Multi-scale characteristics of coal structure deformation during methane adsorption

The surface structure of meso-anthracite samples was observed by scanning electron microscope ( SEM). The multi-scale heterogeneity characteristics of meso-structure of coal caused by the diversity of the mesopores and fractures in the vitrinite were studied. Comparing the results of CT scanning test and the analysis of coal sample sur- face,it is found that the heterogeneity of the meso-structures and the density distribution of anthracite is strong due to the diversity of the meso-pores and fractures in the vitrinite in the small scale. With the scale increase,the heterogeneity of the meso-structures and the density distribution of anthracite becomes weak. Through multi-scale three-dimension- al segmentation and the statistics of CT scan data of coal adsorbed methane under different adsorption pressures,the variation of the correlation between meso-structure swelling / extrusion deformation after coal adsorbed methane and density distribution with structure-scale change was revealed. Based on the statistics and analysis of swelling / extrusion deformation amount and volume of swelling / extrusion deformation of coal structure under different scales and adsorp- tion pressures,the main controlling factors of swelling and extrusion deformation in coal were revealed respectively. It is found that the volume of extrusion deformation is always smaller than that of the swelling deformation due to methane sorption. The attenuation process of swelling deformation and extrusion deformation of coal structures is asynchronous with the increase of scale. At small scale,the extrusion deformation of coal structure is larger than that of swelling de- formation. With the increase of scale,the attenuation rate of extrusion deformation is faster than that of swelling deform- ation. This study has an important theoretical value for the accurate and dynamic evaluation of multi-scale gas seepage characteristics in the process of coalbed methane exploitation.