Research On Characteristics Of Effective Gas Extraction Radius Of Coal Seam Drilling Along Seam

With the continuous increase of mining depth,the characteristics of coal seam heterogeneity,low permeability,low permeability and low saturation will become increasingly prominent,and the hidden danger of coal and gas outburst will become more and more serious.Borehole gas extraction is one of the main measures to prevent gas emission,regional outburst and local outburst and improve the output of coalbed methane.Accurately determining the effective drainage radius of coal seam gas drilling is of great significance to improve the efficiency of drilling gas drainage.In order to reasonably predict the effective drainage radius of bedding hole in coal mine gas control,based on the basic theory of fluid solid coupling of gas bearing coal,a three-dimensional fluid solid coupling seepage model of bedding hole gas drainage is established by using COMSOL simulation software PDE module and solid mechanics module.Simulate the three-dimensional gas pressure distribution cloud map of bedding borehole along the direction of borehole diameter;Combined with the critical value of gas pressure（0.6 MPa）,the effective drainage radius is determined,and the influence law of different gas drainage time,borehole diameter and borehole drainage negative pressure on the effective drainage radius of bedding borehole is studied;At the same time,the gas drainage process of multiple bedding holes in coal seam under different hole spacing is simulated,and the volume of the effective drainage area in the three-dimensional map is visualized by using the residual gas pressure isosurface.By observing the volume change of the effective drainage area,the influence of different hole spacing on the gas drainage effect of coal seam is quantitatively analyzed;The gas drainage volume per unit coal body is introduced as the judgment standard of field test,and the test results are compared with the numerical simulation results;The results show that:（1）The effective extraction radius is basically R=α(1-e^-bt) a function of the extraction time of the borehole,and the extraction days are the key factor to determine the effective extraction radius of the borehole.（2）There is a positive correlation between bedding borehole diameter and effective extraction radius,and the increase is becoming more and more obvious in the process of continuous extraction;When the pumping negative pressure decreases from-20 k Pa to-35 k Pa,the porosity and permeability of the borehole increase slightly,which has a limited impact on the effective pumping radius of the bedding borehole.（3）There is overlapping effect during gas extraction when several adjacent drilling holes along the layer are arranged for gas extraction at the same time;The smaller the spacing between boreholes,the more obvious the overlap effect in the borehole control area,and the faster the gas pressure decelerates with the same extraction time;Considering the law of volume variation of effective extraction area comprehensively,it is determined that the optimum drilling spacing for gas extraction is 3.0 m for four-flower-eye drilling and 2.5 m for three-flower-eye drilling.（4）In the field test,the effective extraction radius of the borehole is judged based on the gas extraction volume of the unit coal body,and compared with the numerical simulation results.The average relative error of the numerical simulation results is9.52%,and the average relative error is less than 10%;It shows that the research work of this paper can provide a certain theoretical basis for underground borehole gas drainage.Because the method of field measurement of effective extraction radius is complex,the effective extraction radius can be reasonably predicted by this numerical simulation method;At the same time,the simulation calculation method of bedding borehole spacing with effective extraction radius,overlapping effect and volume change law of effective extraction area as indicators is proposed,which can provide a certain theoretical basis and reference for rational arrangement of borehole spacing and comprehensive gas control.There are 42 figures,9 tables and 93 references.