Abstract: 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.

Key words: CBM exploitation, physical simulation, relative gas flow speed, deviation angle, exploitation time, length of drilling hole