1. College of Geology and Environment, Xi’an University of Science and Technology, Xi’an 710054, China 2. Shaanxi Provincial Key Laboratory of Geological Support for Coal Green Exploitation, Xi’an 710054, China 3. Geological Research Institute for Coal Green Mining, Xi’an University of Science and Technology, Xi’an 710054, China 4. Aerophoto Grammetry and Remote Sensing Bureau, China National Administration of Coal Geology, Xi’an 710100, China
High- and low-rank coalbed methane (CBM) are both important fields of CBM development in China, but their formation and production mechanisms differ considerably. The adsorption/desorption behavior of high- and low-rank coals under the coupling of coal–water–gas was investigated using two series of samples. Coal samples from Zhangjiamao (ZJM) coal mine, Ordos basin, and Sihe (SH) coal mine, Qinshui basin, were tested by isothermal adsorption–desorption experiment, natural imbibition experiment, nuclear magnetic resonance, mercury injection porosimetry, contact angle test, and permeability test. Isothermal adsorption and desorption experiments under dry, equilibrium water, and saturated water, were performed to explore the differences between the adsorption and desorption characteristics. The results show that the wettability and permeability of the ZJM low-rank coal sample was considerably higher than that of the SH high-rank coal sample. The imbibition process of the ZJM sample exhibited a high imbibition rate and high total-imbibition volume, whereas the SH sample exhibited a slow imbibition rate and low total-imbibition volume. The ZJM sample had a complex pore structure and diverse pore-size distribution with a lower mercury withdrawal efficiency at 59.60%, whereas the SH sample had a relatively uniform pore-size distribution with a higher mercury withdrawal efficiency at 97.62%. The response of adsorption and desorption of the ZJM sample to water was more significant than that of the SH sample. The desorption hysteresis of the ZJM sample was stronger than that of the SH sample and was more prominently affected by water, which was consistent with its strong wettability and complex pore-throat configuration. A comprehensive adsorption and desorption mode was constructed for high- and low-rank coal samples under coal–water–gas coupling condition. The research results are important to enrich the geological theory of high- and low-rank CBM and to guide efficient CBM recovery.
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