| Dense boreholes technology in underground coal mines,especially cross-seam boreholes,is one of the most commonly used measures to improve coal seam permeability for the CBM extraction and gas disaster control.However,the effective range of permeability enhancement is small and the utilization rate of boreholes is low,due to the long rock segments and short coal seams of cross-seam boreholes.In this paper,based on the flexible energy transfer characteristics of water jet,a new self-propelled water jet bend drilling method for improving the coal seam permeability by using water jet to break rock to drill holes and provide self-propelling force was proposed.The bend boreholes can extend the borehole length in coal seams and give full play to the utilization rate of single cross-seam borehole,increasing the gas flow channel in coal seam and reducing the drilling cost.The key to realize this method lies in the development of self-propelled water jet bits,which can match well with the existing cross-seam borehole size.Its performance determines the rock-breaking and drilling efficiency and the hole stability.As a new type of water jet bit,the self-rotatory jet bit has the advantages of high drilling efficiency and good borehole stability by utilizing the water jet to break coal and rock with a circular repetitive impact trajectory.So,the self-rotatory jet bit has its own advantages for drilling the bend boreholes in coal seams.However,the jet flow structural characteristics,failure mechanism of coal and rock and the formation of boreholes for self-rotatory jet bit are still unclear.And the design of self-rotatory jet bit with a limited size and the control mechanism affecting the rock-breaking and hole-forming efficiency are lack.Aiming at the self-rotatory jet bit with a limited size used in bend drilling in coal seams,based on the theoretical deduction,laboratory experiments and numerical simulation,the jet flow characteristics,rock failure mechanism,the self-rotatory jet bit design and the hole-forming mechanism were studied.And the main innovative achievements are as follows:(1)The deflection characteristics of jet flow field for water jet generated by a self-rotatory bit was revealed.Based on the analysis of the time-averaged velocity,vorticity and streamline characteristics of the self-rotatory jet,the its flow field structure of the "non-deflection zone and deflection zone" was proposed.And the jet energy attenuation mechanism and the deflection mechanism of flow characteristics were clarified.The effects of rotational velocity and jet pressure on the time-averaged velocity characteristics,deflection distance,deflection angle and impact angle of the water jet were obtained.The results show that the turbulence and vorticity in the front of the self-rotatory jet are stronger than that in the jet back,which accelerates the jet energy dissipation and makes the jet flow patterns and characteristics gradually deviate to the jet back.With the continuous development of the self-rotatory jet,the jet deflection distance,deflection angle and impact angle increase parabolically and linearly,respectively.And the greater the rotational velocity and the lower the jet pressure,the more severe the jet deflection is.(2)The failure mechanism of coal and rock subjected to the self-rotating jet impact was revealed.Based on the evolution law analysis of the stress and displacement in coal and rock under jet impact,the rock-breaking mechanism of rock broken by jet impact load at water-hammer pressure stage and fractured by jet quasi-static pressure at stagnation pressure stage was revealed.The ranges of crushing zone and tensile fracture zone in rock were obtained,and verified based on the experimental data.And the rock erosion experiments under the water jet rotatory impact was carried out.The effects of jet pressure,rotatory speed and impact angle on the rock-breaking properties of the self-rotatory water jet were obtained,and the mechanism of rock fragmentation subjected to the self-rotator water jet was revealed.The results show that the "water cushion" effect can be weakened when breaking rock by a self-rotatory jet.And the cements and rock particles with low connection strength can be washed out first,and then the brittle tensile fracture dominates rock matrix failure under the impact stress wave.The rock was further fractured by the quasi-static hydraulic pressure,leading to the formation of the annular broken pit with a "V" shaped section and its surrounding damage zone.When breaking coal and rock,the self-rotatory jet has the optimal speed and impact angle;and the higher the jet pressure,the higher the optimal speed.(3)The optimal design criteria of structure and nozzle layout for the self-rotatory jet bit were established.And the self-propelled drilling and hole-forming mechanism of the jet drill was revealed.Based on the calculation models of self-propelled force,rotating torque and rotating speed of the self-rotatory jet bit,a new self-rotatory jet bit with a small size was designed by considering the bit size,rotation realization under pressure,rotating seal and speed limit.The optimal combination of nozzle distribution radius was obtained based on the rock-breaking and drilling experiments.And the effects of the jet pressure and rock strength on the rock-breaking and drilling efficiency were investigated.The rock-breaking and hole-forming mechanism of self-rotatory jet bit was revealed.(4)The self-propelled water jet drilling technology and equipment were developed,and field application tests were carried out.The effects of hydraulic parameters,coal strength and drilling angle on the self-propelled water jet drilling efficiency were obtained.The results show that the drilling speed increases with the increase of jet pressure and decreases with the increase of coal strength.But the limit drilling depth is shallow due to the poor borehole stability when the coal strength is too low.The drilling angle with a certain elevation is beneficial to the backflow of coal slags in the drilling process,thus improving the drilling efficiency.The average drilling speed in coal seams is 0.75m/min,and the drilling depth can reach 45 m.The results proved the high drilling efficiency of self-rotatory jet bit in coal seam and the feasibility of the self-propelled water jet drilling technology and equipment.The research results in this work will enrich the rock-breaking theory of high-pressure water jet to a certain extent.It will have a guiding significance for improving the water jet drilling efficiency.And it will be helpful for innovating the application of water jet bend drilling technology in the permeability enhancement of coal seam. |
PDF Full Text Request