| The No.3 coal seam of Wuyang Coal Mine has the characteristics of high gas pressure and low gas permeability coefficient.Coal seam gas is between difficult to extract and extractable,and the effect of gas pre-pumping is poor.According to the principle of mining pressure relief and permeation,the permeability and gas extaction effect of coal and rock mass within a certain range will be significantly improved by mining process.In order to improve the gas extaction effect of Wuyang Coal Mine,this thesis uses on-site investigation,theoretical analysis,laboratory test,similar simulation,numerical simulation and on-site measurement to study the coal mine gas seepage characteristics,the effective extraction area of gas in the current coal seam and the upper adjacent strata,the coal and gas coordinate mining technology on the working face,and other relevant research work has been carried out.The following conclusions are obtained:(1)Through the investigation and analysis of engineering background such as mine coal seam and gas occurrence characteristics,the discriminating index of coal seam gas extraction difficulty is considered to evaluate the difficulty of the extraction of No.3 coal seam.It is found that the No.3 coal seam gas is between difficult to extract and extractable,and the coal seam gas pre-draining effect is poor.Thus,it is proposed that the pressure relief effect in the coal seam appeared in the mining process can be utilized,to determine the reasonable extraction area and arrange effective pumping measures to improve the gas extraction effect.(2)The results of triaxial seepage test show that the internal cracks of the coal sample specimen show the original pore cracks were gradually closed during the loading and unloading process—microcracks were generated—the cracks expanded and connected—the macroscopic fractures were formed—a macroscopic fracture surface were generated—the law of change in slip damage.With the change of the internal crack structure of the coal sample,the permeability showed a distinct four-stage variation.In the first stage,the sample was in the loading process.As the axial stress increased,the original pores and cracks in the specimen gradually closed.The volume of the sample was continuously compressed and remained a compressed state.The passage of gas flowing in the sample was narrowed,the permeability of the sample was gradually reduced,and the minimum can be down to 56.8%of the original state.In the second stage,the sample was still in the loading process.With the increase of axial stress,the original pores and cracks in the sample were further closed.But at the same time,a small amount of micro-fracture began to generate and develop in the sample,and the sample volume started to expand to a certain extent from the previous stage while the sample was still in the compressed state,and the permeability of the sample continued to decrease with a minimum value of 54.2%of the original state.In the third phase,the sample continued to be loaded.As the axial pressure increased,the development of micro-rupture began to change qualitatively.More and more micro-ruptures continued to develop,forming a convergence and expansion of micro-fractures.When the sample was loaded to 85.91%of the peak pressure,the yield strength was reached,and the deformation of the specimen changes from elastic to plastic,phased into a stage of unstable fracture development.The volume of the sample continued to expand fromthe expansion state to a compressed state.As a large number of new cracks were generated inside the test piece,the gas flow channels in the sample increased,and the permeability began to increase gradually,reaching up to 355.9%of the original state.In the fourth stage,the sample was loaded over the peak pressure.The internal structure was destroyed during the unloading process.As the axial pressure decreased,the original closed pores in the sample and the new fracture crack formed in the previous stage began to expand and connect to each other,and the sample volume continued to expand.The gas flow channels in the sample became more and wider,and the permeability of the sample increased sharply,up to 505.1%of the original state.(3)Through comprehensive analysis of the change law of permeability and the actual situation of gas extraction,it is determined that the yield strength of coal can be determined as the discriminant index of current coal seam gas effective extraction area.At the same time,combined with the distribution characteristics of the leading support pressure in front of the working face,it is pointed out that the gas extraction in front of the working face presents a four-stage variation characteristic,that is,according to the distance to the working face it can be divided into namely original stress naturally declining extraction stage,stress increasing with low permeability and low efficient extraction stage,stress decreasing with high permeability and highly efficient extraction stage,and stress decreasing with increasing permeability disturbing extraction stage.The position of each extraction stage of the two types of parallel drilling and bedding is determined by field measurement,and the gas extraction growth period and gas extraction attenuation period are pointed out according to be the best areas for the pressure relief of coal seam by the gas extraction effect of each stage.When the gas in the coal seam is extracted by the bedding parallel drilling and the bedding oblique drilling,the optimal extraction area is within 24.9m and 44.8m(22.4m below the hole)in front of the working face.In addition,the comparison of the drilling extraction effects of the two layout modes is carried out,and it is pointed out that the gas extraction concentration and extraction velocity of the bedding parallel drilling is higher than the bedding oblique drilling.The main reason is that the bedding oblique drilling holes can be broken by section cutting during the advancement of working face and connected to the working face,causing air to enter the extraction hole.But the gas extraction total volume of the bedding oblique drilling is more than the bedding parallel drilling in the optimal extraction area,the extraction effects of the bedding oblique drilling is better.