Mechanism of adsorption-desorption hysteresis and its influence on deep CBM recovery
The phenomena of methane adsorption-desorption hysteresis in coal has been observed by many scholars. The results indicate that the adsorption and desorption are not fully reversible. This paper analyses the previous studies on this phenomena and proposes an improved hysteresis index (IHI). The influences of maximum pressure and coal particle size on hysteresis degree are studied by sorption test. The mechanism of sorption hysteresis and its impact on deep coalbed methane (CBM) recovery are discussed. Results show that IHI can reflect the degree of sorption hysteresis from a fully irreversible status to a fully reversible status. The hysteresis degree increases with growing maximum pressure and coal particle size. The result of sorption test is an integration of sorption and diffusion, and these two processes cannot be differentiated. Therefore, a 'restricted diffusion hypothesis' is proposed to explain the present phenomena. It is due to the diffusion restriction of narrow pore throat, which is induced by the imbedded gas molecules during high pressure adsorption. The gas content of deep CBM may be very high, however, due to the sorption hysteresis, the minable gas content and the production perspective should be evaluated using desorption isotherm rather than adsorption isotherm. Besides enhancing coal permeability, some novel methods to increase desorption and diffusion rate should be considered for deep CBM recovery.