Study On Pressure Relief Mechanism Of Hydraulic Flushing In Coal Seams And Its Application

The coal industry is one of the high-risk industries in China.According to investigations,the vast majority of coal mines in China have encountered gas problems.Gas problem can be regarded as a primary issue that restricts the safe and efficient mining of coal mines in China.With the increase of coal mining depth,the mining intensity of working face is improved.The amount of gas emission from the working face is increased and gas-induced disasters are more likely to occur.The implementation of gas predrainage through boreholes is currently the fundamental method for the management of gas.However,the traditional borehole drainage method has certain limitations.On the one hand,the perforation phenomenon tends to occur in dense boreholes,which reduces their drilling efficiency.On the other hand,the construction of dense boreholes needs more man power and the comprehensive cost is high,which directly affects the economic benefits of a coal mine company.In addition to the above problems,for some coal seams with high gas permeability,low permeability and high risk,the drainage effect of dense boreholes is often not guaranteed.At present,for low-strength coal seams,hydraulic flushing has achieved initial success,which can effectively increase coal seam permeability and improve drainage efficiency.However,there are still deficiencies in laboratory and field research work,and there is still a need for further research on the pressure relief mechanism of hydraulic flushing.Based on the situation,the research work through the combination of laboratory-site and macro-micro views was carried out.It examined the remolding effect of hydraulic flushing on the macroscopic and microscopic structures of coal,and obtained the pressure relief mechanism of hydraulic flushing.The technology was used in the field for industrial test and obtained good results.The main research results include the following points(1)A hydraulic flushing experimental platform was set up and cement blocks with different mechanical strengths were prepared.The changes of surface morphology of the borehole before and after flushing were compared.The results showed that the depth of the impact groove is influenced by flushing pressure and sample strength.That is to say,the flushing pressure and the strength of the coal body have an important influence on the impact damage.From the perspective of the topography,the high-speed water jet has a certain stripping effect around the impact point.It can be considered that during the flushing process,the high pressure water body not only has a vertical pressure effect on the coal body,but also has a certain stripping effect,which is similar with the drill bit's rolling effect on the coal body during the drilling process.The factors that affect the stress wave characteristics were compared and analyzed.The results showed that the amplitude of stress wave was positively correlated with the degree of compaction of the sample.In real coal body,the coal structure with higher strength was dense and the stress wave propagates farther and has less attenuation.The stress wave can cause disturbances at deep part of the coal body.The coal structure with low strength is loose and the stress wave decays greatly.It only applies disturbance to the coal near the borehole.The amplitude of the stress wave is positive to flushing pressure.Within a certain impact range,the stress wave shape and amplitude is similar,that is to say,for a certain point in coal body,the effect of stress wave vibration will not disappear quickly after the transfer of the impact point.During the actual flushing process,a high-intensity disturbance will take long time at any position inside the coal body.During the disturbance process,the coal body is influenced on different directions at the same time.The internal structure of the coal body is very complex,and there are pores in all directions.The multi-directionality of the stress wave vibration can cause the destabilization and destroy of internal structure of the coal body and achieve the remolding of coal body.(2)The underground flushing process was studied,and a hydraulic flushing test system was set up.Tracking and measurement of coal flushing,gas emission and water consumption were carried out in the 35 th and 37 th groups of boreholes.The results showed that the cavities of flushing boreholes are jagged.For a single drill hole,the hole expansion among the coal segments is constantly changing,which reflects a certain randomness.This randomness gradually increases with the decrease of the inclination angle of the drill hole.For different boreholes,as the inclination of the borehole is reduced,the overall shape of the cavity is more varied and complex.The parting layer in coal seam will cause a sudden decrease in the amount of coal produced by hydraulic flushing,and the cavity will be distorted.However,as the inclination of the borehole is reduced,the effect of the parting layer on the distortion of the cavity will