(4)Using the similar simulation and numerical simulation methods,the variation law and distribution characteristics of the stress field and fracture field of the overlying strata are studied.Similar simulation results show that with the continuous advancement of the working face,the mining area gradually enlarges,the overlying strata subsides and breaks layer by layer,the fracture boundary presents the trapezoidal distribution characteristics,the mining fracture height increases continuously,the maximum development height of the separation fracture basically stabilizes at 95.2 m above the coal seam,and the maximum development height of the connected gas conduction fracture basically stabilizes at 86.3 m above the coal seam.At the same time,when the working face advances a certain distance,the central part of the goaf is re-compacted,and the mining cracks are closed.Only at the broken boundary of the overlying strata is within 31.65 m of the goaf direction and above the coal seam within 70 m,the opening degree,connectivity degree and distribution density of the mining cracks in the overlying strata can maintain a higher level for a long time,and the degree of gas pressure relief and desorption is higher and disintegration is higher.The numerical simulation results show that the maximum height of gas-conducting cracks in overlying strata is 89.6m above the coal seam with the advancing of the working face,but because of the re-compaction in the middle of the goaf,only the broken boundary of the overlying strata is within 32.5 m in the direction of the goaf and within 75.2m above the coal seam,good gas flow and extraction channels are maintained in the overlying strata.At the same time,during the process of coal seam mining,the vertical stress of overlying strata changes steadily from far away from the working face to entering the goaf.At last,due to the re-compaction of the goaf,the stress is restored,and only the overlying strata within 37 m of the broken boundary and within 90 m above the coal seam are fully relieved.Finally,through comprehensive analysis,the effective pressure relief extraction area of the upper adjacent seam in Wuyang Coal Mine No.3 under different mining conditions is predicted.Its spatial location is as follows:during mining,the overburden strata close to the open-off cut at the broken boundary of the overlying strata is within 33 m of the goaf direction and above the coal seam within 75 m,and the overburden strata close to the working face at the broken boundary of the overlying strata is within 55 m of the goaf direction and above the coal seam within 75 m,belong to the area of effective pressure relief and extraction;after mining,the overburden strata at the broken boundary of the overlying strata is within 33 m of the goaf direction and above the coal seam within 75 m are located in the area of effective pressure relief and extraction.(5)Combined with the prediction results of the effective pressure relief extraction area of the upper adjacent strata,the parameters of the adjacent strata gas extraction drilling arrangement on the 7801 fully mechanized caving face were designed.Through the field measurement of the gas extraction effect of the adjacent strata,it is found that the gas extraction effect of the upper adjacent strata shows an obvious three-stage variation characteristic during the advancement of the working face.That is,when the average hole of the gas extraction hole enters the goaf with a range of 37.4 m averagely,it is in the initial pressure relief pumping stage,and the gas extraction effect is relatively poor;when the gas extraction drilling hole enters the goaf with an average range of 37.4m?85.8m,it is in the stage of sufficient pressure relief and extraction,the extraction effect is significantly improved.The maximum gas extraction volume and extraction concentration of single hole are increased by 18.64 times and 13.56 times respectively compared with the initial pressure relief extraction stage;when the end hole of gas extraction drilling hole enters the goaf area with an average range of 85.8m,it is in the stage of compaction and pressure extraction,affected by the re-compaction of the goaf,and the single-hole gas extraction effect is greatly reduced.The change law of gas extraction in the upper adjacent esam further verified the reliability of the pre-judgment results of the effective pressure relief extraction area of the overburden.(6)Through the analysis of the field measurement results of mining face and gas control effect,it is found that the mining technology parameters of working face will have certain influence on the gas control effect.The effect of gas extraction in the effective pressure relief extraction area of the coal body in front of the working face varies with the change of the advancement speed.If the advancement speed is too fast or too slow,the effect of drilling gas extraction will be reduced.Through the analysis of the measured data,it is proposed that the advancecment speed is 4.0-5.6m/d(5-7 knives per day)and the working face length is 185m-259m,which can better ensure the coal production and gas extraction effect.(7)Through the theoretical prediction of the gas emission from the coal wall,falling coal,residual coal and other sources,the estimation formula of the gas emission from the working face is obtained.On the basis of that the production of the working face and the gas of the roadway are not exceeded,it is found that the maximum coal cutting speed of the shearer has a negative power function relationship with the length of the working face,which is y=41.283x-0.482,where y is the maximum coal cutting speed of the shearer and x is the working face length.Based on the formula,the maximum coal cutting speed of the shearer under the existing working face length of No.3 coal seam of Wuyang Coal Mine was calculated and didn't exceed 3.3m/min.Otherwise,the working face or the upper corner gas may exceed the limit.(8)Through the analysis of on-site measured gas extraction and discharge,it is pointed out that the existing extraction mode and extraction parameters have not yet reached the ideal effect of gas control on the working face,and further optimization is needed.It is proposed to use the space cross-drilling to extract the gas in the coal seam and design the parameters of the drilling and drilling arrangement.It is proposed to adopt the high-level gas extraction roadway to extract the gas in the adjacent strata and the goaf.and consider the mutual influence position between the roadways.The high-level gas extraction roadway is arranged on the basis of the fracture development zone.The layout parameters of the high-level gas extraction roadway are calculated and determined,and the pumping effect is numerically simulated.The simulation results show that the high-level gas extraction roadway can better control the adjacent strata and the goaf gas,to meet the requirements of the working face gas.Meanwhile,according to the important role of gas extraction roadway in working face gas management,gas extraction roadway support scheme and working management measures are proposed. |
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