Experimental study on the influence of drainage borehole position on dynamic parameters of coal seam in CBM recovery process

Using a self-developed multi-field coupling testing system for CBM extraction,the physical simulation exper- iments of CBM extraction with different drill hole locations under a non-uniform stress state were carried out,and the gas flow characteristics and the influence law and mechanism with drill hole locations were studied. The results show that gas pressure decreases quickly at the beginning but slowly in the end during CBM extraction,and the variation law is in accord with Weibull distribution function;the variation trend of temperature is consistent with gas pressure. The shorter distance to the drill hole,the faster gas pressure and temperature decrease and the bigger gas pressure gradient is,otherwise the smaller gas pressure gradient is. The stress concentration area seems to hinder gas flow on a certain extent and thus low CBM extraction efficiency,but the pressure relief area which has high permeability is favorable for gas flow. After analyzing the fitted Weibull parameters,the authors canclude that the drill hole locations influence extraction efficiency apparently,location and scale parameters are relatively small when extraction is in pressure relief ar- ea,which suggests a faster gas pressure decrease and more effective extraction,and also the extraction quantity is the biggest. Therefore,when using single drill hole,the extraction in pressure relief area is relatively an optimal method.