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In this work, effective thermal conductivity (λEff) of fractured rock masses was numerically investigated. A two-dimensional Discrete Fracture Network (DFN) model of the fractured rock masses was established based on the statistic results of natural fracture development in a potential area for high level radioactive waste disposal in China. Steady state heat transfer processes in the fractured granite rock masses were numerically simulated using finite element method (FEM). The calculated λEff values of the fractured granite rock masses in dry and saturated conditions are 1.99 W/(m K) and 2.31 W/(m K), respectively. Compared with the thermal conductivity of intact granite [λIntact, 2.5 W/(m K)], the drop rates are 20.4% and 7.6%, respectively. Sensitivity analysis was conducted on the main model parameters including fracture density (FDensity), trace length (FLength), thermal contact resistance (FTCR), and λIntact. The results indicate the relation between λEff and three fracture parameters (FDensity, Flength and FTCR) can be fitted using power law or negative exponent functions with good consistency. When fracture network parameters remain unchanged, λEff is in linear positive correlation to λIntact. The slop of the fitted line is determined by the fracture network parameters. Due to the fact that distribution of generated fractures in different directions are quite uniform, λEff did not show significant difference in different directions. On the basis of the above-mentioned results, an estimation model was proposed for the determination of λEff of fractured rock masses using P21 (total length of fracture traces per unit area), FTCR, and λIntact. The proposed estimation model shows good consistency to the calculated results of FEM model. [ABSTRACT FROM AUTHOR]

The natural fracture system exerts a significant influence on the physical and mechanical properties of rock masses. The heat transfer in fractured rock masses is much more complicated than that of intact rock due to the thermal contact resistance induced by natural fractures with varying size, aperture, roughness, and orientation. This work describes a method for determining the representative element volume (REV) of heat transfer and the 3D thermal conductivity tensor of fractured rock masses. The method is based on the element-wise averaging of temperature and heat flux, which can be obtained from a finite element solution to a steady state heat transfer problem with specific boundary conditions. A case study was conducted based on the geological data of a potential area for high level radioactive waste disposal in China. The results indicate that the size of heat transfer REV of the rock masses in the study area is approximately 18 m × 18 m × 18 m. 3D thermal conductivity tensor, the magnitudes and directions of the three principal thermal conductivities, and anisotropy of the fractured rock masses were obtained. Influences of main fracture parameters including density, disc diameter, and thermal contact resistance on the heat transfer characteristics were investigated. The research results in this work can provide a better understanding of the discontinuous heat transfer processes in fractured rock masses and their homogenization methods. [ABSTRACT FROM AUTHOR]

Convection heat transfer characteristics in a single rock fracture was experimentally investigated in this work. Distilled water was used as working medium for heat transfer in manmade smooth and rough fractures of sandstone and granite samples. The influence of fracture surface roughness on heat transfer intensity was analyzed. During seepage heat transfer process, evolution of samples external surface temperatures was monitored using six resistance temperature detectors to study the local heat transfer characteristics along the flow direction. Subsequently, heat transfer correlations were proposed on the basis of the experimental results. The results indicate that the roughness of rock fracture surface improves the overall heat transfer intensity to a certain extent, whereas lithology has little influence on the heat transfer intensity. For smooth fractures, the local heat transfer intensity decreases with the increase of the distance from the inlet. For rough fractures, evolution of sample external temperatures along the flow direction exhibits strong nonlinear characteristics with multiple peaks. Nu/Pr 1/3 is close to a power function of Re under the experimental conditions in this work. With the increase of experimental temperature, the relationship gradually tends to be linear. [ABSTRACT FROM AUTHOR]

In this work, the feasibility of using enhanced geothermal system correlation techniques (hydraulic fracturing and heat extraction) to develop medium–low temperature geothermal resources for heating purposes in northeast China is investigated. A detailed experiment on field hydraulic fracturing stimulation is conducted. Combining with numerical simulation approaches, heat production potential of this system is forecasted. Field hydraulic stimulation results indicate that the matrix acidizing treatment significantly limits near wellbore friction loss. For network fracturing treatment, however, the formation of branching fractures and its enhancing effects on reservoir flow conductivity needs to be verified by production tests in the future. The geometric dimension and flow conductivity of the induced fracture are determined by numerical simulation of hydraulic fracturing process. The accuracy of the numerical model is verified through matching well head pressure curve. Parameters including production flow rate, flow impedance, production temperature, heat extraction rate and energy efficiency are determined through the simulation results of heat production forecast. The results indicate that thermal energy production from the stimulated reservoir is satisfactory for heating purpose. However, water flow impedance maintains at a relative high level during production period. [ABSTRACT FROM AUTHOR]

In this work, thermal damage effect on the thermal conductivity inhomogeneity of granite was experimentally investigated. To study the influence of different cooling methods, granite samples were heated at different temperature levels (from 100°C to 1000 °C), then cooled in air and water, respectively. Distributed thermal conductivity of granite samples were measured using optical scanning method before and after thermal damage treatment. Besides, some non-destructive tests (open porosity and P-wave velocity) were performed. The results indicate that thermal damage will lead to the decrease of thermal conductivity and the increase of thermal inhomogeneity degree. The decrease degree of thermal conductivity values increases with the treatment temperature. The maximum decrease can be 54.0% (after heated at 1000 °C). When the treatment temperature is higher than the phase transition temperature of quartz (573 °C), the thermal inhomogeneity degree of the damaged samples increased obviously. After heated at each temperature level, the water cooled samples exhibit greater thermal conductivity decrease and greater thermal inhomogeneity factor increase than air cooled samples. This may be attributed to the fact that rapid cooling has extra effects on the damage evolution of rock, which is induced by the high temperature gradient generated when temperature changes occur in a very short time. A thermal damage model was proposed to describe the evolution characteristics of thermal conductivity with treatment temperature. Research results in this work can provide better knowledge to the evolution mechanism of distributed thermal conductivity of engineering rock masses experienced high temperature environment, and the influence of different cooling methods. [ABSTRACT FROM AUTHOR]

• Granites were artificially damaged through cyclic loading and unloading tests. • Temporal and spatial evolution of AE events were obtained. • 2D thermal conductivity images were obtained using optical scanning method. • Influence of mechanical damage on the distributed thermal conductivity was investigated. In this work, influences of mechanical damage and water saturation on the distributed thermal conductivity of granite were experimentally investigated. The samples were artificially damaged using uniaxial cyclic loading and unloading tests, with acoustic emission activity monitored simultaneously. 2D thermal conductivity images were obtained using optical scanning method for both undamaged and damaged samples in dry and saturated conditions. The results indicate that the overall thermal conductivity of the samples decreased after mechanical damage treatment, while the thermal inhomogeneity factor increased. The thermal conductivity difference distribution matched well with the spatial distribution of acoustic emission events. The overall thermal conductivity values increased after water saturation in vacuum condition for both undamaged and damaged samples. Thermal inhomogeneity factor didn't change significantly when undamaged samples were saturated, while thermal inhomogeneity factor of damaged samples decreased to a certain extent when samples were saturated. Research results in this work can provide better knowledge to the evolution mechanism of distributed thermal conductivity of engineering rock masses. [ABSTRACT FROM AUTHOR]