Difference Analysis on Sandstone Permeability After Treatment at Different Temperatures During the Failure Process: A Case Study of Sandstone in Chongqing, China

Abstract

The channels of underground coal gasification (UCG) can be divided into three regions according to different temperatures. In order to explore the permeability characteristics of rocks in different temperature regions, permeability experiments were carried out under cyclic loading of rocks after heating at different temperatures, and the permeability of rock during the cyclic loading and unloading process after high-temperature treatment was obtained. The evolution characteristics of rock permeability in each cyclic loading and unloading process were analysed, and the permeability of different samples were compared. The results show that the initial permeability of rock treated at different temperatures clearly varies. The critical temperature of sandstone is 400 °C. After exceeding this temperature, the permeability of the rock samples generally decreases with the increase in the number of cycles but increases rapidly in the failure stage. The characteristic permeability values ( $${K}_{0}$$ , $${K}_{\mathrm{m}}$$ and $${K}_{u}$$ ) and permeability index ( $$\eta $$ ) of rock treated at different temperatures are dependent on temperature. Compared with 200 °C, the characteristic permeability value of the rock treated at 800 °C tends to gradually stabilize during the whole process of cyclic loading and unloading, and the permeability recovery of the rock is low. By comparing the increment coefficient ( $$\xi $$ ) of the permeability index of each cyclic loading and unloading process, it can be seen that the permeability characteristics of rocks undergoing the same stress path show a completely different change trend under the influence of different temperatures. These results are helpful for understanding the mechanics and permeability of rock under the influence of different temperatures, and provide a reference for site selection of UCG.