Experimental studies on the changing characteristics of the gas flow capacity on bituminous coal in CO_2-ECBM and N_2-ECBM

摘要

Compared with the pre-modification technique, such as hydraulic fracturing, the gas injected for enhanced coalbed methane recovery (ECBM) technique offers not only a good economic advantage by enhancing the CH4 recovery, but also provides an environmental benefit with CO2 storage, and the changing characteristics of the flow capacity in coal reservoir critical affect both the CH4 production and CO2 geo-sequestration. In this pursuit, the present study envisaged the determination of the change in characteristics of the gas flow capacity in ECBM using N2 and CO2 core flood experiments on CH4 saturated bituminous coal. The critical controlling factors of the changing characteristics were discussed. The whole experimental process was divided into two stages, in which the first stage was an unsteady flow in coal that directly led to a sharp drop on both the volumetric strain and the outflow rate, while the second stage was a steady flow in coal. It was found that the changing trend of permeability in both tests followed the same relationship: rise (I), stable (II), slowly drop (III), steady-state (IV). The differences on both the flowing ability and the displacing effect between CO2 and N2 mainly played the role in the outflow rate, volumetric strain, and the duration of each stage in permeability. Total recovery of CH4 was 66% at the stages I + II + III in N2-ECBM test, while it was obtained 80% after the unsteady flow stage in CO2ECBM test, thereby showing the different characteristic between two tests. Based on the effective stress principle, the changing characteristics in stress caused by adsorption were discussed to analyse the differential flowing characteristics of two tests. Our results provide new insight into the changing characteristics of flow capacity on