The research on gas migration law during coal bed methane (CBM) exploitation is beneficial to understand the source of gas

impact of coal seam position on gas production and the law of gas pressure decay

thus providing foundation for design of exploitation time and the position and length of drilling hole. Using the multi-field coupling testing system for CBM exploitation developed by the authors

physical simulation experiments of CBM exploitation under different lengths of drilling hole in distressed zone were carried out

and relative gas flow speed and direction were analyzed. The results show that the initial stage of exploitation is the main contribution period to gas production and the surrounding area of drilling hole is the main contribution area to gas production

because gas pressure gradient is high and thus gas flows fast. Gas flow has the highest speed at the distressed zone. Existence of concentrated stress zone makes the relative flow speed attenuate quickly

and also creates a barrier to original stress zone

leading relative gas flow speed to be zero. Relative gas flow speed decreases with time while the migration direction does not change much because relatively stable seepage channels are formed at the beginning of exploitation. Due to gas pressure drop that leads gas pressure gradient and relative gas flow speed to decrease

the uncertainty of migration direction rises. With the length of drilling hole growing

the relative gas flow speed increases in the distressed zone

so it is meaningful to moderately lengthen the drilling hole.