| How to realize rock efficient fragmentation is a core issure in the field of oil and gas drilling engineering,especially as the drilling work has gradually turned to deep formation,high efficient rock breaking technology is more important.However,due to the traditional way of drilling has brought a series of problems in deep dense formation,such as low rate of penetration,long drilling cycle,high drilling cost,etc.,conventional rotary drilling technology has been unable to meet the needs of high drilling speed in deep well of China’s oil and gas exploration and development,and percussive drilling technology is one of effective ways to improve the deep well drilling speed in hard and brittle formation.Therefore,this paper has completed the following research work based on the percussive drilling process of deep tight sandstone.In view of the dynamic characteristics of tight sandstone,the mathematical relationship among rock dynamic strength,crushing time,strain energy density and strain rate is obtained based on the principal of damage due to inherent defect,and the model parameters conformed to deep tight sandstone are determined through the indoor SHPB impact tests.The results show that within the strain rate range equivalent to percussive drilling,the ratio of dynamic and static elastic modulus of tight sandstone is between 1.20 and 2.13,the ratio of dynamic and static strength is between 1.31 and 1.91,and the higher the strain rate,the greater the dynamic elastic modulus and strength;with the increase of strain rate,the crushing time of tight sandstone shortens and strain energy density increases;Compared with dynamic properties of shallow rock,deep tight sandstone’s dynamic effect of strain rate sensitivity is stronger.Therefore,during the design of percussive drilling parameters in deep dense formation,rock dynamic characteristics should be considered,in order to guarantee that load on bit teeth is greater than rock dynamic strength and teeth-rock contact time is greater than rock dynamic crushing time.In view of energy consumption in rock dynamic fragmentation,a new model of rock fragmentation energy density is developed based on fractal rock mechanics and fracture mechanics theory,which is verified through SHPB impact tests of tight sandstone and granite.This model combines rock crushing energy with rock inherent properties and fragments distribution characteristics,which can be used to determine the energy for rock breaking directly based on fragments fractal characteristics and rock surface energy.In view of formation mechanism of rock fragmentation under impact load,rock crack formation and propagation during spherical indenter dynamic intrusion under different condition of bottom hole pressure is studied from two aspects of theory analysis and two-dimensional discrete element simulation.Results indicate that confining pressure and hydrostatic pressure are the main inhibitory factor of radial/lateral crack propagation and the formation of rock fragmentation;the hydrostatic pressure increases the internal damage of rock and has a promoting effect on lateral crack propagating inside rock;the numerical simulation results visualize the formation process of rock failure.In view of the effect of impact load amplitude and duration on rock breaking efficiency,the numerical model of spherical indenter dynamic intrusion is established by using three-dimensional discrete element method,and the influence of load amplitude and duration on rock crushing efficiency is studied under two kinds of working condition including the same impact velocity and same impact energy.The results find that there exists a optimal force duration or loading rate which can make the best rock fragmentation efficiency when the impact velocity or load amplitude is constant,and the reasonable loading rate shape of force can be produced by changing impactor shape in actual percussive drilling;the force of short duration and high amplitude is more advantageous to high efficient rock fragmentation when the impact energy is constant.In view of the drilling rate equation for rotary percussive drilling,the mechanical process of percussive drilling system is analyzed,and the model of energy efficiency is obtained,and the influence factors of energy efficiency is studied,moreover,a new drilling rate equation for rotary percussive drilling is developed according to rotation broken work and axial penetration work.The results indicate that when the rock is considered as line elastic material and dimensionless parameter α of bit mass and β of impactor length are close to 2,the axial impact system can obtain higher than 90% energy efficiency;when the rock is considered as viscoelastic material(dimensionless parameter n of viscosity is equal to 2)and α and β are close to 0.5,the axial impact system can obtain higher than 80% energy efficiency;compared to bit mass and other parameters,the impactor length has an obvious influence on rock fragmentation efficiency of axial impact system;the drilling rate for percussive rotary drilling can be directly determined by bit working parameters including weight on bit,rotary speed,torque,and impactor working parameters including impact energy,impact frequency,and the particle size distribution. |
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