Study On The Mechanism Of Rock Fragmentation By Laser Irradiation

With the continuous development of various countries in the world,oil and gas in the shallow middle part of the strata is in short supply.Due to the deep geological depth,complicated geological structure and extremely high rock hardness,this poses a great challenge to the drilling.The influence of the laser parameters of the laser rock drilling on the crushing effect and the rock crushing mechanism are all important aspects of the technology.In this paper,a combination of laser linear scanning method and fixed-point irradiation experiment was used to quantitatively analyze the crushing effect of granite irradiated by different laser power density.The laser power density range from0.5W/mm~2 to 5W/mm~2 for granite low-power-density laser rock Drilling(the melting of granite appears with the laser power density above 5W/mm~2).On the basis of the previous experiments,the analysis of different specific energy and rate of penetration with different laser power density shows that when the laser power density is 5W/mm~2,the rate of penetration reaches 0.69mm/s and the efficiency of low-power-density laser drilling is the highest.Research about laser irradiation of granite and sandstone in the high power density range was also carried out.Single-factor controlled variable method is applied to experimentally analyze,to analyze the laser power and irradiation time respectively.From the analysis of specific energy,drilling speed,porosity and crack analysis of the rock surface,the results show that specific energy decreases with the increase of laser power;Specific energy increases first and then decreases with the increase of laser irradiation time.The rock porosity fluctuates with the increase of laser irradiation time,but in all,the rock porosity still has the trend of increase with the increase of the laser irradiation time.With the assistance of MATLAB image processing technology,quantitative analysis of rock surface cracks is researched,as laser power and laser irradiation time increase,the diameter and depth of the boreholes in granite and sandstone rise,the crack appears on the surface of the rock,the values of the crack area and the crack length also increases.In addiation,the crack width continuously increases.The drilling depth of granite is smaller sandstone,the rock fracture tends to pass through laser irradiation zone,to the nearest boundary of the expansion.Through the microscopic analysis of the fractures of the rock,combined with the analysis of the fracture process of the rock,the granite fracture in low power density laser rock drilling is a kind of fracture mechanisms combining fracture and shear fracture,trans-granular fracture and inter-granular fracture and their coupling.The mechanism of micro-fracture is that the weaker surface started to fracture and rupture,macroscopically showed debris eruption.On the other hand,the fracture appearance of rock produced by high-power-density laser irradiation illustrates the following conclusions.The fractures of sandstone and granite are all along the trans-granular fracture and inter-granular fracture and the brittle fracture coupling.The difference between sandstone and granite is that the binding capacity of the cements in the sandstone is much weaker that the cements in the granite,so the fracture of sandstone is mainly along the trans-granular fracture.The granites have a dense structure with a large number of tensile fracture patterns and a small number of shear fracture patterns.The fractures start from the laser irradiation zone and expend to all directions,trigger the flaking of the rock surface.