Mechanism and Application of High Pressure Gas Presplitting Detonation on Coal Rock Fracturing

Author:Yu Hai Ling

Supervisor:zhang yan song


Degree Year:2019





In the production process of coal mine,it is necessary to solve the safety technical problems such as gas disaster,dust prevention and control,rock burst and "two-hard" coal seam.It is often necessary to presplit the coal seam to increase the permeability of coal seam,weaken the hardness of coal and remove the pressure.Based on the existing mechanism of coal rock fracture,a new method of coal crack weakening is proposed,which is high pressure gas pre-split detonation.In this paper,by means of theoretical analysis,experimental study,numerical simulation and engineering test,the theory and technology of the coal rock weakening based on high pressure gas presplit detonation are systematically discussed.This research has important theoretical significance and extensive engineering application prospect.This paper is based on the brittle fracture mechanics,seepage mechanics,combustion and explosion mechanics.In this paper,the crack growth driven by high pressure gas and the crack growth in borehole under pressure are analyzed theoretically.The method to broke the coal and in this paper can be divided into two process.First,under the effect of the high-pressure gas generated by the combustion of gas-producing pre-cracking agent,the initial crack of surrounding rock propagates.Second,with the increase of gas pressure in boreholes,the gas producing pre-cracking agent undergoes detonation reaction,then the detonation shock acts on the surrounding rock.This paper is based on the brittle fracture mechanics,seepage mechanics,combustion and explosion mechanics.In this paper,the crack growth driven by high pressure gas and the crack growth in borehole under pressure are analyzed theoretically.At first,based on the fracture pore structure of the coal body,the stress characteristics of the coal body,gas seepage in the coal seam and the stress state of the borehole in the coal seam are analyzed.Secondly,by analyzing the crack growth condition under the action of quasi-static pressure gas,the stress intensity factor of the drill hole existing crack crack-tip is obtained.Finally,when the drilling pressure reaches 200kg and the hole is filled with high-pressure gas,the crack expansion law of the surrounding coal body caused by detonation is analyzed.Through the gas presplitting and detonation impact in the process of combustion into gas,the fracture network in coal seam is interconnected,and the fracture network is filled with high-pressure gas.High pressure gas surrounds the crushed coal and penetrates continuously to the interior,so that the coal seam can achieve full crushing effect.The physical simulation experiment of fracture expansion driven by high pressure gas was carried out on the platform of triaxial pressure tester.The whole test system includes loading system,gas supply system,test box,equipment numerical control system and data acquisition system.The specimen is prefabricated with similar materials and raw coal.The data were collected using the stress-strain data acquisition system and real-time acoustic emission location technique,and the test results were analyzed through pressure-time curve,acoustic emission characteristics and fractal feature.The test results show that the pressure inside the bore hole is greater than the tensile strength of the specimen and less than the compressive strength of the specimen.This indicates that the tensile strength of the specimen is overcome when the crack expands.The pressure curve and acoustic emission analysis show that the pressure of the sample in the process of rupture presents obvious segmentation characteristics.In addition,the whole similar material samples were rapidly released from pressure after the rupture,while after the rupture of the raw coal sample,a pressure stability interval appears in the process of the borehole pressure decreasing.The reason should be the original fracture development in raw coal,and during the expansion of the main fracture,the internal small cracks are communicated,which makes the pressure in the hole reach a stable process in a period of time.LS-DYNA numerical analysis software is used to simulate the two process of coal rock fracture based on high pressure gas pre-split detonation.It is shown that the initial cracks of coal expand.Detonation occurs on the basis of high-pressure gas pre-cracking.Then,the detonation shock wave enters into the rock along the pre-crack and generates some new cracks at the tip of the crack.The damage of the surrounding rock of the borehole wall caused by detonation shock is obviously smaller than that caused by conventional explosive blasting,that is,and the smashing effect is obviously weaker than that caused by ordinary explosive blasting,which is basically consistent with the theoretical analysis results.Aiming at the collapsing problem of hard top coal,the technology described in this paper is used to carry out the engineering practice of hard top coal weakening in QianShuTa coal mine.Field test results show that the method can ensure the top coal caving in time,guarantee the collapsing of top coal in time,control the periodic weighting,increase the recovery rate by 15%.Aiming at the problem of high coal dust production and explosive tendency in 9309 working face of Nantun Coal Mine,the technology of high-pressure gas pre-splitting detonation combined with coal seam water injection was adopted,which made the dust reduction rate of working face reach to 84.5%.This method effectively solved the problem of dust reduction in working face.