Change of sandstone microstructure and mineral transformation nearby UCG channel

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.