Number Of Heating-cooling Cycles Research Articles

Mesoscopic precursor anomaly characteristics of red sandstone under uniaxial compression after cyclic high-temperatures

The deformation and failure characteristics of rock caused by periodic changes of temperature are not only significantly different from that under normal temperature, but also significantly different from that produced by a single high-temperature event. Here, using a three-dimensional digital image correlation (3D-DIC) integrated test system, mechanical and micro-damage tests of red sandstone are performed under cycles of high-temperature and natural-cooling with a temperature range of 25°C to 300°C and the number of cycles varying between 0 and 25. Macroscopic, mesoscopic and microscopic characteristics such as rock strength, deformation, strain field and fracture morphology are examined and their evolution with the number of heating-cooling cycles is discussed. Using the time-history evolution of the differentiation rate of the precursor anomaly (En), the characteristic coefficient (K) and the damage variable (DF), three kinds of characteristic time points are put forward to quantify the incubation and development time of precursor abnormal characteristics and are used to divide the whole process of deformation and failure. These time points are the precursor abnormal differentiation time (PADT) tE, the precursor abnormal startup time (PAST) tK, and the precursor abnormal failure time (PAFT) tD. Test results show that: (1) The action of cyclic high-temperature natural-cooling not only shows the strengthening effect of temperature, but also shows the strengthening effect of a certain number of cycles. (2) The evolution sequence of the three precursor abnormal characteristic moments is PADT, PAST and PAFT in turn, and this sequence can intuitively describe the whole complex process of rock deformation and failure, and there are wave peaks in the evolution curve with the number of high-temperature cycle. (3) The effect of high temperature (i.e., 300°C) cyclic heating on rock properties is similar to the threshold effect of a single high-temperature heating event.

Experimental investigation on the effects of heating-cooling cycles on the physical and mechanical properties of shale

The deep shale gas reservoirs are characterized by great burial depth, high formation pressure, high temperature. During the hydraulic fracturing process, the fracturing fluids will inevitably exchange heat with the surrounding rock, causing the shale in the near-well zone to undergo temperature alternation, the physical and mechanical properties of shale reservoirs are significantly affected, which brings technical difficulties to fracturing optimization design. In this paper, the Longmaxi Formation shale is used to pay attention to the rock physical and mechanical characteristics of the shale undergo temperature alternation, the microstructure and damage and fracture characteristics of the shale undergo temperature alternation are analyzed. The results show that: (1)As the heat treatment temperature increases and the number of heating-cooling cycles increases, the permeability of the shale increases, the P-wave velocity decreases, the tensile strength (TS) decreases, and the brittleness index rises. (2) Heat treatment temperature and the number of heating-cooling cycles have little effect on the triaxial compressive strength (TCS) of shale. However, it has a significant effect on the failure mode of shale. The increase of temperature and the number of heating-cooling cycles will enhance the degree of fracture of the shale failure surface. (3) Based on industrial CT scanning and scanning electron microscopy (SEM) observations, the thermal cracks caused by heating-cooling cycles are mostly inter-granular cracks and inter-layer cracks, which unfold on the surface of the sample and extend to the interior. Heating-cooling cycles also lead to an increase in the diameter of organic pores. The study results can provide a certain theoretical basis for the optimization design of staged fracturing parameters for horizontal wells in the process of deep shale gas development.

The Effect of Temperature and Heating–Cooling Cycles on Mode I, Mode II and Mixed-Mode I-II Fracture Toughness of Sandstone

In the case of explosions and fires, the rocks undergo a cycle of heating and cooling. For that, first, they are exposed to considerable heat and then cooled after extinguishing the fire. Temperature variations and subsequent contraction and expansion affect the physical and mechanical properties of rocks. Through two series of tests, the effects of temperature and the number of heating-cooling cycles on the mode I, mode II and the effective mixed-mode I-II fracture toughness of Lushan sandstone were investigated. In the first series, the effect of temperature was studied in a heating–cooling cycle at ambient temperature (25°C) and 60, 150, 200, 300, 500, and 700°C. The highest and lowest mode I, mode II and the effective mixed-mode I-II fracture toughness were, observed at 150 and 700°C, respectively. In the second series of tests, the effect of the number of heating–cooling cycles was investigated on the mode I, mode II and the effective mixed-mode I-II fracture toughness of sandstone specimens at 150°C (for hydraulic fracturing modeling) and a crack inclination angle of 45°. According to the results, the mode I, mode II and the effective mixed-mode I-II fracture toughness increased in the first cycle and decreased with increasing the number of heating–cooling cycles. As the crack inclination increased, the effective mixed-mode I-II fracture toughness of the sandstone specimens increased. The mode II fracture toughness increased up to a crack inclination angle of 45° and then decreased. Moreover, the mode of fracture changes from opening mode (mode I) at the crack inclination angle of zero degree to mixed mode (tension-shear) at the crack inclination angle of less than 28.8°. The mode of fracture changes from tensile-shear to compression-shear at the crack inclination angle of greater than 28.8°.

