Investigation of the Mechanical and Permeability Evolution Effects of High-Temperature Granite Exposed to a Rapid Cooling Shock with Liquid Nitrogen

Abstract

Liquid nitrogen (LN2), which can greatly improve the efficiency of hot dry rock (HDR) mining, is commonly used as a cooling material in the enhanced geothermal system (EGS). Physical property, triaxial compression, and permeability tests were undertaken on treated granite samples, for a better scientific understanding of the effect of the LN2 cooling method on the mechanical and permeability properties of the rocks after heat treatment. The experimental results indicated that the physical properties of the treated granite change significantly, such as the density and wave velocity are substantially reduced. Meanwhile, with the increase of treatment temperature, the macroscopic cracks on its surface are gradually generated and the volume is expanded clearly. In addition, the surface wettability of granite gradually increases with increasing temperature. Compared with the air/water cooling methods, under LN2 cooling condition, the mechanical properties decrease markedly. When the temperature exceeds 600°C, the granite strength decreases significantly to only 56.16% of the reference value. The deformation properties also change significantly, with a final strain of about 3% at failure for a sample at 800°C, showing an obvious ductile deformation characteristic. Further, an appreciable correlation also exists between the initial permeability of granite and temperature. Once the temperature exceeds 200°C, the increase in temperature contributes to the increase in initial permeability. In addition to the effect of temperature, the increase in load also leads to a change in the permeability coefficient. When the temperature reaches 600°C, the permeability of granite first decreases and then increases with the increases in axial stress. The results of this paper are valuable in understanding the effect of thermal shock by LN2 on the fracturing efficiency and permeability characteristics of dry hot rocks.

Effect of Liquid Nitrogen Cooling and Heating on Mechanical Properties and Acoustic Emission Characteristics of Coal

“…In addition, no matter the stress-strain graph or the load-displacement graph, the phenomenon and law of the mechanical effect of liquid nitrogen cycle freeze-thaw on coal samples under the condition of Brazil splitting can be obtained. Generally speaking, the load-displacement curve of coal will go through four different stages [30,31].…”

Heat Transfer Analysis of Enhanced Geothermal System Based on Heat-Fluid-Structure Coupling Model

“…(3) The rock matrix is treated as a homogeneous porous medium with significantly lower permeability compared to fractures. The primary flow pathways for fluids are considered to be the surfaces of fractures (4) The flow of water follows Darcy's law (5) Heat transfer between water and the surrounding rock is achieved through convection and conduction (6) Chemical reactions that may occur between fluid and hot dry rock are ignored…”

“…Geothermal energy, with its advantages of cleanliness, low carbon emissions, abundant resources, stable and reliable supply, direct utilization without conversion, and high efficiency, has gained broad recognition from countries around the world [4]. Geothermal energy is regarded as a highly promising renewable energy source for the future, given its vast development potential and widespread resource distribution [5][6][7].…”

Investigation of Fracture Evolution and Failure Characteristics of Rocks under High-Temperature Liquid Nitrogen Interaction

“…This suggests that the cryogenic shock caused by liquid nitrogen weakens the mechanical properties of the rock, making it more susceptible to fracture failure. Additionally, the thermal stress induced by low-temperature liquid nitrogen causes microfractures to form within the rock, facilitating the propagation of rupture along these microfractures and resulting in shear failure [39,40].…”