Structural Control Of Gas Seepage And Prediction Of Residual Coal Seam Gas After Extraction

After pre extraction of coal seam gas,the danger of gas outburst cannot be completely eliminated in coal mine.Therefore,it is very important to study the occurrence regularity of residual gas after extraction and its geological influencing factors for further reducing coal mine gas disasters.Against this background,the Xutuan coal mine,a high gas mine was taken as an example for the study about residual gas.The structural complexity of mine was evaluated quantitatively by fractal dimension method.The tectonically deformed coals（TDCs）types of study area was divided according to the characteristics of coal sample micro-deformation.The development thickness and distribution range of the TDCs were discussed.The conclusion can be drawn that the structural complexity of in the study area is obviously affected by the large scale reversed faults and the TDCs development is mainly controlled by those reversed faults.Research shows that in TDCs,the crumpled and cracked microstructures are well developed and the mesopores and macropores account for large proportions.The permeability of TDC is mainly controlled by structural deformation degree,effective stress（or effective confining pressure）,adsorption swelling effect and slippage effect.On the premise of only effective stress increasing,the internal space of deformed coal is compressed and the slippage effect is weakened.The both factors contribute to the permeability reducing in an exponential form with the increase of effective stress.On the premise of only pore pressure increasing,the gas seepage channels are narrowed and obstructed because of the adsorption swelling effect and the slippage effect is also weakened.In this case,the permeability will also be reduced.Thus,the permeability of TDC is larger than that of undeformed coal.Generally,the permeability change rate of TDCs is larger than that of undeformed coal.The stress sensitivity coefficient of permeability also shows a negative correlation to the deformation degree of coals.A quantitative evaluation of the structural complexity of mining area No.3₂33 in the study area was carried out through comprehensive consideration of fault fractal dimension,structural curvature,and coal seam floor dip angle.By comparing the change of gas content after extraction and the change with their influence factors,this paper discussed the influence of structural comprehensive index,buried depth,coal thickness,and raw coal gas content on the unit gas drainaged volume（UGDV）.The result shows that the structural comprehensive index can more accurately reflect the complexity of the structure of the mine and can be used to characterize the coal seam permeability.The UGDV is mainly controlled by these geological factors such as permeability of coal seam（structural comprehensive index）,buried depth,coal thickness,raw coal gas content,and permeability of coal rock.Except for the permeability of coal rock,the UGDV is positively correlated with the permeability of coal seam,buried depth,coal thickness and raw gas content of coal seam.A BP artificial neural network model was established with a structure of 5×10×1layers and nodes for the prediction of UGDV.The model has good convergence and high precision,and the average deviation is 2.96%-6.15%.The results of the model analysis show that there is a close internal relationship between the influence factors（the permeability of coal seam,buried depth,coal thickness,raw coal gas content and permeability of coal rock）and the unit gas extraction rate,and it is feasible to predict the residual gas by BP neural network method.