Experimental study of coal rank effect on carbon dioxide injection to enhance CBM recovery

Coal seam gas extraction is an important part of green mining. At the same time gas injection for enhanced coalbed methane recovery (ECBM) has been proposed as an environmentally friendly, low-carbon and effective technology to increase methane production. To study the influence of coal rank on carbon dioxide (CO2)-ECBM efficiency, lignite, bituminous coal, and anthracite were tested using the Gas Flow and Displacement Testing Apparatus (GFDTA). Data indicated that: (1) in lignite and anthracite, outflow CO2 and CH4 fractions changed at higher rates as compared with bituminous coal. At the end of the experiments, lignite and anthracite displayed CH4 outflow fractions of 0.19% and 7.73%, respectively; (2) at the initial stage of the experiments, permeability rapidly decreased. Later, a fast increase was followed by a stable phase. In lignite and anthracite, the increment in permeability during the rapid increase stage was significantly larger than that of bituminous coal. (3) The CH4 flow rate of the three coal ranks showed a decreasing trend. Before the breakthrough, CH4 flow rate decreased rapidly and slowed down. After the breakthrough, CO2 flow displayed a linear increasing trend and later reached stability. (4) In general, CH4 production increased with the increase in coal rank. The variation of CH4 production is manifested in two phases with different dominant factors. In the first stage, CH4 production was mainly affected by permeability, and followed the order lignite > anthracite > bituminous. In the second stage, CH4 production was mainly controlled by the displacement effect, following the order anthracite > bituminous coal > lignite.