Understanding the mechanical behaviors of granite after high temperature exposure and under confining stress conditions is an important issue in deep rock engineering projects such as the mining of deep underground solid mineral resources, deep geothermal energy exploitation and deep nuclear waste repositories. In this research, conventional triaxial compression experiments were conducted on Nanan granite after thermal treatment from 200 °C to 600 °C. Based on the experimental results, the influences of pressure and temperature on the deformation and strength characteristics were analysed. The physico-mechanical change mechanisms of the heat-treated granite were revealed by optical microscopy. The test results show that under 600 °C, granite volume increases by 4.11%, whereas the mass and density decrease by 0.28% and 4.21%, respectively. Average values of triaxial compressive strength and elastic modulus, cohesion and internal friction angle all reduce with temperature, decreasing rapidly by 54.99%, 39.81%, 49.39% and 27.51% from 500 to 600 °C, respectively. Granite specimens are less brittle and have higher ductility and plasticity as the temperature increases. However, confining pressure improves the mechanical properties of granite. Optical microscope images show that microcracks in granite specimens are generated and extend gradually with temperature, causing the deterioration of the physico-mechanical behaviors of heat-treated granite.

Experimental investigation on mechanical behaviors of Nanan granite after thermal treatment under conventional triaxial compression

Coupled thermal–hydraulic–mechanical model for an enhanced geothermal system and numerical analysis of its heat mining performance

The present work focuses on the dynamic characterization of oil-free wire mesh dampers. The research was focused on determining nonlinear stiffness and damping coefficients while varying the excitation amplitude, excitation frequency, and static eccentricity. Force coefficients were extracted using a forced response method (FRM) and also a transient vibration method. Due to the nonlinearity of the dampers, controlled amplitude tests were required for the forced excitation method, whereas the transient response was analyzed using a Hilbert Transform procedure. The testing using both methods showed that eccentricity has minimal influence on force coefficients, whereas increasing excitation amplitude and frequency yield decreasing stiffness and damping trends. In addition to the parameter identification tests, a rotating test was performed demonstrating high-speed damping capability of the oil-free wire mesh dampers to 40,000 rpm, which was also simulated using an nonlinear rotordynamic response to imbalance analysis.Copyright © 2007 by ASME

Nonlinear Dynamic Characterization of Oil-Free Wire Mesh Dampers

Laboratory experiments are performed on Laizhou granite samples after heating and rapid water-cooling treatment to investigate the thermal effects on the physical and mechanical properties and fracture initiation of granite during the utilization of hot dry rock. Experimental results show that P-wave velocity and tensile strength monotonously decrease because of the generation of thermally induced microcracks when the thermal treatment level increases from 25 to 500 °C. Scanning electron microscopy analysis indicates that few and isolated microcracks are observed before 300 °C, whereas microcrack networks are generated at 400 °C and 500 °C. Porosity and gas permeability sharply increase with the enhancement in the density and connectivity of microcracks. The variation trend of compressive strength tends to show a transition from increasing to decreasing at the threshold temperature. Uniaxial compressive strength and cohesion increase and the Young’s modulus slightly changes before 300 °C. Afterward, the three parameters decrease, indicating that the dominant mechanism transforms from thermal hardening to thermal cracking. Failure process of uniaxial compression shows that the proportion of crack closure region in the pre-peak stage gradually increases, whereas those of the elastic and stable crack growth regions decrease after 300 °C with the increase of the thermal treatment level. Experimental results of laboratory fracturing tests show that the decreasing tendency of breakdown pressure is similar to that of tensile strength. The pressure build-up rate substantially decreases because of the enhancement in fluid leak-off, and hydraulic fracture initiation occurs before the breakdown of granite samples when the thermal treatment levels are higher than 300 °C. Shear failure becomes dominant based on the hypocenter mechanism analysis of acoustic emission events.

