Experimental investigation of methane adsorption / desorption behavior in coals with different coal-body structure and its revelation

The adsorption / desorption experiment of bulk coal sample can be more effective to characterize the flow state of methane (CH4 ) in coal seams compared to powder coal sample,since CH4 flow through a coal seam consists of fluid flow,diffusion and adsorption / desorption process. Thus,the low-pressure CH4 adsorption / desorption properties of bulk primary coal ( PC), cataclastic coal ( CC) and mylonitic coal ( MC ) with identical volume collected from Hancheng Mine,Weibei coalfield were analyzed using self-designed adsorption / desorption apparatus of bulk coal. In combination with micro-focus CT and scanning electron microscope images,the influence of coal-body structure on CH4 adsorption / desorption behavior was discussed. The results indicate that the obvious difference can be observed in coals with different coal-body structures. Long CH4 adsorption / desorption equilibrium time,small adsorption capacity and low desorption rate were recorded in PC due to its dense structure and low porosity. In comparison with PC,a shorter adsorption / desorption equilibrium time,higher adsorption capacity and faster desorption rate were occurred ow- ing to developed and connected fracture system and increased porosity in CC under brittle tectonic stress. Under duc- tile deformation,the coal composition of MC was folded and concomitant with increased pore number and closed frac- ture,which induced isolated pore structure and poor permeability. These characteristics result in the shortest adsorp- tion / desorption equilibrium time and the fastest desorption rate,indicating most of CH4 in bulk MC may merely adsorb on inter-granular pores formed during deformation process. Therefore,the CC reservoir is a favorable area for CBM de- velopment. Nonetheless,for both PC and MC reservoirs,the increase in the well yield and efficiency can be achieved through reservoir reformation by means of multi-scale fracturing,heat injection and other techniques which can increase the channels within coal fracture.