Effects of Different Conditions of Water Cooling at High Temperature on the Tensile Strength and Split Surface Roughness Characteristics of Hot Dry Rock

To investigate the effects of the different conditions of water cooling at high temperature on the tensile strength and split surface roughness characteristics of hot dry rock in the Songliao Basin, the physical characteristics, tensile strength, and split surface roughness of granite under different conditions of water cooling at high temperature were studied. In addition, the relationship between tensile strength and split surface roughness under different conditions of water cooling at high temperature was established. The results showed the following: (1) as the rock temperature increased, the number of water injection cycles increased or the water injection temperature decreased, the mechanical properties of the specimen weakened, and the roughness of the split surface increased. The threshold for the effect of the rock temperature on the split surface roughness of granite was 300°C. At 400°C, the tensile strength greatly decreased. At 600°C, the tensile strength, height mean square error (MSE), fluctuation difference, roughness coefficient, and roughness profile index of the specimen were 0.21, 2.51, 2.57, 8.92, and 1.06 times those at 100°C, respectively. After five heating‐cooling cycles, the tensile strength, height MSE, fluctuation difference, roughness coefficient, and roughness profile index of the specimen were 0.57, 1.33, 1.49, 1.29, and 1.01 times those after one cycle, respectively. (2) The roughness angle calculated using the root mean square of the first derivative of the profile was always greater than that derived using the roughness profile index. In addition, the higher the temperature, the lower the water temperature, the more high‐temperature‐water cooling cycles, the greater the difference between the above two calculations. (3) When the tensile strength varies, the factors affecting the variation in the height MSE and surface roughness were in the following descending order: rock temperature, number of heating‐cooling cycles, and water temperature. In addition, the higher the tensile strength, the lower the roughness coefficient. This study is expected to provide a reference for the selection of different conditions of water cooling at high temperature for thermal recovery in the Songliao Basin.

Reaction of Zirconium and Its Dihydride with Ammonia

The reaction of zirconium and its dihydride (particle size 40-100 μm) with ammonia was studied under the following conditions: 300-550°C, initial ammonia pressure 0.6-0.8 MPa, 10 wt % NH4Cl (relative to the initial amount of Zr and ZrH2). Depending on the pressure, temperature, size of the solid phase particles, and number of heating-cooling cycles, it is possible to prepare finely dispersed zirconium nitride or hydride.

Influence of isothermal and cyclic annealing on structure and swelling of neutron-irradiated beryllium

The influence of cyclic and isothermal annealing on structure and swelling of neutron-irradiated beryllium was examined. At cyclic annealing the base temperature was equal to 150°C, the maximum cycle temperature was 600°C (700°C in the second experimental set), the number of heating-cooling cycles was from 10 to 100. Isothermal annealing of beryllium was performed at 600°C, 700°C and 800°C. The annealing time in the isothermal regime was the same as well as the total annealing time of beryllium samples at appropriate temperatures in the cyclic regime. It was shown that there are no significant differences in the influence of cyclic and isothermal annealing on structure and swelling of irradiated beryllium.

Thermal-deformation curves for carbonized carbon plastics at elevated temperatures

1. QGCP shows thermal expansion on heating; an increase in the heating rate produces an increase in the relative value of the thermal deformation in the working temperature range in neutral and oxidizing media. 2. An increase in the number of heating-cooling cycles with a constant heating rate reduces the thermal deformation no matter what the nature of the gas medium, where the reduction in the relative maximum deformation in a neutral medium is not more than 0.3%, as against 0.1% for an oxidizing medium, in each case in terms of the deformation in the previous test. 3. An increase in the number of heating cycles in this temperature and time range produces the same changes in the thermal deformation as a reduction in the heating rate in the first test, which means that the necessary heating rate for a single thermal loading can be selected in order to determine the thermal deformation of carbon plastic under repeated loading.