Rock Mechanics and Rock Engineering | 2021
Failure Analysis of High-Temperature Granite Under the Joint Action of Cutting and Liquid Nitrogen Jet Impingement
Abstract
Hot dry rock (HDR) is an important renewable energy resource within the overall energy mix. However, its popularization is restricted by drilling costs due to the high hardness and abrasiveness of the rock. As a type of cryogenic fluid, liquid nitrogen (LN2) could aggravate the damage of high-temperature rocks and provide a novel insight in improving the drilling efficiency of HDR. To investigate the rock failure process under the joint action of cutting and jet impingement, a series of experiments were conducted in the present paper. The cutting force was monitored in real-time to evaluate the characteristics of rock-breaking. Then, the 3D topography of cutting grooves was analyzed to determine the rock failure modes (brittle failure or ductile failure). Finally, the rock failure mechanism was investigated by observing micro-structures of cutting grooves including cracks on surfaces and inside the rock. Results indicate that rock strength degrades obviously during the heating process, resulting in a 31.13% decrease in normalized cutting force (F). Meanwhile, the brittle failure of rocks is enhanced at elevated temperatures. This phenomenon can be validated by the fluctuation of cutting force and roughness of cutting grooves. The jet impact can induce thermal stress, thereby, further intensifying the volumetric breakage of rocks. Compared with the water jet, the LN2 shows better performance in improving the rock-breaking efficiency due to its cryogenic feature and lower viscosity. The F reduces by 43.96 and 52.53% with the assistance of water jet and LN2 jet, respectively. Besides, micro-cracks generate with the increasing temperature. The fluid penetrates the rock along these cracks, which rises the width of subsurface cracks and facilitates the subsequent rock cutting.