Multiscale Transport Characteristics of Coalbed Methane in Abandoned Mines and Its Applications in Gas Recovery

A considerable amount of coalbed methane (CBM) that can be extracted by surface vertical wells exists in gobs of abandoned mines. A model of CBM enrichment and extraction in abandoned mine gobs during mine closure and methane extraction by surface vertical wells was established based on the migration law of CBM occurrence and time-varying diffusion in the dual-porosity medium of coal as well as the nonlinear seepage characteristics of CBM in gobs. The results are as follows. (1) During mine closure, the CBM pressure in the gob recovers gradually and stabilizes eventually; it decreases gradually from the gob interface with coal pillars to the gob center. The CBM pressure in the gob is correlated positively with the matrix pressure and diffusion coefficient of coal seam; compared with the permeability of the gob, these two factors exert greater impacts on CBM pressure recovery in the gob. (2) In the early stage of extraction, residual coal in the gob is the dominant mass source of CBM seepage. The closer the extraction well bottom is to the gob bottom, the wider the influence range of CBM extraction is, and the larger the CBM flow in the well is. The layout of surface wells, the negative extraction pressure and the extraction time can affect the production and recovery of CBM. The model in this study can provide a theoretical basis for the optimization of the layout of surface wells and the prediction on CBM enrichment and extraction in abandoned mine gobs.

Automated summary

A model of coalbed methane enrichment and extraction in abandoned mine gobs was established to study the migration law and seepage characteristics of CBM. It was found that during mine closure, CBM pressure gradually recovers and is positively correlated with the matrix pressure and diffusion coefficient of coal seam. Residual coal in the gob is the dominant source of CBM seepage during extraction, and the layout of surface wells, negative extraction pressure, and extraction time can affect CBM production and recovery.