is concluded that the kaolin in the sandstone gradually transforms into metakaolin with the increase of temperature, meanwhile the carboxyl group and the oxygenic functional group decrease. The generation of metakaolin and the precipitation of organics increase the number of fractures in samples, which is the main reason for the compressive strength and elastic modulus for the sandstone decreases with increasing temperature for temperatures greater than 500 °C. The microscopic manifestation is the appearance of a large number of flocs on the surface. Macroscopically, the exterior appears as a grayish shell. The flow resistance and the adsorption property are weakened, and the permeability is enhanced.

中文翻译：

---

UCG河道附近砂岩微观结构变化及矿物转化

摘要 为研究煤炭地下气化围岩微观结构演化规律，从岩石中矿物成分和有机物的变化规律揭示其变化规律，以输气通道附近的砂岩为试验岩石。在本文中，岩石在 300、400 和 500 °C 下加热，模拟气体传输通道的热对流环境。通过XRD、FTIR、SEM等手段对砂岩转化产物的矿物、有机官能团和微观结构进行了检测。结论是砂岩中的高岭土随着温度的升高逐渐转变为偏高岭土，同时羧基和含氧官能团减少。偏高岭土的生成和有机物的沉淀增加了样品中的裂缝数量，这是砂岩的抗压强度和弹性模量随温度升高而降低的主要原因，温度高于 500 °C。微观表现是表面出现大量絮状物。从宏观上看，外观呈灰色外壳。流动阻力和吸附性减弱，渗透性增强。