Study On Fissure Development Law Of High Pressure Nitrogen Impacting Fracture For Coal And Rock Mass

Nitrogen is an inert gas.Coal shows limited adsorption capacity to nitrogen.Nitrogen under high pressure won't leave a large number of expansion energy to the coal seam after impacting,and won't cause any risk of outburst as well.With the depth of mining increasing,the content of coal mine gas rises while the permeability descends in our country.It will increases the gas drainage rate and reduce the content of gas by using high pressure nitrogen impacting.Therefore,the study on high pressure nitrogen impacting and fracturing has a important significant.Based on the study of high pressure nitrogen' air-jet process and quasi-static expansion process,the energy change has been analyzed.Taken it as theory foundation,experiment research and numerical simulation of high pressure nitrogen blasting have been lunched.The relevant law of high pressure nitrogen impacting and fracturing under different condition has been obtained.Regarding high pressure nitrogen impacting and fracturing device as core,experimental system includes blasting system and real-time monitoring system.Highpressure nitrogen blasting system includes source of pressure,the device of high pressure nitrogen impacting,pneumatic high pressure cut-off ball value,briquette and linkage unit.Pressure change and internal absolute energy of briquette will be monitored by pressure transmitter and acoustic emission monitoring system.Through strength measurement experiment,material ratio of different intensity and the 28-days curing cycle have been confirmed.It will add stress to the briquette to simulate the actual stress field in the coal seam.By using 1L container,there are 3 kinds of initial pressure(5MPa,7.5MPa,10MPa)to develop high-pressure nitrogen impacting and fracturing experiment for 3kinds of strength briquette.And then it will change the stress to repeat the experiment under 5MPa.From the observation of experiment results,with the pressure increasing,it extends from initial longitudinal main crack to more cracks which present divergence around the releasing hole.The crack is perpendicular to the minimum principal stress at first and then equidistribution.The stress difference also influence the distribution of crack.The smaller difference will lead cracks to equidistribution while the bigger difference will make cracks perpendicular to the least stress.On this basis,the result of study on the least breaking pressure is about 2MPa to 3MPa.Based on the analysis of pressure monitoring results,the whole process of high pressure nitrogen fracturing can be divided into 3 parts,gas jet impingement stage,crack initiation stage and dynamic crack arrest stage with the observation on the pressure rising rate.From the results of acoustic emission monitoring,with the increase of gas pressure,internal absolute energy of briquette when fracturing will increase as well.It will be more obvious when the pressure rise from 7.5MPa to 10 MPa.Based on the theory research and field practice,by using ANSYS/LS-DYNA to build models and LS_PREPOST to deal with it,the high pressure nitrogen fracturing numerical simulation under 5MPa,7.5MPa,10 MPa has been in progress.From the analog results,the development of fractures is obviously improved with the increase of initial gas pressure,and the fracture first appears at the axial direction and the expand following the radial direction.The result is also authenticated in stress nephogram.From the analysis of stress wave transmission results,in the process of cracking,the stress wave presents vibration rise.When the stress value is greater than the failure stress of the coal seam,the crack will occur.If the stress is less than that of the coal seam,the high stress state will be kept until the crack happens.