Mechanism Of Removing Rock By Laser And The Study On Drillability

Improving drilling efficiency is important,and laser assisted drilling is an emerging technology,which has its unique advantage in developing oil and gas,however,it is still in the exploratory stage.Optimization parameters and analsis the rock drillability before and after the laser drilling are the key of the technology.Therefore,it is necessary to research on the mechanism of laser assisted drilling and rock drillability.In this paper,we have revealed the influence of parameters,fluid and gas on laser assisted drilling by the experiment.We have conducted that: the effect of laser drilling get better with the action time increasing;it is important to optimize the deposition and laser power for getting the best effect.Compared with non fluid environment,the water and mud can reduce the crushing effect of laser on rock,while the saturated salt water is helpful.And compared with non gas environment,the compressed nitrogen can reduce the crushing effect of laser on rock,while the compressed air is helpful.We have carried out the experiment of rock properties variation before and after laser drilling to research on the macro and micro evolution mechanism of the laser drilling process,which includes rock density,porosity,mineral composition,mechanical properties and internal structure.It is shown that: compared with the rock properties before the laser drilling,the rock density reduced,the porosity increased and the strengths of uniaxial compressive、 uniaxial tensile and triaxial shear decreased;and the species of mineral did not change.By using the CT scan,we observed that: the effect of laser caused internal crack,and the granite cracks are big crisscrossed,while the sandstone cracks are mostly subtle small along the radial direction.Through the experiments we have found that: the crushing mechanism of granite is: the thermal effect of laser high temperature caused internal cracks in granite,and finally macro cracks developed from crack initiation,propagation and evolution;the the crushing mechanism of sandstone is: the thermal effect of laser high temperature caused the water loss and sandstone shrink、exfoliation between particle and particle,and eventually resulted in the inside rock crack.In this paper,we have comprehensive assessed the effect of laser assisted drilling by the experiment of drillability.It is shown that: laser drilling can reduce the series of rock drillability to improve the drilling efficiency;the rock uniaxial compressive strength and drillability series decreased with the laser distribution density increasing;the laser drilling with nitrogen could improve the rock series of drillability,while the laser drilling with compressed air could reduce the rock series of drillability,so the compressed air is helpful in the laser drilling.We can also concluded that from the drillability experments: in order to create the favorable conditions for rotary drilling and improve the drilling efficiency in the hard rock,it is feasible to use the laser of the high temperature melting,gasification and thermal stress to produce cracks、weaken the rock strength and accelerate the rock breaking by controlling the laser energy.