The Effects of High Temperature on Crack Propagation and Failure Characteristics of Sandstone

Mechanical properties of reservoirs and overlying strata in the condition of underground coal gasification (UCG) are vital to the stability of surrounding rock. In this work, sandstone samples of seam roof were collected and a series of experiments including X-ray computed tomography (CT) scanning, X-ray diffraction (XRD) and uniaxial compression tests with synchronous acoustic emission (AE) monitor were conducted to study crack distribution, propagation, minerals composition and failure modes of sandstone samples after different temperature treatments. As the treatment temperature increases, the threshold ratios of crack initiation stress and rupture stress gradually decline. Tensile failure mode of the biplane symmetric is dominant for sandstone samples treated with 25 °C and 200 °C. Single plane shear failure mode is dominant for 400 °C and 600 °C. Nevertheless, for 800 °C and 1000 °C, comprehensive features of the above temperatures are exhibited, resulting in a complex failure mode. 800 °C is the threshold temperature for failure behavior and uniaxial compressive strength (UCS). The primary fracture morphology plays a critical role in crack propagation and failure mode of sandstone samples. When axial stress reaches 96% of peak stress, peak frequency abruptly declines. It discloses the precursory features of the critical failure of sandstones after different temperature treatments. Meanwhile, various forms of water evaporation, the muscovite and kaolinite decomposition as well as phase transformation of quartz due to high temperature affect the mechanical properties and failure modes of sandstone. The results can guide the safety evaluation and roof disaster prevention of UCG.