Thermal Effects on the Physical and Mechanical Properties and Fracture Initiation of Laizhou Granite During Hydraulic Fracturing

A comprehensive review on mechanical responses of granite in enhanced geothermal systems (EGSs)

When exploiting the deep resources, the surrounding rock readily undergoes the hole shrinkage, borehole collapse, and loss of circulation under high temperature and high pressure. A series of experiments were conducted to discuss the compressional wave velocity, triaxial strength, and permeability of granite cored from 3500 meters borehole under high temperature and three-dimensional stress. In light of the coupling of temperature, fluid, and stress, we get the thermo-fluid-solid model and governing equation. ANSYS-APDL was also used to stimulate the temperature influence on elastic modulus, Poisson ratio, uniaxial compressive strength, and permeability. In light of the results, we establish a temperature-fluid-stress model to illustrate the granite’s stability. The compressional wave velocity and elastic modulus, decrease as the temperature rises, while poisson ratio and permeability of granite increase. The threshold pressure and temperature are 15 MPa and 200°C, respectively. The temperature affects the fracture pressure more than the collapse pressure, but both parameters rise with the increase of temperature. The coupling of thermo-fluid-solid, greatly impacting the borehole stability, proves to be a good method to analyze similar problems of other formations.

/pdf/thermophysical-and-mechanical-properties-of-granite-and-its-2u7hic3zlv.pdf

Thermophysical and Mechanical Properties of Granite and Its Effects on Borehole Stability in High Temperature and Three-Dimensional Stress

Stiffness of rubber and metal rubber (MR) changes nonlinearly. Based on hysteretic damping theory and energy conservation equations, a unified stiffness model is developed. The ring type of isolators made from rubber and metal rubber are studied. The isolator samples are tested on the electro-hydraulic loading system, which is fixed by a clamping device. We aim to study the deformations under different loading rates, loading forces and other quasi-static loading states. The dynamic stiffness affected by the preload and vibration frequency is studied on the sonic drilling processing. It can be concluded that the metal rubber isolators deform larger plastic than the rubber. The damping, plastic deformation and elastic deformation of the metal rubber and rubber material are inversely proportional to the loading rate. The total deformation of the rubber is larger than the metal rubber, no matter when the load increases or decreases. The dynamic stiffness of the metal rubber isolator is proportional to the vibration frequency. However, the dynamic stiffness of the rubber remains the same under different preloads, while decreases when the preload increases.

https://www.jvejournals.com/article/14743/pdf

Stiffness characteristic comparison between metal-rubber and rubber isolator under sonic vibration

Turbodrill is a type of hydraulic axial turbomachinery which has a multistage blade consisting of stators and rotors. In this paper, a turbodrill blade that can be applied in crystallized section under high temperature and pressure conditions is developed. On the basis of Euler equations, the law of energy transfer is analyzed and the output characteristics of turbodrill blade are proposed. Moreover, considering the properties of the layer and the bole-hole conditions, the radical size, the geometrical dimension, and the blade profile are optimized. A computational model of a single-stage blade is built on the ANSYS CFD into which the three-dimensional model of turbodrill is input. In light of the distribution law of the pressure and flow field, the functions of the turbodrill blade are improved and optimized. The turbodrill blade optimization model was verified based on laboratory experiments. The results show that the design meets the deep hard rock mineral exploration application and provides good references for further study.

/pdf/design-and-development-of-turbodrill-blade-used-in-4rpp1sbhii.pdf

Design and development of turbodrill blade used in crystallized section.

The supporting plate is one of the key parts of the sonic vibration head, and the regular fatigue and fracture make the support plate be one of the most venerably damaged units, resulting in huge loss in economy and security. After analyzing the loading states of the supporting plate on sonic vibration head, the mechanical stresses of three kinds of supporting plate under different operation load cases are simulated by the method of finite element method (FEM). The effects of stress of supporting plate are studied by means of fatigue testing on actual drilling processing. The results show that the steel structures fatigue failure is caused by stress concentration and unbalance. The optimal supporting plate structure is selected by the simulation and test.

Study on Fatigue Crack of Supporting Plate on Sonic Vibration Head

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Papers

Thermophysical and Mechanical Properties of Granite and Its Effects on Borehole Stability in High Temperature and Three-Dimensional Stress

Stiffness characteristic comparison between metal-rubber and rubber isolator under sonic vibration

Design and development of turbodrill blade used in crystallized section.

Study on Fatigue Crack of Supporting Plate on Sonic Vibration Head