To comprehensively understand the bulk macro conduction characteristics of typical geomaterials existing in deep underground coal mines, i.e. coal, rock, and concrete (shotcrete), meso-scale composition (solid mineral and fluids such as water and air) and structure (particle and pore size, shape, and contact form) analyses were conducted via water absorbability and thermal conductivity measurements, scanning electron microscope, and X-ray diffraction technologies. The results indicate that coal, rock, and concrete's respective meso-scale composition and structures have different degrees of influence on their macro bulk effective thermal conductivity at various conditions. First, the fluid (water and air) composition significantly impacts thermal conductivity at a certain bulk effective porosity: coal < rock < concrete. The ranking of thermal conductivity shows two different forms: concrete < coal < rock in a dry state and coal < concrete < rock in a water-saturated state. Second, the surface morphology and face porosity obtained by scanning electron microscope imaging can explain thermal conductivity fluctuations at various moisture contents and magnification times. The face porosity as well as its quadrant anisotropy degree can also be achieved via scanning electron microscope digital image processing technology, and both of them have a certain relationship with magnification times (scale effect) and effective volumetric porosity, as obtained by water absorbability experiments. In addition, apart from the fluid composition, the mineral composition (species and content of mineral) of the solid skeleton or matrix also significantly influences the macro-scale thermal conductivity of geomaterials. Furthermore, a quantitative comparison analysis can be performed by X-ray diffraction, but further research is needed to improve its accuracy. This study's detailed meso-characterization of the macro effective thermal conductivity for typical geomaterials in an underground coal mine may be of reference value to establish the relationship between different properties of differing scales.

Efficient fracturing is the key issue for the exploitation of geothermal energy in a Hot Dry Rock reservoir. By using the laser irradiation cracking method, this study investigates the changes in uniaxial compressive strength, ultrasonic characteristics and crack distributions of granite specimens by applying a laser beam under various irradiation conditions, including different powers, diameters and moving speeds of the laser beam. The results indicate that the uniaxial compressive strength is considerably dependent on the power, diameter and moving speed of the laser beam. The ultrasonic-wave velocity and amplitude of the first wave both increase with a decreased laser power, increased diameter or moving speed of the laser beam. The wave form of irradiated graphite is flattened by laser irradiation comparing with that of the original specimen without laser irradiation. The crack angle and the ratio of the cracked area at both ends are also related to the irradiation parameters. The interior cracks are observed to be well-developed around the bottom of the grooving kerf generated by the laser beam. The results indicate that laser irradiation is a new economical and practical method that can efficiently fracture graphite.