Experimental analysis of methane adsorption-diffusion property in high-maturity organic-rich shale and high-rank coal

Methane adsorption-diffusion property in coalbed/shale is one of the key factors for coalbed methane/shale gas resource evaluation. With No. 3 coal seam of Permian Shanxi Formation in Sihe coalfield Jincheng mine area and shale of Longmaxi Formation in Lower Silurian of Paleozoic in south China as study objects, the nano-pore system determination using low temperature liquid nitrogen adsorption method and methane adsorption-diffusion experiments of manometer method was conducted, methane adsorption-diffusion properties of high-maturity organic-rich shale samples and high-rank coal sample were elucidated, and then a mathematical model of methane adsorption-diffusion in coal/shale was established, finally the difference of methane adsorption-diffusion in coal and shale and its controlling mechanism were analyzed. It turns out that, meso-pores and macro-pores contribute in a major way to the pore volume of high-maturity organic-rich shale and coal samples and micro-pores and meso-pores contribute in a major way to the specific surface area; the nano-pore system is more developed in shale samples than that in coal sample and the specific surface area and pore volume in shale samples are bigger than that in coal samples; the methane adsorption-diffusion in coal and shale samples can be governed by Langmuir Equation, high-maturity organic-rich shale has a much lower methane adsorption capacity than high-rank coal and the methane adsorption capacity of shale increases with the increase of total organic carbon; open pores dominate in high-maturity organic-rich shale samples while semi-sealed pores dominate in high-rank coal sample; for semi-sealed pores, the dissolution of strong adsorbate into adsorbent will alter the macromolecular structure of adsorbent and result in the irreversibility of adsorption and desorption, that is, desorption hysteresis; therefore the diffusion properties during methane desorption-diffusion are different from that during methane diffusion-adsorption in high-maturity organic-rich shale and coal. ©, 2015, Science Press. All right reserved.