Experimental study of influence of gas pressure on coal shale gas adsorption characteristics based on nuclear magnetic resonance spectrum

Gas pressure is the major impact factor on gas adsorption of coal shale. Based on the typical high gassy mine, the coal shale of Qinghemen Mine in Fuxin is taken as research object. The sample is placed into the low field Nuclear Magnetic Resonance(NMR) instrument. Through increasing gas pressure, the adsorption process of coal shale gas is simulated. The amplitude integration of T-2 spectrum of nuclear magnetic resonance is used as a quantitative index to reflect the gas adsorption capacity. From the micro-quantitative perspective, the influence law of gas pressure on adsorbed gas and free gas is presented. The results show that (1) The ranges of T-2 spectrum curve for adsorbed gas and free gas can be determined by the two cutoff thresholds. (2) Gas pressure has significant influence on the increment of adsorbed gas and free gas in coal shale. The incremental change of adsorbed gas is controlled by the interaction force between coal shale and gas molecule. The increment of free gas is mainly related to the pore structure of coal shale. (3) The relationship between adsorbed gas and gas pressure is in accordance with the Langmuir isothermal adsorption equation. The relation between free gas and gas pressure is cubic function. (4) The mean value of gas T-2 spectrum is used to quantitatively describe the expansion deformation of the average pore radius. With the increment of gas pressure, the medium-macro pore structures are expanded, and the average radius increases 1.47 times. But the micro-pore structures are not affected.