Experimental Study On Thermal Cracking Characteristics Of Hot Dry Rock Irradiated By Moving Laser Beam

The geothermal resources are abundant in China.The highly efficient cracking of Hot Dry Rock to formation the enhanced geothermal system is the key problem.Traditional hydraulic fracturing is a common and effective method.However,there are still many technical limitations,such as high-water consumption and uncertain initiation fracturing site for the hydraulic fracturing.Therefore,it is of great significance to investigate the thermal cracking technology on HDR induced by laser irradiation for the development and utilization of geothermal resources.The variations of temperature,temperature gradient and thermal crack efficiency of irradiated granite samples with irradiation parameters including different laser power,laser beam diameter and beam moving speed were investigated in this thesis,as well as the characteristics of open porosity and fracture morphology.The ultrasonic characteristics and mechanical properties of irradiated samples were also tested by ultrasonic detector and universal testing machine.The cracks in the granite samples were reconstructed by using 3D-XRM to study the cracks distribution.The main conclusions are shown as:(1)The influence of laser power,beam diameter and beam moving speed on granite temperature field,specific energy and thermal cracking efficiency,open porosity and cracks distribution were studied by using the control variable method.The results show that the high temperature area of rock sample increases with the increase of laser power.However,the specific energy and thermal cracking efficiency decrease with increasing laser power.The higher the laser power,the deeper and wider the grooving.In addition,the surface crack density and open porosity also increase with the increase of the laser power.The surface temperature decreases with the increase of beam diameter.However,the specific energy and modified specific energy increases with increasing beam diameter.In addition,the grooving width and depth increases and decreases with the increasing beam diameter.The surface cracks density and open porosity both decreases with the increase of beam diameter.The area with high temperature decreases with increasing moving speed of laser beam.The specific energy and modified specific energy decreases and increases with the increasing beam moving speed respectively.Both the grooving depth and width decrease with the increasing beam moving speed.In addition,the surface crack density and the open porosity also decrease with the increase of the beam moving speed.(2)The ultrasonic results show that the lager laser power and smaller beam diameter and smaller movement speed resulted in the greater wave velocity and the lower first wave amplitude.In addition,the first waveform of the pulse signal fluctuated gently.However,the distortion of the first and second cycle waveform increased.The attenuation of the high frequency parts is larger than that of the low frequency parts,which leaded to the slippage of the receiving signal from high frequency to low frequency.Based on the above four acoustic parameters,it was determined that the larger thermal damage and more internal cracks would be caused by the larger laser power,the smaller beam diameter and moving speed.(3)XRD and XRF results showed that the quartz content of the molten material increased due to the laser irradiation,while its diffraction intensity decreased and the diffraction angle widened,indicating that the rock structure changed from crystal state to amorphous state and the rock sample strength lost its bearing capacity.Uniaxial compression results further verified this conclusion.With the increase of laser power and the decrease of laser beam diameter and moving speed,the slope of the straight section of the stress-strain curve of rock sample decreases,the elastic modulus and peak stress of rock sample decrease correspondingly,and the axial strain increases gradually.(4)According to the internal structure reconstructed by 3D-XRM,it is found that the crack density in the rock sample after laser irradiation is relatively large,and the crack extends to the opposite direction of the laser beam,and the distribution is relatively uniform.The laser slit profile width decreases from top to bottom and a Ushaped groove is formed.There are 69 Figures,8 Tables and 102 References in this thesis.