Study On Damage Characteristics And Fragmentation Mechanism Of Deep Granite Under Impact Loading

Energy is an important material basis for the survival,production and development of human society.With the rapid growth of global population and the rapid development of the world economy,the problems of global warming and energy shortage are becoming more and more prominent.Therefore,the utilization and development of renewable energy is of great significance in promoting the transformation of energy structure and achieving carbon neutrality.Deep geothermal energy has great development potential due to its huge reserves,stable supply and high thermal quality.At present,conventional rotary drilling method is commonly used for geothermal drilling.However,this drilling method has some problems,such as low efficiency in rock fragmentation,serious bit wear,long drilling cycle and high drilling cost,which seriously restricts the large-scale development of deep geothermal energy.Percussion drilling technology is considered to be one of the most effective drilling techniques in hard rock formations,which is expected to increase geothermal drilling efficiency exponentially and reduce drilling costs significantly.To this end,accurately assessing the damage characteristics of rocks under impact loads and revealing the rock fragmentation mechanism can provide reference and guidance for optimizing the percussion drilling process parameters,improving the drilling tool structure and guiding the layout of button bit,which is of great significance in eliminating the negative impact of well depth on the percussion drilling efficiency and improving the rate of penetration(ROP).Based on the above scientific problems and engineering needs,this Ph.D.thesis adopts a combination of laboratory tests,theoretical analysis and numerical simulation to investigate the damage characteristics and fragmentation mechanism of deep granite under impact loading,and the specific research contents and findings are summarized as follows.(1)Laboratory drilling tests and numerical simulations of impact stress wave evolution in granite rock were carried out,and macro-scale evaluation of crack distribution in damaged granite near the borehole was performed using dye penetrant detection method.Meanwhile,micro-scale qualitative and quantitative assessment of impact cracks was carried out using2 D image technique and fractal theory.The results showed that the cracks near the borehole after percussion drilling showed regionalization characteristics,and the attenuation of impact stress waves was the main reason for the regionalization pattern of cracks in the damaged rock.There are seven basic forms of impact cracks,and the morphological differences are mainly related to the proportion of mineral components in the rock.The impact cracks have good fractal characteristics,with fractal dimension ranging from 1.27 to1.67.The analysis and comparison of crack characteristics at macroscopic and microscopic scales reveals that the damage range of percussion drilling to the rock near the borehole is about 1.5?1.7 times the diameter of the borehole,and the radius of the rock damage area caused by percussion drilling is about 5?7 times that of conventional rotary drilling under atmospheric pressure.(2)The change law of physical and mechanical properties of damaged rocks was studied.Taking the change of longitudinal wave velocity of damaged rock as the damage reference index and Brazilian tensile strength,uniaxial compressive strength,peak stress and elastic modulus of damaged rock as the damage auxiliary reference index,the relevant physical and mechanical properties were measured.The results show that the physical and mechanical properties of mechanically damaged rocks are influenced by their internal cracks.There is a positive correlation between the longitudinal wave velocity variation of the damaged rock and the dimensionless distance.There is a positive linear relationship between the Brazilian tensile strength and the dimensionless distance,and the compressive strength and elastic modulus have good consistency with the tensile strength in terms of trend and degree of variation.The evolution law of physical and mechanical properties of damaged rocks shows that the range of damage to the rock near the borehole caused by percussion drilling is about 5?7 times of that by conventional rotary drilling under atmospheric pressure.The changes of physical and mechanical properties of damaged rocks and cracks on macroscopic and microscopic scales reflect a good agreement.(3)A series of laboratory hard rock fragmentation tests was conducted,and the rock fragmentation performance under dynamic load with different confining pressure was analyzed.The results show that there is a good negative linear relationship between the confining pressure and the crater parameters,including projected area,perimeter,diameter and depth.Most importantly,removal volume of the crater decreases as an exponential polynomial with the increase of the confining pressure.The variation of specific energy indicates that the ROP of percussion drilling in hard rock will not change significantly in shallow formation,but it will decline sharply once it enters into the medium and deep hard rock formation.Confining pressure can lead to insufficient rock fragmentation beneath adjacent button,which will aggravate the wear of buttons,shorten the service life of the bit,increase the drilling cycle and the drilling cost.Moreover,the observation of cracks inside the damaged rock reveals the mechanism of rock fragmentation under confining pressure,that is,the confining pressure can inhibit the propagation of side cracks around the crater and hinder the expansion of radial cracks to the free surface.Side cracks became the dominant crack type as the confining pressure increases,but the number of side cracks also gradually becomes less.The number of fragments is dominated by 1 mm>d>0.18 mm,and the smaller fragments represent over 99% of the total number of fragments generated.(4)The limitations and shortcomings of the rock fragmentation device with full-size drill bit were analyzed,and a small-size test system for rock fragmentation with single button bit was established by numerical simulation using the equivalent stress approach.In this thesis,the performance of a single spherical button penetration into granite rock under dynamic impact loading was studied considering the confining and hydrostatic column pressure.Then,the dynamic damage evolution and rock fragmentation mechanism were discussed.Based on the cavity expansion model,the elastic-plastic boundary radius was predicted,and the key parameters of crater under different confining pressure and hydrostatic column pressure were compared and quantified.The results show that the hydrostatic column pressure has a certain negative effect on the rock fragmentation performance,but the influence is much less than the confining pressure.The modified cavity expansion model can reasonably predict the radius of the elastic-plastic boundary in the damaged rock,and the radius of the elastic-plastic boundary has a negative linear relationship with the confining pressure and a positive linear relationship with the hydrostatic column pressure.Hydrostatic column pressure can aggravate the damage inside the rock,but the lateral cracks generated at the elastic-plastic boundary cannot extend to the free surface to form radial cracks,which makes the formation of fragments more difficult.(5)The penetration performance of rock indentations under dynamic and quasi-static loading was compared and evaluated,and the similarities and differences between dynamic and quasi-static loading in penetration performance were revealed.The effects of rock type,mineral particle size and mineral component content on rock penetration performance under dynamic and quasi-static indentation were explored.The mechanical response between indenter and rock in indentation process was established.The results show that the damage characteristics of rock under quasi-static loading are similar to those under dynamic loading,but the penetration performance under dynamic loading is better than that under quasi-static loading.Rock types have a significant influence on the penetration performance,different types of rocks show different failure modes in the fragmentation process.When the brittleness index of the rock is less than 20,the rock fragmentation performance is particularly sensitive to the brittleness index.The mineral particle size will affect the period of rock failure.The failure of the rock with small mineral particle size is mainly completed in the early stage of load peak,and that of rock with large mineral particle size is mainly completed in the late stage of load peak.The mineral particle size of 900 ?m is the threshold of rock damage.The mode ratio of feldspar to quartz will also affect the penetration characteristics.When the content of feldspar is more than 5 times that of quartz,the damage and fragmentation performance will not be much different.The relationship between penetration force and displacement under quasi-static loading follows a power law,which can be used to describe the dynamic mechanical response of bit and rock in the process of percussion drilling.