be gradually weakened.The reduction in the inclination angle of the borehole weakens the influence of external factors such as the parting layer,but it amplified the effect of instability of coal itself.The final result is that as the inclination of the borehole decreases,the borehole cavity becomes more and more complex.The amount of water in the flushing process is not related to the amount of coal and the number of outburst.However,as the inclination of the borehole is reduced,the average water consumption is gradually reduced by affecting the instability of the coal body and the severity of outburst.The relationship between the gas flow and the borehole angle is not obvious.For a single coal section,under normal conditions,the concentration of gas in the separation tank gradually rises over time.The effect of blocking and outburst on gas emission is opposite.The block of hole will cause the gas concentration to decrease,but the outburst will make the gas concentration rise greatly in a short time.Through the analysis of the mechanism of outburst,it is believed that the outburst in the flushing process is mostly caused by the hole blocking.With the reduction of the borehole inclination,the concentrated stress of the cavity surface is more complex,leading to the instability on the cavity surface and accelerating the formation of the macroscopic fracture field around the borehole.(3)Based on the raw,drilled and flushed coal samples collected from the field,the coal samples were measured experimentally for particle size distribution,chemical functional groups and microscopic pore structures.The results indicated that the particle size distributions of three samples are similar and only differ in the large particle size range.It can be considered that the impact damage in the flushing process is similar to that of other samples,belonging to the category of mechanical damage,and mechanical damage can only affect within the macroscopic size and cannot affect the microscopic size.For the three samples,the chemical functional group structure does not change.That is to say,the mechanical destruction will not affect the functional groups of coal.The flushing process will have a certain effect on the internal micro-porous structure of the coal,mainly concentrating in the meso-pores.The increase of meso-pores greatly improves the microscopic pore network inside the coal body.However,the flushing effect also has its limitations.For large pores more than 1?m and micro-pores less than 2nm,the remolding effect is relatively limited.The remolding process of the coal body does not occur at the impact and discharge process.Instead,it occurs during the stress wave is disturbing.(4)The analysis on the pressure relief mechanism of hydraulic flushing was carried out.The results showed that the hydraulic flushing process remolding the coal seam from two perspectives: macroscopic cracks and microscopic pores.At the level of macroscopic fractures,the high-pressure water is used to extend boreholes in the coal seams.Under larger borehole diameters,the stress around the borehole is more concentrated on the coal body.A wider range of plastic zone is formed.At the same time,the cavity in real coal is uneven.The concentrated stress not only acts on the roof and floor,but also exists in the uneven surface of the borehole wall.Under the effect of concentrated stress,the irregularities on the wall of the borehole will be the first to cause instability and damage.Together with the concentrated stress acting on the coal body by the roof and floor,a large number of fracture networks will be generated around the borehole to provide more channels for the gas seepage process.At the level of microscopic pores,the pore structure inside the coal body is continuously disturbed by stress waves.For the larger pore network,its connectivity is good,and the surrounding coal body strength is low.Under the stress wave disturbance,it gradually loses its stability and destroys,forming a new microscopic pore network.These new microscopic networks provide more microscopic pores for gas desorption and diffusion,resulting in high level of gas release.(5)The flushing plan for industrial trial was determined and the experiment was carried out.The results showed that the effective radius of hydraulic flushing can reach 4m,which is higher than that of ordinary borehole.The gas flow attenuation coefficient of flushed borehole is 0.003,which is only 1/7~1/8 of ordinary boreholes.The permeability coefficient of flushed borehole is 1.775,which is almost double of ordinary boreholes.Besides,the efficiency and effectiveness of gas drainage of flushed boreholes are obviously better than those of ordinary boreholes.After that,the cost and benefit of hydraulic flushing technology was analyzed,which showed that the hydraulic flushing technology can effectively reduce the construction cost,increase the efficiency of gas drainage and ensure the safe production of working face.The hydraulic flushing method is proved to be an effective way to solve the gas problem of coal mines.